Adenosine 3 Œ, 5 Œ-monophosphate (cyclic AMP) could not replace epinephrine in

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1 J. Biochem., 69, (1971) Effect of Thyroid Hormones on Epinephrine-induced Lipolysis in Adipose Tissue of Rats Atsushi ICHIKAWA, Hisashi MATSUMOTO, Nobuo SAKATO and Kenkichi TOMITA The Faculty of Pharmaceutical Sciences Kyoto University, Kyoto, Received for publication, November 11, 1970 The effect of thyroid hormones on fat mobilization induced by epinephrine in adipose tissues of fasted rats was studied both in vivo and in vitro. Thyroidectomy markedly diminished, but did not completely abolish, the lipolytic response to adipose tissue to epinephrine. Triiodo-L-thyronine (L-T3) added to a suspension of fat cells at a concentration of as low as 0.01 mm stimulated epinephrine-induced lipolysis. Even in the absence of epinephrine, L-T3 at concentrations above 0.1 mm clearly stimulated the basal lipolysis of fat cells from both thyroidectomized and euthyroid rats. Various com pounds structurally related to L-T3 were tested, but only diiodo-l-thyronine (L-T2) showed a similar stimulatory effect on lipolysis. Adenosine 3 Œ, 5 Œ-monophosphate (cyclic AMP) could not replace epinephrine in stimulating lipoly sis, but increased the release of free fatty acid from fat cells in the presence of L-T3. Theophylline stimulated lipolysis, and its effect was enhanced by L-T3. At a high concentration of theophylline (10 mm), the lipolytic response of fat cells was almost maximal regardless of the absence or presence of L-T3, with or without epinephrine. Furthermore, both L-T3 and L-T2 were found to inhibit cyclic AMP phospho diesterase effectively, indicating that thyroid hormones might elicit a lipolytic action by preventing destruction of cyclic AMP. The potent fat mobilizing action of epinephrine observed in vivo (1, 2) could also be demon strated in vitro by incubating isolated fat pads in the presence of about physiological concen trations of epinephrine (3). With regard to the effect of the thyroid on this in vitro action of epinephrine, several investigators reported that thyroidectomy or administration of pro pylthiouracil to rats completely abolished the release of free fatty acid (FFA) induced by epinephrine, indicating the absolute require ment of thyroid hormones for the action of epinephrine (4E). However other workers reported that FFA release from adipose tissue was often reduced by thyroidectomy, but was still stimulaled by added epinephrine (7-10). Furthermore, while pretreatment of rats with thyroid hormones markedly restored the epinephrine-induced FFA release from adipose tissue of hypothyroid animals (7, 9), and Vol. 69, No. 6,

2 1056 A. ICHIKAWA, H. MATSUMOTO, N. SAKATO and K. TOMITA greatly elevated that of tissue from euthyroid animals (6, 8), addition of thyroid hormones in vitro in the presence of epinephrine either resulted in no stimulation of lipolysis in the presence of epinephrine (4), or in its eleva tion, depending on the amount of thyroid hormone used (11). In studies on the effect of thyroid hor mones on lipolysis induced by epinephrine, we observed that thyroidectomy of rats reduced, but did not completely abolish FFA release induced by epinephrine and that even in the absence of epinephrine, addition of high con centration of triiodo-l-thyronine (L-T3) in vitro markedly stimulated basal lipolysis in free fat cells from both thyroidectomized and euthy roid rats. Furthermore, we also observed that adenosine 3 Œ, 5 Œ-monophosphate (cyclic AMP) acted synergistically with L-T3 in stimulating FFA release from normal fat cells. The present paper reports results of studies on the effects of thyroid hormones, cyclic AMP, theophylline and other compounds on lipolysis induced by epinephrine in vitro and on the effect of thyroid hormones on cyclic AMP phosphodiesterase. METHODS AND MATERIALS Animals-Normal or thyroidectomized adult male Wistar and Donryu rats were fed on standard laboratory diet (NIHOH CLEA, CE-2 pellet) and tap water ad libitum until 24 hr before sacrifice, and then allowed free access to tap water only. Rats weighing g were thyroidecto mized at least 3 weeks before experiments and were given tap water containing 1 o Ca gluconate and 0.1% 2-thiouracil to drink for 1 week after the operation. The operation was judged to be satisfactory when animals showed a marked retardation of growth (weight gain less than 1 g/day in contrast to the gain of ca. 4 g/day of euthyroid rats), and when no detectable remnants of thyroid were found on postmortem examination of the neck. Sodium triiodo-l-thyronine (L-T3), sodium L-thyroxine (L-T4) (both purchased from Aldrich Chemical Co., Milwaukee) and diiodo-l-thyro nine (L-T2) (Sigma) were dissolved in a minimal volume of 0.1 N NaOH and diluted with 0.15 M saline. Thyroidectomized animals re ceived a single injection of thyroid hormones (100 ƒêg in ml per 100 g body weight) into the tail vein, while control animals received an equivalent volume of the vehicle. Incubation Procedure-Animals were fasted for 24 hr and then killed by decapitation and their epididymal fat pads were quickly removed and weighed. In experiments using intact fat pads, one lobe of paired tissue from the same animal was transferred to a 25-m1 Erlenmeyer flask containing 3 ml of incubation medium, while the other lobe was incubated as an appropriate control with or without epineph rine. The incubation medium was 3 % bovine serum albumin (BSA, Armour Fraction V) in Krebs-Ringer bicarbonate buffer, ph 7.4, con taining half the recommended amount of CaCl2. In experiments with minced tissues, fat pads were cut into 4-5 pieces and weighed, and each piece ( mg) was placed in a test tube (1 x 10 cm) containing 1 ml of incu bation medium. Isolated fat cells were prepared by the method of Rodbell (12 ), and the cells obtained from mg of tissue were suspended in 1 ml of incubation medium and placed in a plastic tube (1.4 x 7 cm). All incubations were performed for 1-2 hr at 37 Ž with gentle shaking in air. For addition in in vitro experiments, L-T3 and other derivatives of thyroid hormones synthe sized in our laboratory (13) were dissolved in dimethyl sulfoxide and added to the medium at a final concentration of this solvent of 2.5% (v/v). Epinephrine solution (Sankyo Phar maceutical Co., Tokyo) containing 1 mg of epinephrine-hcl per ml was diluted to the desired concentration with medium. Prosta glandin E, (a gift from Ono Pharmaceutical Co., Osaka) was dissolved in 95% ethanol containing K2CO3 (1 mg/ml) and diluted to the desired concentration with medium. Other chemicals used were standard com mercial products of reagent grade. Assays of Glycerol and Fatty Acid-The amount of glycerol released was measured J. Biochem,

3 THYROID HORMONES IN EPINEPHRINE-INDUCED LIPOLYSIS 1057 colorimetrically at 570 nm after periodate oxi dation and color development with chro motropic acid following the method of Korn ( 14). Free fatty acids (FFA) were extracted by the method of Dole (1) and assayed color imetrically by the cobalt soap method of Novak ( 15 ). The amount of FFA released is express ed as microequivalents (ƒêeq) of palmitic acid per gram of tissues per hour. All values listed for FFA or glycerol release during incubation were corrected for those of the corresponding O-time controls. Cyclic AMP Phosphodiesterase-All steps of enzyme preparation were performed at 0-4 Ž. Cyclic AMP phosphodiesterase [3 Œ, 5 Œ cyclic-nucleotide phosphodiesterase, EC 3.1.4c] eluted with a linear gradient obtained with 300-ml each of the same buffer mixture used above and 20 mm imidazole (ph 7.8) containing 400 mm KCl and 1 MM MgSO,. Then the enzyme (275 mg of protein, 98 units/mg) was dialyzed against 100 volumes of 10 mm imida zole (ph 7) containing 1 mm MgSO, to remove KCl. Cyclic AMP phosphodiesterase was also prepared from rat adipose tissue by a similar method, except that Sephadex G-200 was used instead of DEAE-cellulose. The enzyme dis solved in 10 mm imidazole buffer (ph 7)-1 mm MgSO, was applied to a column (3 x 50 cm) of Sephadex G-200 and eluted with the same buffer. of pig heart was purified to Step 4 of the method of Butcher and Sutherland for prep aration of the beef heart enzyme (16) (80 units' mg). Enzyme activity was assayed by their method (16) except that E. coli alkaline phosphatase [EC (Type III, Sigma) was used in place of Crotalus atrox venom. Inorganic phosphate released from AMP was determined by the method of Fiske and SubbaRow (17 ). One unit of enzyme was defined as the amount catalyzing the hydrol ysis of 1 ƒêmole of cyclic AMP to AMP in 30 min at 37 Ž. Specific activity was express ed as units of enzyme per mg of protein, determined by the method of Lowry et al. (18 ). This enzyme was also prepared from rat liver. Rat liver (50 g) was homogenized with 200 ml of 0.25 M sucrose in a Teflon homog enizer, and centrifuged at 17,000 ~g for 15 min. Solid ammonium sulfate was added to the supernatant fraction (6.4 g of protein, 20.5 units/mg) to 45% saturation at ph 7. The precipitate was collected by centrifugation 2.6 mg of protein, 35 units/mg) and dissolved in imidazole buffer (10 mm, ph 7) containing 1 mm MgSO,, and was dialyzed against 100 volumes of the same buffer for 15 hr. Solid KCl was added to the dialyzed solution to a concentra tion of 80 mm, and then the solution was applied to a column (3.2 X 15 cm) of DEAE cellulose equilibrated with the same imidazole buffer containing 1 mm MgSO, and 80 mm KCl. The column was washed with two bed volumes of the same buffer and then the enzyme was RESULTS Effects of Thyroidectomy and T3 Adminis tration on FFA Release As shown in Fig. 1, the release of FFA from intact or minced adipose tissue or from fat cells of normal rats was markedly stimulated in the presence of epinephrine, and was roughly proportional to the concentration of epinephrine added from 0.01 to 50 j g/ml. Thyroidectomy of rats markedly diminished the lipolytic response of their adipose tissue and fat cells to epinephrine. Thus, the dose of epinephrine required for release of 5 ƒêeq/g/hr of FFA was 0.1 ƒêg/ml with normal fat cells and 5 ƒêg/ml with those from thyroid ectom ized rats. Furthermore, FFA release from the adipose tissue of thy roidectomized rats reached a maximum with 10 ƒêg/ml of epinephrine and did not increase significantly with higher doses of epinephrine. A single intravenous injection of L-T3 (200 ag) to thyroidectornized rats 24 hr before sacrifice resulted in about 2-fold increase in FFA release from adipose tissues and cells in response to epinephrine concentrations above I fig/ml, but the sensitivity of thyroidectomized adipose tissue to epinephrine was not fully restored to the normal level by a single treat ment. Over the dose range tested epinephrine had more effect on free fat cells than on whole or minced adipose tissues. As shown in Fig. 2, FFA release from minced fat pads of thy Vol. 69, No. 6, 1971

4 1058 A. ICHIKAWA, H. MATSUMOTO, N. SAKATO and K. TOMITA Fig. 1. Effect of epinephrine on lipolysis in adipose tissues of normal, thyroidectomized and T3-treated thyroidectomized rats. (A) Intact fat pads, (B) minced fat tissues, and (C) free fat cells were incubated as described in "METHODS AND MATERIALS" in the presence of various concentrations of epinephrine. O, Normal rats;, thyroidectomized rats ; thyroidectomized rats 24 hr after intravenous injection of L-T3 (200 ƒêg). Each point is the mean of values in 3-6 experiments. Fig. 2. Effect of L-T3 administration to thyroid ectomized rats on lipolysis in minced adipose tissues. The epididymal fat pads were removed from thyroid ectomized rats at intervals after a single intravenous injection of L-T3 (200 ƒêg). They were minced and incubated in the presence of 0.1 and 0.5 fig/ml of epinephrine (EP). 0-Time values were obtained with fat tissues from rats injected with saline. Each point represents the mean of values in 5 experiments±se. Lipolysis in all samples after epinephrine treatment was significantly higher than in the 0-time controls (p<0.05). Fig. 3. Effect of addition of L-T3 in vitro on FFA release from fat cells. Fat cells obtained from 38 mg of fat pads of normal rats and 36 mg of those of thyroidectomized rats were incubated with (0 and or without ( and A) 0.1 ƒêg/ml of epinephrine for 1 hr at 37 Ž. Each point represents the mean of values in at least 3 experiments. -, normal; ----, thyroidectomized. J. Biochem

5 1 THYROID HORMONES IN EPINEPHRINE-INDUCED LIPOLYSIS 1059 TABLE I. Effect of fasting on T3-stimulated lipolysis. Minced alipose tissue (ca. 200 mg wet weight) from fasted normal rats was incubated for 1 hr in 1 ml of medium (see "METHODS AND MATERIALS") con taining the additions indicated. Each value represents the mean of values in 6 experiments+standard error. TABLE II. Effects of T3 and related compounds on FFA release from free fat cells. Fat cells from mg of fat pads of normal rats were incubated for 90 min with or without epinephrine (0.1 ƒêg/ml) and with the other additions indicated (0.1 ƒêmole/ml). Data represent the means of values in 5 experiments±se. ) P< ) p<0.02. Vol. 69, No. 6, 1971

6 1060 A. ICHIKAWA, H. MATSUMOTO, N. SAKATO and K. TOMITA roidectomized rats started to increase as early as 4 hr after a single intravenous injection of L-T3 (200 ƒêg) and continued to increase to a maximum level after 24 hr. Without added epinephrine, the basal lipolysis was only slightly elevated 24 hr after L-T3 administra tion. Effect of L-Ts in vitro-as shown in Fig. 3, even in the absence of epinephrine, L-T3 added in vitro at concentrations above 0.1 mm clearly stimulated basal FFA release from fat cells of both normal and thyroidectomized rats, but at lower concentrations it had no significant effect. In the presence of epinephrine, L-T3 at concentrations of as low as 0.01 mm definitely stimulated lipolysis induced by epinephrine, and the magnitude of stimulation was propor tional to the T3 concentration. As reported by Vaughan (11), in these experiments, the in vitro effect of T3 was immediate, being clearly observed as early as min after addition of the hormone, and it continued to increase linearly for about 1 hr. As reported by Gordon and Cherkes (3), basal lipolysis in vitro was greatly elevated in adipose tissue from fasting animals. Table I shows that the degree of enhancement of basal lipolysis by L-T3 added alone in vitro was also affected by the nutritional state of animals. The adipose tissue of animals after 24-hr fast was very sensitive to L-T3 and that of animals after 48-hr fast was most sensitive. More prolonged fasting did not augment the effect of L-T3 any further. For routine assays, rats were fasted for 24 hr before sacrifice, as de scribed in "METHODS AND MATERIALS." Effects of Ts Analogues in vitro-as shown in Table II, among the various compounds structurally related to T3 tested, only diiodo L-thyronine (L-T2) had a similar stimulatory effect on both basal and epinephrine-induced lipolysis of isolated fat cells of normal rats. L-T4 and triiodo-d-thyronine (D-T3) had only slight effects, and the other compounds tested, L-thyroxine, thyroacetic acid and its iodinated derivatives (3, 5-diiodo-, 3, 5, 3 Œ-triiodo-, 3, 3 Œ, 5 Œ triiodo-, and 3, 5, 3 Œ, 5 Œ-tetraiodo-thyroacetic acids), and their 4 Œ-OH substituted derivatives (0-methyl and O-ethyl ethers, and 0-phos phate ester) were without effect. L-T,2 is usually regarded as a weak thyro mimetic agent (19 ). However, the effect of L-T2 in these experiments was not due to contamination with L-T3, because the L-T2 preparation used appeared to be essentially pure and free from L-T3 according to the paper-, thin layer and gas-chromatographic techniques routinely used in our laboratory (13 ). In contrast to L-T3, injection of L-T2 into thyroidectomized rats did not restore the reduced basal FFA release from adipose tissue to the level of euthyroid animals. However, lipolysis induced by epinephrine was distinctly elevated in adipose tissues from T2-treated animals as in those of T3-treated ones (Table III). Effect of Cyclic AMP-As reported by Vaughan (11), cyclic AMP alone at a concen tration of 1 mm could not replace epinephrine and did not stimulate lipolysis in normal fat TABLE III. Effects of injection of T3 and T3 on lipolysis. Thyroidectomized rats received L-T2 or L-T3 (200ƒÊg each per animal) i.v. 24 hr before sacrifice. Paired fat pads were incubated with, and without epinephrine (0.1 ƒêg/ml), respectively, for 1 hr at 37 Ž. Each value represents the mean of values in 6 ex periments±se. 1) p< ) p<0.02. J. Biochem.

7 THYROID HORMONES IN EPINEPHRINE-INDUCED LIPOLYSIS 1061 cells. However, when L-T3 was added to the medium containing cyclic AMP, it increased FFA release from cells. As shown in Fig. 4, in the presence of 5 mm cyclic AMP which itself was slightly simulatory, L-T3 at concen trations from 1 ƒêm to 1 mm clearly had an additive stimulatory effect on FFA release. Effects of Theophylline, Insulin, Prosta glandin and Other Compounds-Theophylline itself stimulated lipolysis, as reported by Weiss et al. (20), but fat cells became more sensi tive to the lipolytic effect of theophylline in the presence of L-T3. However, with a high concentration of theophylline (10 mm), the Fig. 4. Effect of L-T3 on lipolysis in fat cells in the presence of cyclic AMP. Free fat cells from 48 mg of fat pads of normal rats were incubated for 1 hr at 37 Ž in 1 ml of the incubation medium containing 1 or 5 mm cyclic AMP (C-AMP) and various concen trations of L-T3. Each point represents the mean of values in at least 3 experiments. lipolytic response reached almost the same maximum level regardless of the absence or presence of L-T3, or of the presence of both L-T3 and epinephrine (Table IV). Lipolysis stimulated by T3 was effectively suppressed by addition of insulin, prostaglandin E, or dichloroisoproterenol (DCI), a beta TABLE IV. Effects of theophylline, insulin, prostaglandin E,, dichloroisoproterenol and phenoxybenzamine on T3 stimulated lipolysis. Fat cells from mg of adipose tissue of normal rats were incubated for 1.5 hr at 37 Ž in 1 ml of incubation medium containing various concentrations of the test compounds with or without L-T.3 or/and epinephrine. Vol. 69, No. 6, 1971

8 1062 A. ICHIKAWA, H. MATSUMOTO, N. SAKATO and K. TOMITA TABLE V. Inhibition of cyclic AMP phosphodiesterase by T3 and related compounds. Cyclic AMP phos phodiesterases were prepared from pig heart, rat liver and adipose tissue as described in "METHODS AND MATERIALS." The reaction mixture (0.9 ml) contained 1 ƒêmole of cyclic AMP, 18 ƒêmoles of MgSO4, 38 ƒê moles of Tris-HCl buffer, ph 7.4, and 0.5 mg of enzyme (78, 95 and 75 units/mg for pig heart, rat liver and adipose tissue, respectively). After incubation at 30 Ž for 20 min, 0.1 ml of alkaline phosphatase (E. coli, type III, 0.02 mg) in 10 mm Tris-HCl, ph 8.5, was added and incubation was continued for another 10 min. The reaction was stopped by addition of 0.5 ml of 20% trichloroacetic acid. The mixture was centrifuged and inorganic phosphate released was determined by the method of Fiske and SubbaRow (17 ). 1) See Table II. adrenergic blocking agent at fairly high con centration (1 mm), but not by phenoxybenz amine, an alpha-adrenergic blocking agent. Inhibition of Cyclic AMP Phosphodiesterase by T3 and Related Compounds-The effect of theophylline on T3-stimulated lipolysis indicated that T3 might have a lipolytic action by preventing destruction of cyclic AMP, as reported by Mandel and Kuehl (21). In fact, it was found that T3, T4 and T2 effectively inhibited cyclic AMP phosphodiesterase in rat adipose tissue and liver, and pig heart (Table V). The effects of these compounds varied with the source of the enzyme. L-T3 was most effective in inhibiting the enzyme from rat adipose tissues, while L-T4 was more effective in suppressing the enzymes from rat liver and pig heart. Other compounds which did not show any stimulatory effect on lipolysis induced by epinephrine in vitro (Table II) were not potent inhibitors of the enzymes from either adipose tissues, liver or heart. The inhibition of cyclic AMP phosphodi esterase of rat adipose tissue by T3 was competi tive and its K, value was calculated as 0.43 mm (Fig. 5), which was almost identical with the value of 0.4 mm reported for the beef heart phosphodiesterase by Mandel and Kuehl (21). Fig. 5. Inhibition of cyclic AMP phosphodiesterase of rat adipose tissues by L-T3. The standard assay mixture (see Table V) was incubated with 0.52 mg of cyclic AMP phosphodiesterase (72 units/mg) and various concentrations of L-T3. Other conditions were as described in Table V and "METHODS AND MATERIALS." DISCUSSION The lipolytic action of epinephrine has been reported to be either completely absent (4-6) or greatly reduced (7-10) in adipose tissues of hypothyroid animals. Our results support the latter opinion; i.e., under our conditions J. Biochem.

9 THYROID HORMONES IN EPINEPHRINE-INDUCED LIPOLYSIS 1053 the fat pads of thyroidectomized-thiouracil fed rats showed a slight but definite lipolytic response in vitro to epinephrine at a concen tration of 1 Đg/ml and a maximum response to 10 Đg/ml (Fig. 1). However, our results differed from those by the latter group of workers with regard to the sensitivity of tissues to epinephrine. In our experiments, the response to hypothyroid tissue to epinephrine only increased linearly over a narrow dose range (1-10 Đg/ml), and the maximum response obtained with 10 Đg/ml of epinephrine was about 1/3 of that of euthy roid tissues, regardless of whether intact or minced tissues, or free fat cells were used. On the other hand, the above workers reported that when exposed to high concentrations of epinephrine, e.g., 10 Đg/ml (8) or 50 Đg/ml (9, 10), differences in tissue sensitivity were abolished and FFA release from both hypo thyroid and euthyroid tissues approached es sentially the same maximum value. The former group of workers mentioned above reported that there was little or no lipolytic response of hypothyroid tissue to epinephrine at concentrations of 2.5 Đg/ml (4, 5) or 0.1 Đg/ml (6), while in our in vitro studies, the lowest effective dose of epinephrine was 1 Đg/ml. As clearly shown in Fig. 1, these dis crepancies do not seem to be due to whether intact or minced tissue or free fat cells are used. Moreover, the difference does not seem to be due to the composition of the incubation medium, because we obtained essentially similar results on FFA release from hypothyroid tissue using Krebs-Ringer-bicarbonate or -phosphate buffer with or without glucose, which was reported to antagonize to effect of epinephrine in adipose tissue from hyperthyroid animals (5). Other experimental conditions such as the age (22) or nutritional state of the animals or the thyroidectomy technique may cause these differences. L-T3 acts synergistically with low levels of epinephrine in stimulating lipolysis in normal fat cells both in vivo and in vitro ; L-T3 itself at concentrations of 1-10 Đm had no effect on lipolysis, but markedly potentiated epinephrine stimulated lipolysis (Fig. 3). At higher con centrations ( MM), L-T3 caused increased FFA release from fat cells. This result is in good accordance with that reported by Mandel and Kuehl (21). The direct effect of L-T3 on lipolysis was slightly stimulated by concen trations of Đm of dichloroisopreterenol (DCI), which inhibits epinephrine-stimulated lipolysis (Table IV). These results suggest that higher levels of L-T3 act at a different site from that affected by epinephrine. The effect of L-T3 on basal lipolysis and that stimulated by epinephrine was effectively inhibited by both insulin and prostaglandin E1. These compounds are reported to inhibit both stimulation of lipolysis and generation of cyclic AMP by various hormones (23, 24) and lipolysis stimulated by theophylline (25, 26). Cyclic AMP itself did not have any significant lipolytic effect, probably due to its rapid degradation by cyclic AMP phospho diesterase present in tissues (16). However, it stimulated lipolysis in the presence of L-T3 (Fig. 3). A similar stimulatory effect of L-T3 on lipolysis was also observed with theophylline. Furthermore, the maximum lipolytic response to a high concentration of theophylline was not affected by the presence or absence of L-T3, or the presence of both L-T3 and epinephrine (Table IV). These results suggest that L-T3 and theophylline might both act as inhibitors of cyclic AMP phospho diesterase and so elevate the tissue level of this nucleotide. Vaughan (11) reported that L-T3 had little or no stimulatory effect on lipolysis in the presence of theophylline and cyclic AMP. However, she used fat pads from fed animals while our animals had been starved for 24 hr and the level of L-T3 in her experiments was lower (37 Đm vs. 100 Đm). We found that L-T3 competitively inhibited the activity of cyclic AMP phosphodiesterase from rat adipose tissue as well as from rat liver and pig heart (Fig. 5) (Table V), as reported by Mandel and Kuehl (21). Stimula tion of epinephrine-induced lipolysis by L-T2 (Table II) might be due to its strong inhibitory effect on cyclic AMP phosphodiesterase. On the other hand, L-T4 which caused less stimu lation of lipolysis in vitro inhibited this enzyme as effectively as L-T2. Vol. 69, No. 6, 1971

10 1064 A. ICHIKAWA, H. MATSUMOTO, N. SAKATO and K. TOMITA The K1 value of 0.43 mm calculated for L-T3 is much higher than the concentration required for hormone activity in vivo. L-T3 might be specifically concentrated in adipose tissue during stimulation of lipolysis. According to Krishna et al. (27), thyroid hormones might control norepinephrine-induced FFA mobilization by regulating the amount of adenyl cyclase in adipose tissue without affect ing the amounts of lipase and cyclic AMP phosphodiesterase. Our results suggest that thyroid hormones might regulate the tissue concentration of cyclic AMP by inhibiting the phosphodiesterase of this nucleotide. The physiological significance of this effect requires further investigation. REFERENCES 1. V.P. Dole, J. Clin. Invest., 35, 150 (1956). 2. R.S. Gordon, Jr. and A. Cherkes, J. Clin. Invest., 35, 206 (1956). 3. R.S. Gordon, Jr. and A. Cherkes, Proc. Soc. Exp. Biol. Med., 97, 150 (1958). 4. A.F. Debons and I.L. Schwartz, J. Lipid Res., 2, 86 (1961). 5. V. Felt, B. Schovanec, P. Benes, F. Plzak and V. Vrbensky, Experientia, 18, 379 (1962). 6. D. Deykin and M. Vaughan, J. Lipid Res., 4, 200 (1963). 7. G.A. Bray and H.M. Goodman, Endocrinology, 76, 323 (1965). 8. P.S. Rosenfeld and I.N. Rosenberg, Proc. Soc. Exp. Biol. Med., 118, 221 (1965). 9. M. Goodman and G.A. Bray, Am. J. Physiol., 210, 1053 (1966). 10. G.A. Bray, Endocrinology, 79, 554 (1966). 11. M. Vaughan, J. Clin. Invest., 46, 1482 (1961). 12. M. Rodbell, J. Biol. Chem., 239, 375 (1964). 13. N. Sakato and K. Tomita, J. Biochem., 67, 421 (1970). 14. E.D. Korn, J. Biol. Chem., 215, 1 (1955). 15. M. Novak, J. Lipid Res., 6, 431 (1965). 16. R.W. Butcher and E.W. Sutherland, J. Biol. Chem., 237, 1244 (1962). 17. C.H. Fiske and Y. SubbaRow, J. Biol. Chem., 66, 375 (1925). 18. O.H. Lowry, N.J. Rosebrough, L.A. Farr and R.J. Randell, J. Biol. Chem., 193, 265 (1951) ; E. Layne, "Methods in Enzymology," ed. by S.P. Colowick and N.O. Kaplan, Academic Press, New York, Vol. III, p. 447 (1957). 19. W.L. Money, S. Kumaoka, R.W. Rawson and R.L. Kroc, Ann. N.Y. Acad. Sci., 86, 512 (1960). 20. B. Weiss, J.I. Davies and B.B. Brodie, Biochem. Pharmacol., 15, 1553 (1966). 21. L.R. Mandel and F.A. Kuehl, Jr., Biochem. Biophys. Res. Commun., 28, 13 (1967). 22. W. Benjamin, A. Gellhorn, M. Wagner and H. Kundel, Am. J. Physiol., 201, 540 (1961). 23. D. Steinberg, M. Vaughan, P.J. Nestel, O. Strand and S. Bergstrom, J. Clin. Invest., 43, 1533 (1964). 24. R.W. Butcher and C.E. Baird, J. Biol. Chem., 243, 1713 (1968). 25. M. Rodbell and A.B. Jones, J. Biol. Chem., 241, 140 (1966). 26. D. Steinberg, Ann. N.Y. Acad. Sci., 139, 897 (1967). 27. G. Krishna, S. Hynie and B.B. Brodie, Proc. Natl. Acad. Sci. U.S., 59, 884 (1968). J. Biochem.