The Role of Calcium in the Mechanism of Relaxation of Cardiac Muscle*

Transcription

1 THE Jomtmr. OF Bmmarcar, CHEMSTRY Vol. 239, No. 7, July 1964 Ptintea in U.S.A. The Role of Calcium in the Mechanism of Relaxation of Cardiac Muscle* B. FANBURG,~ R. M. FNKEL,$ AND A. MARTONOS From the Department of Muscle Research, nstitute of Biological and Medical Sciences, Retina Foundation, and Cardiac Biochemistry Laboratory, Department of Medicine, Massachusetts General Hospital, Boston 14, Massachusetts (Received for publication, December 11, 1963) Over the last few years, the concept has evolved that calcium ions might play an important role in the regulation of the contraction-relaxation cycle of skeletal muscle. The strongest experimental evidence in support of this idea was the demonstration by Weber (l-4) with her associates that superprecipitation and ATPase activity of skeletal muscle myofibrils and actomyosin require the presence of about 5 pm Ca++ in addition to magnesium and ATP, and that removal of the calcium from the medium by EGTAl or Chelex resin results in the reversal of superprecipitation (clearing) and inhibition of ATPase activity. t has been established that the active components of skeletal muscle-relaxing factor preparations are vesicular elements (6-9),2 probably originating in the sarcoplasmic reticulum, which in the presence of magnesium and ATP are capable of accumulating Ca++ against a large concentration gradient (10-12). The lowering of the free Ca++ concentration by these vesicular elements may account for the effect of relaxing factor preparations on myofibrils, glycerinated fibers, and actomyosin (4, 13, 14). Earlier observations on the relaxing effect of a preparation obtained after the removal of particles from an incubation mixture containing grana, magnesium, and ATP (15, 16) have now been shown to be interpretable in terms of removal of Caf+ from the medium rather than in terms of the formation of a soluble relaxing substance (17). However, other recent studies have produced further evidence suggesting that a soluble relaxing substance from skeletal muscle may exist (18-20). While a more specific identification of a possible soluble relaxing substance requires further evaluation (a claim that cyclic adenosine * This work was supported by research grants from the National nstitutes of Health, United States Public Health Service (H5949), Life nsurance Medical Research Fund, National Science Foundation and American Heart Association. t Trainee, Graduate Training Program HTS 5391 of the National nstitutes of Health, United States Public Health Service. t Present address, University of Minnesota Hospital, Minneapolis, Minnesota.. 6 This work was carried out during the tenure of an Established niestigatorship from the American Heart Association. Present address, Department of Biochemistry, University of Birmingham, Birmingham, England. 1 The abbreviation used is : EGTA, 1,2-bis-(2-dicarboxymethylaminoethoxy)ethane (5) ( ethylene glycol bis (P-aminoethylether)-N,N -tetraacetic acid (2)). 2 These vesicular elements will be referred to as grana throughout the paper and this is synonymous with other terms such as microsomes, fragmented sarcoplasmic reticulum, particulate relaxing factor, and relaxing granules used by various authors. 3 ) 5 -monophosphate is the physiological relaxing substance has recently been withdrawn (21)) the removal of Ca* by grana, with or without the participation of a soluble relaxing substance, appears to be an important feature in the relaxation mechanism of skeletal muscle. n view of the many similarities in the physicochemical and qualitative enzymatic properties of cardiac and skeletal contractile proteins (22, 23), a basically similar mechanism for the regulation of contraction and relaxation could be expected to operate in these two muscles. Yet, Finkel and Gergely found the ATPase activity of cardiac myofibrils to be largely unaffected by EDTA under conditions where the ATPase of skeletal muscle myofibrils was strongly inhibited (24) and thus suggested that the cardiac myofibrils differ from their skeletal counterparts in some important aspect related to the relaxing mechanism. Furthermore, reports from various laboratories indicated that particulate relaxing factor preparations obtained from heart muscle are considerably less effective, both with regard to inhibition of the contraction of glycerinated muscle fibers (25) and the uptake of calcium (26) than those of skeletal muscle. Stam and Honig reported the formation of a soluble relaxing substance by incubation of heart grana with ATP and magnesium (27,28). n contrast with comparable preparationsobtained from skeletal muscle (15, 16), the relaxing effect of their cardiac soluble substance was not inhibited by calcium. From a review of the literature, one gains the impression that there is considerable evidence against the participation of calcium in the mechanism of relaxation of cardiac muscle. The purpose of the present experiments was to reinvestigate the role of calcium in the regulation of the ATPase activity of cardiac myofibrils and the possible involvement of cardiac grana in the regulation of contractility of cardiac myofibrils. The results support the view that calcium plays the same role in the mechanism of contraction and relaxation of cardiac myofibrils and actomyosin as in the skeletal system. The inhibition of syneresis of cardiac myofibrils by cardiac grana depends on the uptake of calcium by grana resulting in a lowering of the free calcium concentration below a critical level. While these experiments were in progress, a similar view was expressed by Weber, Here, and Reiss (29). EXPERMENTAL PROCEDURE Calf hearts, removed 5 to 15 minutes after the animals were killed, were immediately immersed and transported in 0.1 M Tris buffer, ph 7.5, at 2. Fat and connective tissue were 2298

2 July 1964 B. Fanburg, R. M. Finkel, and A. Martmosi 2299 trimmed away, and the muscle was cut into small pieces for rabbit skeletal muscle were also prepared as described above. homogenization in a Waring Blendor. This and all subsequent Sucrose was omitted from the solutions in some cardiac and in steps were performed at 2. all skeletal grana preparations. Preparation of MyojibrilsThe muscle was homogenized with ATPase Measurements-ATPase activity was measured by de- 3 volumes of 0.1 M Tris, ph 7.5, for 2 minutes. The homogenate termining the liberation of inorganic phosphate according to Fiske was centrifuged at 600 X g for 20 minutes, and the top layers of and SubbaRow (31) after termination of the reaction with 10% sediment were suspended in 0.1 M Tris, ph 7.5, and passed trichloroacetic acid. The amount of ATP hydrolyzed did not through a strainer or two layers of cheesecloth for removal of exceed 15% of that originally present. coarse material. The myofibrils were washed by repeated Syneresis of MyoJibrilsSyneresis was measured essentially as resuspension in 0.1 M Tris, ph 7.5, and centrifugation at 600 X described by Mueller (32) and by Seidel and Gergely (14). g, and each time the coarse bottom layer of the sediment was Myofibrils were incubated with 1 mm EGTA in a solution of 0.1 M discarded. Finally, the sediment was suspended in 0.1 M Tris, Tris, ph 7.5, at 2 for 30 to 60 minutes for the removal of exph 7.5, and was stored at a final protein concentration of 10 to changeable calcium from the myofibrils. The myofibrils were 15 mg per ml in 50% glycerol and 0.05 M Tris, ph 7.5, at -20. then washed three times with 0.1 M Tris, ph 7.5 (50 to 100 times Before the experiments, the myofibrils were washed 3 times with the volume of the myofibrillar sediment), for the removal of 0.1 M Tris, ph 7.5, and resuspended in this medium. The sus- EGTA and were resuspended in 0.1 M Tris, ph 7.5, to a protein pension was stored at 2 for use over the next 3 or 4 days. Myo- concentration of approximately 10 mg per ml. Of this suspenfibrils from rabbit skeletal muscle were prepared in a similar sion, 3 ml were centrifuged at 1000 X g for 10 minutes in Kolmer manner. n an effort to remove the nuclear fraction from the tubes for separation of myofibrils, and the solution in which myofibrils, in some cases the method described by Perry and syneresis was to be studied was added to the myofibrils. When Grey (30) with the separation of heavier elements at 400 x g the effect of grana on the syneresis of myofibrils was studied, t.he in a M KCl M sodium borate isolation medium was grana were preincubated at room temperature (21-22 ) for used. varying intervals with the solution to be added to the myofibrils. Preparation of Calf Heart Actomyosin-The muscle was homog- The suspension was then incubated for 5 minutes at room temenized for 1 minute with 3 volumes of Weber-Edsall solution perature and centrifuged at 1000 X g in a swinging rotor of the (0.6 M KC, 0.04 M NaHC03, and 0.01 M Na&Os), and the homog- nternational centrifuge for 1 minute, and the sedimented myofienate was allowed to stand for 20 hours with constant stirring. brillar volume was recorded. The insoluble material was removed by centrifugation at 1,000 X Superprecipitation and Clearing of Actomyosin-The absorbg for 1 hour, the ph of the supernatant was adjusted to 7.0, and ance of an actomyosin suspension was measured at 660 rnp in a 9 volumes of distilled water were added. The precipitate formed Zeiss spectrophotometer following the addition of ATP. The was allowed to settle for 2 hours and was collected by centrifuga- content of the cell was mixed by inversion before each reading. tion at 1,000 x g for 30 minutes. The sediment was dissolved Superprecipitation corresponds to an increase, clearing to a with the addition of 2.4 M KCl, and adjustment to 0.6 M KC1 decrease, in optical density (13). was made by adding distilled water. The solution was cen- Measurement of Calcium Uptake-Grana were incubated in trifuged at 40,000 x g for 1 hour, the precipitate was discarded, solutions which contained 46Ca for varying times at 22, followed and the ph of the supernatant was adjusted to 7.0. Five vol- by the removal of grana by filtration through Millipore filters umes of distilled water were added, and after 30 minutes the (type HA with an average pore diameter of 0.45 p) (33). The precipitate formed was separated by centrifugation at 1,000 X g radioactivity of the particle-free filtrate was measured by the for 30 minutes and was dissolved with 2.4 M KC1 with adjustment method of Loftfield and Eigner (34). The amount of bound to 0.6 M KC1 as described above. Undissolved material was calcium was calculated from the difference in the radioactivity of removed by centrifugation at 40,000 X g for 1 hour. The the original solution and the particle-free filtrate. actomyosin was stored in a solution of 0.3 M KC1 and 50% Chemical Determinations-DNA was determined according to glycerol at -20. Before use, the glycerol was removed by the method described by Schneider (35). Protein concentration precipitation of actomyosin by dilution to 0.05 M KCl, centrifuga- was determined by the biuret method standardized against tion, and subsequent washings of the precipitate with about 100 crystalline bovine serum albumin. volumes of 0.05 M KC1 before resuspension in 0.05 M KCl. Preparation of Subcellular Particles-The muscle was homog- RESULTS enized in a Waring Blendor for 45 seconds with 3 volumes of a solution of 0.1 M KCl, 5 mm histidine (ph 7.4), and 0.32 M sucrose. Eg ect of EGTA and onic Strength on A4 TPase of Cardiac Myojbrils and Actomyosin The homogenate was centrifuged at 1,000 X g for 20 minutes for the sedimentation of myofibrils and nuclei. The supernatant The ATPase activity of cardiac myofibrils prepared by widely was centrifuged at 12,000 x g for 10 to 20 minutes for the re- used methods differs in important aspects from that of skeletal moval of mitochondria. The particulate fraction used in most myofibrils. The rate of AT!? hydrolysis by cardiac myofibrils of the experiments was obtained by centrifugation of the super- at P/ is lower and its inhibition by EGTA is considerably natant at 40,000 X g for 30 minutes. n some preliminary less pronounced than that of the skeletal enzyme (Fig. 1). experiments, a fraction was prepared by centrifugation of the Whereas the magnesium-activated ATPase of the cardiac prepasupernatant at 30,000 to 100,000 X g for 1 hour. The sediment ration is decreased by only about 65% when the ionic strength is was homogenized in a solution of 0.1 M KCl, 5 mm histidine, ph increased from 0.07 to 0.6, that of the skeletal myofibrils is almost 7.4, and 0.32 M sucrose and the final protein concentration was completely inhibited and at P/2 0.6 is actually exceeded by the adjusted to 2 to 5 mg per ml. The preparations were either cardiac ATPase. f one compares cardiac actomyosin (Fig. 2) stored at 2 or kept frozen at -20. Subcellular fragments of with cardiac myofibrils under identical conditions, a marked

3 2300 Ca* and Relaxation of Cardiac Muscle 0 \ Y \ :i?q& \ \ \ \ b \ J 0.6 /ON/CSTRENGTH FG. 1. Effect of EGTA and ionic strength on the magnesiumactivated ATPase of cardiac and skeletal myofibrils. ncubation was performed for 5 minutes at 22 in a medium which contained 5 mm MgC&, 5 mm ATP, 20 mm Tris (ph 7.5), and various concentrations of KCl. Cardiac and skeletal myofibril protein concentrations were 0.6 and 0.25 mg per ml, respectively. wm, cardiac myofibrils without EGTA; O-0, cardiac myofibrils with 0.1 mm EGTA; X, cardiac myofibrils with 0.1 mm EGTA and 0.1 mru calcium; l , skeletal myofibrils without EGTA; O , skeletal myofibrils with 0.1 mm EGTA. difference between the two preparations is noted. The ATPase activity of actomyosin is markedly inhibited by 0.1 mm EGTA or by increasing the ionic strength and these effects more closely resemble those seen with skeletal actomyosin or myofibrils. Syneresis of Cardiac Actomyosin and Myoj%rils Similarly to that which previously has been shown for skeletal actomyosin and myofibrils (3, 13), syneresis of cardiac actomyosin (Fig. 3) or myofibrils (Table ) is inhibited by 0.1 mm EGTA and at high ionic strength. Fractionation of ATPase Activity of Cardiac Myojibrik The gross difference in ATPase activity of skeletal and cardiac myofibrils suggests the presence of a contaminating ATPase in the cardiac preparation which is not influenced by EGTA or high ionic strength. Since myofibrils are obtained and purified by repeated sedimentation at 600 x 9, a possible source of contamination would be the nuclear fraction. n fact, the DNA content of cardiac myofibrils was found to be considerably higher than that of skeletal myofibrils (average DNA content of five cardiac myofibrillar preparations is 2.46 pg of DNA phosphorus per mg of protein with s.d. 0.46; average content of three skeletal preparations is 0.26 pg of DNA phosphorus per mg of protein with s.d ) and the presence of a large number of unbroken nuclei (considerably more than in similar skeletal preparations) was /ON%.S?RENG?Ti FG. 2. Effect of EGTA and ionic strength on the magnesiumactivated ATPase of cardiac actomyosin. ncubation was performed for 5 minutes at 22 in a medium which contained 5 mm MgCL, 5 mm ATP, 20 mm Tris (ph 7.5), varying concentrations of KCl, and 0.6 mg of actomyosin per ml. O--O, cardiac actomyosin without EGTA; , cardiac actomyosin with 0.1 mm EGTA: 0. cardiac actomvosin with 0.1 mm EGTA and 0.1 mm CaCL. L------A TMEAFJFR ATPADDlrlonl,MlNUTES Fig. 3. Effect of EGTA and ionic strength on the superprecipitation of cardiac actomyosin. Superprecipitation was followed by optical density measurements in a Zeiss spectrophotometer at 660 rnp aa described in Experimental Procedure. The incubation medium contained 5 mm MgCL, 5 mm ATP, 20 mm 7.5), varying concentrations of KCl, and 0.7 mg of actomyosin O-O / without EGTA; A--& r/ %ih% EGTA; X-X r/ without EGTA. +m / without EGTA; Cl-- -6, / with 0.i mm EGTA; A A, '/ with 0.1 mm EGTA; , r/ with 0.1 mm EGTA; J , r/ with 0.1 mm EGTA. demonstrated by microscopic examination of cardiac myofibrillar preparations stained by the Feulgen technique (36): The absence from cardiac actomyosin preparations cf the EGTA-insensitive ATPase indicates that this ATPase is either not solubilized by Weber-Edsall solution or is differentially removed from actomyosin during the subsequent purification steps. Evidence for the presence of a contaminating ATPase in cardiac myofibrillar preparations was obtained by differential extraction of actomyosin or myosin from myofibrils with Hasselbach- a Although cardiac myofibrils are contaminated by a large number of nuclei, it now appears that the contaminant ATPase is of mitochondrial origin. Evidence for this will be presented in a subsequent paper. -

4 July 1964 B. Fanburg, R. M. Finkel, and A. Martonosi 2301 Schneider solution or 0.6 M K and the demonstration of a residual ATPase activity with properties different from those of actomyosin ATPase. Extraction with Hasselbach-Schneider Solution-Cardiac myofibrils (protein concentration, 5.0 mg per ml) were extracted with Hasselbach-Schneider solution (37) for 30 minutes at 2. The insoluble residue was separated by centrifugation at 10,000 X g for 20 minutes and was washed repeatedly with 0.1 M Tris, ph 7.5, and resuspended in this medium. The supernatant was dialyzed against 0.04 M KC1 and 0.6 mm KHZPO~, ph 7.2, for 3 hours, and the precipitate was collected by centrifugation at 1,000 X g for 30 minutes and was subsequently washed with and resuspended in 0.1 M Tris, ph 7.5. The ATPase activity of untreated myofibrils, the extract and the insoluble residue are compared in Fig. 4. The ATPase activity of the extract is much more strongly inhibited by EGTA or by increasing the ionic strength than that of the untreated myofibrils. The insoluble residue contains an ATPase which is unaffected by EGTA, is only moderately inhibited by increasing the ionic strength to 0.6 TABLE Ej ect of EGTA on syneresis of cardiac myo$brils The experiments were carried out as described in Experimental Procedure. The test mixture had a final volume of 6 ml and contained 30 mm Tris (ph 7.5), 5 mm MgCL, 28 mg of myofibrillar protein, and other additions indicated in the table. The calcium concentration present in the system without added calcium was approximately 10-S M, that contributed by the ATP (14). Additions Control, without ATP. + 5 mrvr ATP + 5 mm ATP mivr EGTA + 5 mrvr ATP mm EGTA mm calcium Volume of sediment ml and is equally active in the presence of calcium or magnesium. The ATPase activity of the original myofibrils and especially that of the extract, in contrast with the residue, is much higher in the presence of calcium than in the presence of magnesium. Extraction with 0.6 M K-Extraction of cardiac myofibrils (2 mg of protein per ml) was carried out with 0.6 M K and M sodium thiosulfate at ph 7.2 for 30 minutes at 2. The insoluble residue was separated from the extract by centrifugation at 10,000 X g for 30 minutes, washed repeatedly with 0.1 M Tris, ph 7.5, and resuspended in this medium. The extract was diluted with 5 volumes of distilled water, and the precipitate was collected by centrifugation at 1,000 X g for 20 minutes, repeatedly washed with and finally resuspended in 0.1 M Tris, ph 7.5. The ATPase activity of the original myofibrils, the extract, and the insoluble residue are compared in Fig. 5. Treatment of myofibrils with 0.6 M K and M sodium thiosulfate causes the solubilization of myosin and actin. Since ATP was not present during the extraction, the F-actin irreversibly depolymerized (38), and, as is shown, the extract contained only calcium-activated and magnesium-inhibited, i.e. myosin-type, ATPase. The ATPase of the insoluble residue obtained under these conditions, on the other hand, is greater in the presence of 5 mm magnesium than in the presence of 5 mm calcium. Attempts to remove the non-actomyosin ATPase from cardiac myofibrils by various methods, such as differential centrifugation, digestion with phospholipase C or DNase, and treatment with 0.2 y. deoxycholate were unsuccessful. Uptake of Calcium by Cardiac Ghana Cardiac grana are capable of accumulating calcium although at a slower rate and to a lesser extent than their skeletal counterparts (Fig. 6). The ability of cardiac grana to accumulate calcium is rapidly lost during storage at 2 in 0.1 M KC1 and 5 mm histidine, ph 7.4, with or without 0.32 M sucrose (Fig. 7). The addition of 1 mm glutathione, ascorbic acid, sodium cyanide, sodium fluoride, sodium nitrite, potassium oxalate, MgCL, CaCh, ADP, or Na2HAS04 does not prevent this decay in s $ 0.25 $ P 0.20 s k 0.O? ONC STRfA GiY FG. 4. ATPase activity of cardiac myofibrillar fractions ob- bation at 22 for 5 or 10 minutes in a medium which contained 5 tained by extraction with Hasselbach-Schneider solution. Ex- mm ATP, 20 mm Tris (ph 7.5), KC1 at varying concentrations, traction of cardiac myofibrils was done as described in the text, 0.5 to 1.0 mg of protein per ml and 5 mm MgClz or 5 mru CaCL. and the ATPase activities of the original myofibrils (A), the ex- O-O, with 5 mm MgC12; X----X, with 5 mm CaC12; tract (B), and the insoluble residue (C) were determined by incu- O , with 5 mm MgClz mm EGTA.

5 2302 Ca++ and Relaxation of Cardiac Muscle Vol. 239, No. 7 C m -----x.07 J f 50 FG. 5. ATPase activity of cardiac myofibrillar fractions obtained by extraction with 0.6 M K and M sodium thiosulfate. Extraction of cardiac myofibrils was carried out as described in the text and the ATPase of the original myofibrils (A), the extract (B), and the insoluble residue (C) were determined by incubation f at 22 for 5 or 10 minutes in a medium which contained 5 mm ATP, 20 mm Tris (ph 7.5), KC1 at varying concentrations, 0.5 to 1.0 mg of protein per ml and 5 mm MgC or 5 mm CaC12. O-O, with 5 mm MgC12; X X, with 5 rnm CaCl*; C , with 5 mm MgClz mm EGTA. FQ. 6. The rate of calcium uptake by cardiac and skeletal grana. Calcium uptake was measured as described in Experimental Procedure in a medium which contained 0.1 M KCl, 5 rnru histidine (ph 7.4), 5 mm MgCL, 5 mm ATP, 5 mm potassium oxalate and 0 1 mm WaC12. The protein concentration of cardiac g&a was 0.25 mg per ml and that of skeletal grana was 0.13 mg per ml. The test system contained 0.32 M sucrose in the experiments with cardiac grana. n separate experiments, it was found that sucrose had no effect on the calcium uptake of cardiac grana. ncubation was carried out at 22 for times indicated on the abscissa. The rate of Ca++ uptake by different preparations of both skeletal and cardiac grana was somewhat variable, but these variations were rather small compared with the differences between cardiac and skeletal grana shown in the above figure. O-0, cardiac grana; O-O, skeletal grana. TMOFSTORAG& h!rs. FG. 7. Effect of storage on the calcium uptake of cardiac grana. The calcium uptake was measured as described in Experimental Procedure. ncubation was performed for 45 minutes at 22 in a medium which contained 0.1 M KC, 5 mm MgCL, 5 mm ATP, 5 mm potassium oxalate, 5 mm histidine (ph 7.4), 0.1 mm 46CaC11, 0.32 M sucrose, and 0.32 mg of grana protein per ml. Zero time corresponds to the time of killing the animals. O-0, grana preparation which contained 6.4 mg of protein per ml stored in 0.1 M KCl, 5 mm histidine (ph 7.4), and 0.32 M sucrose at 2 ; O-O, same as o-o, but the preparation was stored at -20 beginning 2 hours after killing the animals. ability to accumulate calcium. However, with storage at -20 in the presence of 0.1 M KU, 0.32 M sucrose, and 5 mm histidine, ph 7.4, very little activity is lost even after a few weeks. Omission of sucrose during storage at -20 accelerates the loss of the ability to accumulate calcium and results in visible clumping of particles at the time of thawing.

6 July 1964 B. Fanburg, R. M. Finkel, and A. Martonosi 2303 E$ect of Oxalate, Pyrophosphate, and norganic Phosphate an Calcium Uptake of Cardiac Grana As is seen in Table, there is almost no calcium uptake by cardiac grana in the absence of oxalate, PPi, or Pi. Uptake is enhanced, similarly to that of skeletal grana, when oxalate, PPi, or Pi is present, and the largest uptake is seen in the presence of oxalate. Effect of Cardiac and Skeletal Grana on Syneresis of Cardiac Myoj%rils As shown in Table, cardiac grana inhibit the syneresis of cardiac myofibrils. This effect increases with increasing grana concentrations and is observed only in the presence of oxalate and only if the grana are preincubated for 10 to 20 minutes with the MgATP-oxalate-Tris medium before being added to the myofibrils. n view of the correlations between removal of Ca+f and the relaxing effect in the case of the skeletal system (3) and the effect of EGTA on syneresis of cardiac myofibrils, we sought to relate the inhibition of syneresis of cardiac myofibrils by cardiac grana to the calcium concentration of the test system. This relation is most clearly brought out by comparing experiments with and without oxalate (Table V). Without oxalate, TABLE Effect of oxalate, pyrophosphate, and inorganic phosphate on Ca++ uptake of cardiac and skeletal grana Ca++ uptake was measured as described in Experimental Procedure in a medium containing 0.1 M KC, 5 mm hiatidine (ph 7.4), 5 mm MgCl*, 5 rnm ATP, 0.1 mm 4sCaC11, 0.13 mg of either cardiac or skeletal grana per ml, and other additions as indicated in the table. n the experiments with cardiac grana, the test system contained 0.32 M sucrose. Time of incubation was 30 and 10 minutes with cardiac and skeletal grana, respectively. Additions Cardiac grana Skeletal grana None... Cxalate, 5 mm Potassium pyrophosphate, 5 mm Pi,5rnM..,....,,._._... jo?mles/mg TABLE Calcium uptake of cardiac and skeletal grana and their eflect on syneresis of cardiac myofibils Syneresis tests were performed as described in Experimental Procedure and Table. After preincubation with grana, an aliquot of the medium was separated by Millipore filtration for determination of the remaining calcium concentration in the test system. Preincubation was carried out for 10 minutes at 22 in a medium which contained 5 mm MgCl2, 5 mm ATP, 20 mn Tris (ph 7.4), and 10m6 M 6CaClz with and without 5 rnn potassium oxalate. Cardiac and skeletal grana protein concentrations were 0.34 and 0.16 mg per ml, respectively. Calcium is expressed in the table as calculated concentrations remaining in the test system after the lo-minute preincubation period with grana. n the experiments without oxalate, there was no further fall in the calcium concentration after 20 minutes of incubation. The packed volume of myofibrils after the 1 minute of centrifugation at 1,000 X g in a grana-free control system was 2.1 ml in the absence of ATP, 0.5 ml with 5 mm ATP, and 1.4 ml with 5 rnhr ATP and 0.1 mm EGTA. Sediment ml Without oxalate 0.45 With oxalate 0.88 V Cardiac grana (Calcium) Sediment PM ml Skeletal grana (Calcium) PM TABLE EJect of cardiac grant on syneresis of cardiac myo$brils The experiments were performed under conditions described in Table and Experimental Procedure. All samples contained 5 mn oxalate except the one marked with the asterisk (*) in which oxalate was omitted. The grana were preincubated for 20 minutes with the Tris-MgATP-oxalate medium prior to addition to myofibrils. When calcium was added, it was added after this period of preincuhation. Gram protein m&?/ml : Volume of sediment Without added calcium With 0.1 xm4 CaClr ml ONC SrRENern Fro. 8. ATPase activity of cardiac grana. ncubation was carried out at 34 for 5 minutes in a medium which contained 30 mm Tris (ph 7.5)) 5 mn ATP, varying concentrations of KC, and 0.25 mg of grana protein per ml with the following additions: o-o,5 mm MgC12; ,5 mm MgClz rnm EGTA; X----X, 5 mm CaC12.

7 2304 Ca++ and Relaxation of Cardiac Muscle Vol. 239, No. 7 4 //p / / 1 / s 0.3- / 6 /,.8 $ g s Z ME OF STORRGtJ h R5, FQ. 9. Correlation between calcium uptake and inhibition of grana ATPase by EGTA during aging. Cardiac grana were prepared in sucrose-free medium as described in Experimental Procedure. The end of the preparation was taken as zero time and the grana were stored at 2 in 0.1 M KC1 and 5 mm histidine, ph 7.4, for times indicated on the abscissa prior to the start of the calcium uptake and ATPase measurements. The calcium uptake was measured in a medium which contained 0.1 M KCl, 5 mm histidine (ph 7.4), 5 my ATP, 5 mm oxalate, 5 mm MgC12, 0.1 mm WaC12, and 0.11 mg of grana protein per ml. ncubation was performed at 22 for 30 minutes. ATPase activity was measured as described in Experimental Procedure in a medium containing 30 mm Tris, 5 mm MgC12, 5 mm ATP, 0.05 M KCl, and 0.25 mg of grana protein per ml with and without 0.1 mm EGTA. The reaction was performed at 34 and incubation time was 5 minutes. O-0, Ca++ uptake in micromoles of Ca++ bound per mg of protein; O-O, percentage of inhibition of the ATPase activity by 0.1 mm EGTA; c ---M, ATPase activity without EGTA; q , ATPase activity with 0.1 mru EGTA. there is no inhibition of syneresis by grana and the calcium level remains essentially unchanged. The inhibition of syneresis which takes place in the presence of oxalate is accompanied by a reduction of the calcium concentration to the order of about lop7 M. Skeletal grana acted in a similar way on cardiac myofibrils, wit.h the exception that a shorter period of preincubation and a smaller amount of protein were sufficient to reduce the calcium level and inhibit syneresis. This could be expected since skeletal grana accumulate calcium more rapidly than cardiac grana (Fig. 6). ATPase Activity of Cardiac Grana n view of the proposed involvement of the ATPase of skeletal grana in the transport of calcium (lo), a comparison of the ATPase of cardiac and skeletal grana seemed of interest. n the presence of 5 mm magnesium, cardiac grana hydrolyzed 0.25 to 0.4 pmole of ATP per mg of protein per minute at 34 (Fig. 8), i.e. about one-tenth of the ATP hydrolyzed by skeletal grana under identical experimental conditions4 The rate of ATP 4 A. Martonosi, unpublished results. ONCSTREA'G JH FQ. 10. Calculated actomyosin and non-actomyosin components of cardiac myofibrillar C , non-actomyosin. ATPase. O-0, The contribution actomyosin; of the two components was calculated from the data presented in Fig. 1 by assuming that the percentage of inhibition by EGTA of the cardiac actomyosin component was the same as that of skeletal myofibrils (Fig. 1). For each ionic strength, the difference between the uninhibited and the inhibited rate of cardiac myofibrillar ATPase was divided by the corresponding percentage of inhibition of skeletal myofibrils shown in Fig. 1 and multiplied by 100 to obtain the actomyosin contribution. The non-actomyosin contribution was obtained by subtracting the actomyosin value from the total activity. hydrolysis wit,h 5 mm calcium is somewhat lower than with 5 mm magnesium (Fig. 8). Whereas 0.5 mm EGTA reduces the ATPase activity of skeletal grana in the presence of magnesium to about 10% of the original value: the EGTA inhibition of cardiac grana rarely exceeds 45 to 50% (Fig. 8), and inhibition of the ATPase of cardiac grana rapidly decreases during storage at 2 over a a-day period, concomitantly with an increase in ATPase activity (Fig. 9). The decrease in the EGTA inhibition of the ATPase activity on aging closely parallels the decrease in the ability to accumulate calcium (Fig. 9). f the grana are stored at -20 in the presence of 0.32 M sucrose, the calcium uptake (Fig. 7) and the EGTA inhibition remain unchanged. DSCUSSON While EGTA inhibits the syneresis of cardiac myofibrils and the superprecipitation and ATPase activity of cardiac actomyosin it has a relatively small effect on the myofibrillar ATPase. The lack of inhibition of cardiac myofibrillar ATPase by EDTA was the basis of an earlier conclusion that there may be fundamental differences between cardiac and skeletal muscle with regard to the role of calcium in relaxation (24). The apparent discrepancy between the effect of EGTA on cardiac myofibrils and actomyosin was resolved by our demonstration of an ATPase in cardiac myofibrils that is insensitive to EGTA and is not inhibited by magnesium at high ionic strength. Thus, cardiac myofibrillar ATPase does not fully reflect the properties of actomyosin, and one cannot expect that various modifiers of skeletal actomyosin ATPase, such as chelating agents or relaxing grana, that affect skeletal actomyosin and myofibrillar ATPase in a parallel fashion, should do so in the case of the cardiac system. f one assumes that the inhibition by EGTA of the actomyosin

8 July 1964 B. Fanburg, R. M. Finkel, and A. Martonosi component of cardiac myofibrils is similar to that of skeletal myofibrils, which have no non-actomyosin ATPase, one can recalculate the contribution of the actomyosin component of cardiac myofibrils from the EGTA inhibition. The calculated actomyosin ATPase activity is shown in Fig. 10 and it displays essentially the same strong inhibition by increased ionic strength characteristic of isolated cardiac and skeletal actomyosin and of skeletal myofibrils. By the difference, one can obtain the contribution of non-actomyosin ATPase, and the variation with ionic strength is quite similar to that found directly with the residue from extracted myofibrils (Fig. 4). The assumption of this analysis, that the non-actomyosin ATPase is not affected by EGTA, is borne out by the experiments shown in Fig. 4. Although the calculated behavior with respect to EGTA and ionic strength of the actomyosin and non-actomyosin components agrees qualitatively with that obtained experimentally on the separated components, the quantitative relations are not quite clear. f one assumes that specific iltpase activity of the isolated components is the same as in the untreated myofibrils, one cannot find a ratio of actomyosin to non-actomyosin residue that would lead to the observed specific ATPase activity of the myofibrils. This discrepancy must be due to loss of specific ATPase activity of one of these components during extraction of the myofibrils, since in experiments in which the extract and residue were recombined the ATPase activity of these two components was found to be additive.5 The syneresis of cardiac myofibrils is also inhibited by cardiac or skeletal grana. The cardiac grana, as has been shown for skeletal ones, accumulate calcium in the presence of ATP, Mg, and oxalate or Pi or PPi. There is good correlation between the lowering of the calcium concentration by grana and the inhibition of syneresis of the cardiac myofibrils. f oxalate is omitted from the test system, there is little accumulation of calcium by the grana and no inhibition of syneresis. The rate of calcium uptake by cardiac grana is much slower than by skeletal grana, and this explains the fact that, in order to observe inhibition of syneresis within the B-minute period used in our experiments, cardiac grana had to be preincubated with ATP which was the chief source of calcium. The loss of the ability of cardiac preparations to accumulate calcium after 1 or 2 days storage is another important difference between cardiac and skeletal grana; the latter show little change after several days. n view of the lability of cardiac grana, a large decay in calcium uptake function may occur during preparation which might explain their generally weak relaxing effect when compared with skeletal grana. As magnesium-activated ATPase of grana may be involved in their accumulation of calcium (lo), it is of interest that EGTA inhibition of the cardiac grana ATPase decreases with aging parallel with the disappearance of the calcium uptake. The calcium uptake function and the inhibition of grana ATPase by EGTA are preserved without change for at least several days if grana are stored at -20 in a medium containing 0.1 M KCl, 5 mm histidine, and 0.32 M sucrose. This observation might point to the loss at 2 of an important component which is required for the transport of calcium and which confers EGTA sensitivity on the ATPase activity. Observations reported here are consistent with the view that the mechanism of relaxation in heart muscle, as in skeletal muscle (l-4), is based on the regulation of the calcium concentration. 6 B. Fanburg, unpublished results. There seems to be no doubt that whenever the Ca++ concentration is lowered, either by EGTA or by grana, to about lo+ M, syneresis and ATPase are inhibited. There appear to be two open questions: (a) Can grana, or, in L&O, the sarcoplasmic reticulum, produce a soluble relaxing substance with an activity that is not inhibited by Ca++ concentrations normally required for contraction or myofibrillar ATPase activity? Various reports (19, 20, 27, 28) have indicated this possibility, but settling of the problem for both cardiac and skeletal muscle requires more experimental work. (b) s the sarcoplasmic reticulum the physiological regulator of the cellular Ca+f level in heart muscle? While the present work is in accord with such a possibility, the recently described ATP-dependent calcium uptake by cardiac mitochondria (39) suggests that, mitochondria might play an important role in this regulation. SUMMARY 1. Cardiac myofibrils prepared in the usual way were found to be contaminated by an adenosinetriphosphatase which is not inhibited by 1,2-bis-(2-dicarboxymethylaminoethoxy)ethane (EGTA) and only moderately affected by increasing the ionic strength. The presence of this ATPase appears to account for the observation that there is only minimal inhibition of ATPase activity of cardiac myofibrils by ethylenediaminetetraacetate or EGTA. 2. Cardiac actomyosin ATPase which is free from the contaminating ATPase is inhibited by EGTA to a similar extent as are skeletal myofibrils and actomyosin. 3. EGTA inhibits the syneresis of cardiac actomyosin and myofibrils; the inhibition is counteracted by calcium. 4. Fragments of sarcoplasmic reticulum (grana) accumulate Ca++ in the presence of magnesium, ATP, and oxalate. 5. Cardiac grana inhibit syneresis of cardiac myofibrils and there is a good correlation between this inhibition and concomitant lowering of the calcium concentration. Added calcium prevents the inhibition of syneresis. 6. Storage at 2 for 2 days causes the loss of Ca++ uptake by cardiac grana which is paralleled by the loss of the EGTA inhibition of the ATPase activity. The rapid decay of the Ca++ uptake might explain the variability of the results in the literature concerning the relaxing effect of cardiac grana. Acknowledgments--We wish to express our thanks to Dr. John Gergely for his suggestions and support, to Miss Rebecca Feretos and Miss Alice Sholz for their excellent technical assistance, and to Mr. Jerome Reicher for his patient cooperation in the documentation of this paper. REFERENCES 1. WEBER, A., J. Bid. Chem., 234, 2764 (1959). 2. WEBER, A., AND WNCUR, S., J. Biol. Chem., 236,3198 (1961). 3. WEBER, A., AND HERZ, R., Biochem. and Biophys. Research Communs., 6, 364 (1962). 4. WEBER, A., HERZ, R., AND RESS,., J. Gen. Physiol., 46,679 (1963). 5. CHABEREK, S., AND MARTELL, A. 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10 The Role of Calcium in the Mechanism of Relaxation of Cardiac Muscle B. Fanburg, R. M. Finkel and A. Martonosi J. Biol. Chem. 1964, 239: Access the most updated version of this article at Alerts: When this article is cited When a correction for this article is posted Click here to choose from all of JBC's alerts This article cites 0 references, 0 of which can be accessed free at