Endothelin-1 (ET-1) has been found in renal

Transcription

1 845 Effect of Edotheli-1 o Glomerular Hydraulic ressure ad Rei Release i Dogs Huabao Li, Mariem Sagmal, Mais J. Smith Jr., ad David B. Youg The preset study was desiged to aalyze quatitatively the effects of a wide rage of edotheli-1 levels o real hemodyamlcs ad rei release i the caie oftlterig kidey, icludig their effects o glomerular hydraulic pressure. Itrareal ifusio of edotheli-1 produced dose-depedet reductios i real bloodflow,but it did ot affect glomerular hydraulic pressure util the ifused dose reached high rates. At the rate of 1.0 g/kg per miute, edotheli-1 reduced real bloodflowby 23% (><0.01), whereas glomerular hydraulic pressure was ot sigificatly chaged from 68.1 ±1.3 to 67.4 ± 1.2 mm Hg. However, with a higher rate of edotheli-1 ifusio (5.0 ad.0 g/kg per miute), glomerular hydraulic pressure fell to 59.5±1.3 ad 5L5±1.8 mm Hg (p<0.01), whereas real bloodflowwas reduced from 154.5±15 to 83.0±9.5 ad 53.5±9.9 ml/mi, respectively. Edotheli-1 ifusio also produced a ihibitory effect o rei release. With ifusio at 1.0 g/kg per miute, rei release fell from the cotrol level of 47.9±5.6 to 26.6±4.9 uits/mi per gram kidey weight (p<0.01), ad it fell further to 16.1 ±4.3 uits/mi per gram kidey weight with ifusio at.0 g/kg per miute. I summary, edotheli-1 ifusio did ot affect glomerular hydraulic pressure despite a fall i real blood flow at low doses, but at high doses it reduced both, suggestig that edotheli-1 exerts separate, dose-depedet effects o preglomerular ad postglomerular resistaces. I additio, the preset study demostrated that edotheli-1 ifusio has a ability to ihibit rei release i vivo whe the macula desa-medlated pathway is elimiated. (Hypertesio 1993^1: ) KEY WORDS kidey glomerular filtratio rate real circulatio edothelis rei Edotheli-1 (ET-1) has bee foud i real glomerular edothelial cells 1 ad other kidey cells, 23 ad it has specific bidig sites i the glomeruli; therefore, the possibility has bee raised that ET-1 may play some fuctioal role i the regulatio of real hemodyamics. 4 I vivo, ET-1 has bee reported to have potet real vasocostrictor effects, actig o both real blood flow (RBF) ad glomerular filtratio rate (GFR) i dogs, rats, ad rabbits. 5-7 Some studies have reported that i low doses the effects of ET-1 o RBF were much greater tha those o GFR. I the studies by Kig et al 6 ad Miura et al, 8 ET-1 reduced RBF by as much as 20% ad did ot sigificatly alter GFR. These fidigs suggested that ET-1 had a predomiat effect o the efferet arterioles, so that the glomerular capillary hydraulic pressure could be elevated or maitaied despite the fall i RBF. Kig ad Breer 4 also foud i rats that the trasglomerular capillary pressure gradiet was greater i the ET-1 ifusio group tha that i the cotrol group. However, the direct effect of ET-1 o regulatio of glomerular hydraulic pressure (GH) over the complete dose rage has ot bee fully aalyzed. From the Departmet of hysiology ad Biophysics, Uiversity of Mississippi Medical Ceter, Jackso. Supported by grats HL ad HL from the Natioal Heart, Lug, ad Blood Istitute, ad grat MS-90F22 from the America Heart Associatio, Mississippi Affiliate. Address for correspodece: Huabao Li, hd, Departmet of hysiology ad Biophysics, Uiversity of Mississippi Medical Ceter, 2500 North State Street, Jackso, MS Received December 3,1992; accepted i revised form February 5,1993. I additio to its potet real vasocostrictio, ET-1 may have effects o the rei-agiotesio system. Although may i vitro studies have foud that ET-1 exerts a ihibitory effect o rei release, 9-11 its role i cotrol of rei release remais cotroversial i light of its i vivo effects. Some ivestigators 12 foud that ET-1 icreased plasma rei activity, but others reported that a relatively low dose of ET-1 sigificatly reduced it. 13 Such discrepat effects o the rei-agiotesio system are probably due to the complexity of the ature of cotrollig rei release i vivo. It is most likely that the several cotrol mechaisms kow to play importat roles i the regulatio of rei release are affected by admiistratio of ET-1, such as the macula desamediated pathway, the itrareal baroreceptor, ad possibly a direct effect of ET-1 o the juxtaglomerular cells. Furthermore, each cotrol mechaism may have a differet threshold for the activatio by ET-1, so that the et rei secretory respose i vivo varies markedly with the ET-1 doses. Therefore, the preset study was desiged to aalyze quatitatively the effects of a wide rage of ET-1 levels o the regulatio of real GH usig a stop-flow techique to determie the effects o preglomerular ad postglomerular vascular resistaces. I additio, we aalyzed the effect of ET-1 o rei release i a acute ofilterig kidey model so that the effects resultig from the macula desa mechaism could be isolated. Methods Experimets were performed o mogrel dogs of either sex obtaied from the research aimal facilities of

2 846 Hypertesio Vol 21, Mo 6, art 1 Jue 1993 the Uiversity of Mississippi Medical Ceter (body weight, 19.6±0.6 kg; «= ). Aimals were housed i the aimal facilities before use ad fed a stadard laboratory diet. The food was removed from their cages 15 hours before surgery, but the dogs were give free access to water. The aimals were iitially sedated with mg i.m. acepromazie maleate. Te miutes later, they were aesthetized with approximately 30 mg/kg i.v. sodium petobarbital. Surgical rocedure ad Experimetal Measuremets After aesthesia, a respiratory pump was used to permit artificial vetilatio through a trachea tube as eeded to maitai ormal blood gas values. Both femoral arteries ad oe femoral vei were caulated with Tygo (Norto, Akro, Ohio) catheters for measuremet of arterial blood pressure above ad below the real artery, samplig of arterial blood, ad itraveous ifusio. Blood pressure was determied by a pressure trasducer (Cobe, Lakewood, Colo.) placed at the same level as the dog's heart ad coected to a polygraph (Grass Istrumet Co., Quicy, Mass.). Via a left retroperitoeal flak icisio, a portio of the aorta above the left real artery was getly isolated so that a silicoe rubber cuff occluder could be placed aroud the aorta. The occluder was coected to a servo-cotrol device that was used to maitai arterial pressure below the occluder (i.e., real arterial pressure) at a costat level. The left real artery was also isolated, ad a electromagetic flow probe was placed aroud the real artery. A electromagetic flowmeter (model FM-501, Carolia Medical Electroics, Ic., Kig, N.C.) was used to measure RBF. The left ureter was caulated with a E-90 catheter for measuremet of uriary pressure by a Cobe pressure trasducer. A 22-gauge L-shaped eedle attached to a catheter was iserted ito the real vei for samplig of real veous blood. Fially, a 23-gauge L-shaped eedle was also iserted ito the left real artery for a itrareal ifusio. Blood gas measuremet was made with a ph/blood gas aalyzer (model 1304, Istrumetatio Laboratories, Lexigto, Mass.) to adjust the rate of the respirator. To determie glomerular capillary hydraulic pressure, we used a stop-flow pressure method. I this method, the left kidey was acutely redered ofilterig by techiques described previously. 1 *- 16 The techique icluded two steps: first, a osmotic diuresis was established, ad secod, the ureter was occluded to elevate its pressure util the filtratio pressure (F) was zero. Filtratio pressure was determied from the equatio F=GH-(OSM+T) where OSM is plasma colloid osmotic pressure, ad T is proximal tubular hydrostatic pressure. If T ad OSM are icreased to a poit at which GH=OSM+T the the F will be equal to zero, ad itratio will ot occur. The plasma colloid osmotic pressure was calculated by the method of Navar ad Navar 17 : OSM=1.4C+0.22C C 3 where C is plasma protei cocetratio measured by a refractometer (AO Reichert Scietific Istrumets, Buffalo, N.Y.). T is assumed equal to the stopped-flow uriary pressure (U) i the preset study. 18 Therefore, the glomerular hydraulic capillary pressure was determied by the formula GH=OSM+U After completio of surgery, a dose of 300 ml 6.0% maitol solutio was itraveously ifused over miutes, followed by a sustaiig ifusio of 2.0 ml/ mi. After diuresis had occurred for approximately miutes, the ureteral catheter was clamped ad ureteral pressure was measured by a pressure trasducer. The pressure reached a plateau i approximately miutes. Absece of filtratio was cofirmed by measuremet of arterial ad real veous [ 125 I]iothalamate activities. The preglomerular vascular resistace (RVR^) ad postglomerular vascular resistace (RVRpou) were calculated by the equatios ad =(RA - GH)/RBF RVR pob =GH/RBF where RA is real arterial pressure. Resistace is expressed as millimeters of mercury per milliliter per miute. A idex of rei release was determied from the differece i plasma rei activity betwee the real veous ad arterial plasma ad the real plasma flow. Blood samples for plasma rei activity measuremets were collected i iced sodium EDTA tubes ad coldcetrifuged for more tha 30 miutes. lasma (1 ml) was used for the assay usig the radioimmuoassay procedure of Haber et al. 19 The rei release idex (RR) from the left kidey was calculated from the product of the veous mius arterial plasma rei activity (RA) differece ad the real plasma flow (RF) divided by the kidey weight. It ca be expressed as RR=(RA v -RAJxRF/g kidey weight where RA, is rei activity i the real veous sample, ad RA, is rei activity i the real arterial sample. Oe uit of rei release was take to be equal to 1 g agiotesi I/mL per hour, ad the rate of rei release is expressed as uits per miute per gram of kidey weight. Experimetal rotocol Withi-subject experimet desig. Experimets were begu after ureteral pressure had stabilized for at least 30 miutes. Real perfusio pressure was maitaied at 80 mm Hg durig the experimet with the servo-cotrol device. 20 The experimet cosisted of a 60-miute cotrol period ad a 60-miute ET-1 ifusio period (=5). Durig the cotrol period, 0.9% NaCl solutio was ifused itrareally at a rate of 0.5 ml/mi. Data were collected every 15 miutes, icludig 3.0-mL blood samples obtaied from the arterial ad real

3 Li et al Glomerular ressure ad Re i Release 847 TABLE 1. Edothell-l Ifusio Experimet Systemic arterial pressure (mm Hg) lasma protei cocetratio (g%) Filtratio fractio (%) Hematoct (%) Cotrol 121± ± ± ± ± ± ± ±1.0 Edotheli-1 ifusio [(g/kg)/mi] ± ± ± ±1.1 <0.05 Values of p are for compariso betwee cotrol groups vs. edotheli-1 ifusio groups ± ± ± ±1.1 < ± ± ±0.9 37±1.1 <0.01 veous catheters for measuremet of hematocrit, plasma protei cocetratio, plasma rei activity, ad [ 123 I]iothalamate activity. Three rei samples for each dog were collected durig the 60-miute cotrol period. I additio, ph ad blood gas were aalyzed i the arterial blood samples. I the ET-1 ifusio period, four differet doses (0.2, 1.0, 5.0, ad.0 g/kg per miute) of ET-1 (Sigma Chemical Co., St. Louis, Mo.) solutios were ifused itrareahy, ad each dose was maitaied for approximately 15 miutes. The same measuremets were made as i the cotrol period. Betwee-subject experimet desig. Experimets were performed i two groups of dogs: cotrol dogs ad ET-1-ifused dogs ( = ). Experimets lasted miutes. I the cotrol group, 0.9% NaCl solutio was ifused, ad i the ET-1-ifused group, four differet doses of ET-1 were ifused itrarealh/ as described above. Measuremets i the two groups of dogs were similar to those described above. After completio of the experimet, the ureteral clamp was released, ad 45 miutes were allowed to permit fluid accumulated durig ureteral occlusio to be removed before the kidey was weighed. Data Aalysis Group meas ad SEM are preseted i the text ad figures. Statistical comparisos of all data were performed with a sigle-factor aalysis of variace. The values i each group of cotrol ad ET-1 ifusio were compared. First, aalysis of variace was used to determie whether there were differeces betwee the populatio meas ad group meas with betwee-subject or withi-subject desig. Secod, whe the aalysis of variace led to a rejectio of the ull hypothesis, Duett's test was used to determie the statistical probability of differeces betwee the groups. A probability value of <5% was accepted as statistically sigificat. If a probability value was <1%, it was also idicated i the results. Results I the preset study, real perfusio pressure was maitaied throughout the experimet at 80 mm Hg by the servo-cotrol device. The systemic arterial pressure (Table 1) was 121 ±6 mm Hg ad remaied stable durig the itrareal ifusio of ET-1. Table 1 also shows filtratio fractio, which demostrated that the left kidey was i a ofilterig state. lasma protei cocetratios were ot affected by the ET-1 ifusio i the experimet, whereas hematocrit i the ET-1 ifusio groups was slightly greater tha i the cotrol group, but icreases were o more tha 4%. Withi-Subject Desig Figure 1 shows that RBF ad GH i the salie ifusio period remaied stable durig the 60 miutes of observatio; GH was virtually uchaged throughout this period. RBF was 163 ±25 ml/mi at the ed of the 60 miutes compared with 148 ±17 ml/mi at the begiig (/»0.05). GH was 67.3±2.0 mm Hg at the begiig versus 67.3 ±2.1 mm Hg at the ed of the observatio. However, there were sigificat chages durig ET-1 ifusio. RBF was reduced approximately 70%, from 148±22 to 48±15 ml/mi (p<0.0l), ad GH fell from 67.5±2.2 to 52.9±2.4 mm Hg (p<0.01). Betwee-Subject Desig The experimet was performed i two differet groups of dogs. Figure 2 shows that RBF ad GH i the ET-1-ifused dogs were sigificatly differet compared with values i the cotrol aimals. RBF i the cotrol dogs was ot differet durig the 60 miutes of observatio, whereas i the ET-1-ifused aimals, it fell from 136±17 to 59±14 ml/mi (/><0.01). GH was reduced 28%, from 69.1±2.1 to 50.0±2.7 mm Hg i the ET-l-ifused group (p<0.01), whereas there was o differece i the cotrol group durig the observatio. The relatios betwee ET-1 ifusio rate ad RBF ad GH from the two exper-

4 848 Hypertesio Vol 21, No 6, art 1 Jue , \ \ Silie Ifusio " ISO- I B D Experimetal Time Course (rai) FIGURE 1. Lie graphs show giomerular hydraulic pressure ad real blood flow of ofuterig kideys i withi-subject desig experimet. Mea ad SEM are idicated Asterisks deote differece betwee the last cotrol measuremet ad those i the edotheli-1 (ET-1) ifusio period- *p<0.05, "p<0.01. imetal desigs are preseted i Figure 3. The relatios were idepedet of study desig ad therefore of time. 80-, 200-, ^ - I -r M MM fr K i S Cotrol dogs ET Ifusio dogs I Eiperlmetil Time Cout (mta) MM FIGURE 2. Lie graphs show giomerular hydraulic pressure ad real blood flow of ofilterig kideys i betwee-subject desig experimet. Mea ad SEM are idicated. Asterisks deote differece betwee the measuremet i cotrol dogs ad those i edotheli-1 (ET)-ifuseddogs: *p<0.05, **p< ^ ET Ifusio I I- ET Ifusio EdotbeH 1 Ifusio Rate (g/kg/mi) FIGURE 3. Lie graphs show giomerular hydraulic pressure ad real blood flow i both withi- ad betwee-subject desigs i respose to differet doses of edotheli-1 (ET) ifusio. Relatio betwee edotheli-1 ifusio rate ad real blood flow ad giomerular hydraulic pressure from the two experimetal desigs was idepedet of study desig ad time. Edotheli-1 Dose-Respose Relatios Figure 4 presets the combied data from the two study desigs plotted as ET-1 ifusio rate versus RBF ad GH values as percet of the cotrol level. The preset study showed that ET-1 ifusio could sigificatly alter real hemodyamics, icludig RBF ad GH. However, the respose of RBF to ET-1 was quite differet from that of GH. RBF was altered with the ET-1 ifusio startig at 0.2 g/kg per miute; it was reduced approximately % after 15 miutes of ET-1 ifusio. With the ET-1 ifusio rate of 1.0 g/kg per miute, RBF was reduced from 154.5±15.0 to ml/mi, a 23% reductio from the level durig salie ifusio (p<0.01). However, GH was ot sigificatly chaged util the ET-1 ifusio rate reached 5.0 g/kg per miute. At this ifusio rate, GH was reduced approximately 12%, from 68.1±1.4 to 59.5±1.3 mm Hg (/7<0.01), whereas RBF was decreased more tha 45%. With a higher dose of ET-1 (.0 g/kg per miute), RBF was reduced further to 53.5±9.9 ml/mi, ad GH was reduced to 51.5 ±1.8 mm Hg. Figure 5 shows the preglomerular ad postglomerular resistaces i respose to the differet doses of ET-1. With a low dose of ET-1, both the preglomerular ad postglomerular resistaces were icreased at a similar pace. However, with higher doses (5 ad g/kg per miute), ET-1 ifusio raised preglomerular resistace much more tha postglomemlar resistace (70% versus 232% ad 175% versus 775%, respectively; p<0.05).

5 Li et al Glomerular ressure ad Rei Release =. 0- a ' * * S 60- o ^ 40- : 20- -O RBF GH = * * * * Experimetal Time Course (mi) S.O.0 Edotheli 1 Ifusio Rate (g/kg/mi) FIGURE 4. Lie graph shows combied data from the two study desigs plotted as edotheli-1 ifusio rate vs. real blood flow (RBF) ad glomerular hydraulic pressure (GH). All data show were ormalized by takig values i the salie ifusio period as 0%. RBF ad GH were 154.5±15.5 mllmi ad 68.1 ±1.4 mm Hg i the salie ifusio period, respectively. Statistical comparisos were performed betwee the value i the cotrol period ad those durig the differet edotheli-1 doses. *p<0.05, "p<0.01. Effects of Edotheli-1 o Rei Release Figure 6 presets rei release data i respose to salie ad ET-1 ifusios as real perfusio pressure was costatly maitaied at 80 mm Hg by a servocotrol system. I the ofilterig kidey preparatio, the rate of rei release i cotrol dogs did ot chage durig the 60 miutes of observatio (Figure 6, pael A). The rei release rate was 29.1 ±25, 31.0±4.1, ad -I II Ji rtfkmtrilar Boistct ostgtoxrilu' Rcslitice Edotbdi 1 Ifusio Rate (g/kg/mi) FIGURE 5. Bar graph shows preglomerular ad postglomerular resistaces i respose to differet edotheli-1 doses. With a low dose of edotheli-1 (<.1.0 g/kg per miute), both preglomerular ad postglomerular resistaces icreased at a similar pace. However, with higher doses (^5 g/kg per miute) edotheli-1 raised preglomerular resistace much more tha postglomerular resistace Edothdi 1 Ifusio (g/kg/mi) FIGURE 6. Bar graphs show data of rei release i respose to salie ad edotheli-1 ifusios. ael A: With salie ifusio, rate of rei release was ot differet durig the 60-miute observatio period. Data show were ormalized by takig values i the sample 15-miute period as 0%, which was 29.1 ±2.5 uitsimi per gram kidey weight. ael B: With edotheli-1 ifusio, rate of rei release was reduced sigificatly i a dose-depedet fashio. Data show were also ormalized by takig cotrol period values as 0%, which was 47.9±5.6 uits/mi per gram kidey weight. *p<0.05, **p< ±3.3 uits/mi per gram kidey weight after 15, 30, ad 60 miutes of observatio, respectively (/?>0.05). However, i aimals ifused with ET-1, rei release decreased progressively as the ifusio rate was raised. These data are preseted i Figure 6B, with rei release rate expressed as percet of the cotrol value. With ifusio at 1.0 g/kg per miute, rei release fell from the cotrol level of 47.9±5.6 to 26.6±4.9 uits/mi per gram kidey weight (/><0.01), whereas GH remaied uchaged (Figure 4). Yet, with a higher rate of ET-1 ifusio at.0 g/kg per miute, rei release fell further to 16.1 ±4.3 uits/mi per gram kidey weight, 34% of the cotrol level, despite the fall i GH. Discussio The preset study has demostrated that ET-1 had profoud effects o real hemodyamics ad rei release i the ofilterig kidey preparatio. The results also idicated that ET-1 ifusio ca cause oparallel effects o RBF ad GH uder these coditios. revious studies showed that with a low dose of ET-1, real plasma flow was reduced by more tha 20%, whereas GFR was uchaged i rats ad dogs. 6-8 Furthermore, Kig ad Breer* ad Kig et al 6 also reported that low doses of ET-1 icreased the mea

6 850 Hypertesio Vol 21, No 6, art 1 Jue 1993 trasglomerular capillary hydraulic pressure gradiet i rats, suggestig that the icrease i GH is probably the reaso that GFR was maitaied despite reductios i RBF ad filtratio coefficiet i rats. However, direct effects of ET-1 o the glomerular hydraulic capillary pressure i dogs i vivo have ot bee reported. Other ivestigators have foud that with a ifusio rate of 1.15 g/kg per miute, ET-1 caused parallel reductios i RBF ad GFR without affectig the filtratio fractio. 5 To determie GH i respose to various doses of ET-1, we used a i vivo stop-flow pressure method i the preset study, which had bee used i previous studies. 15 ' 21 The method was based o a model i which the kidey was redered ofilterig. The glomerular capillary hydraulic pressure was expected to be elevated due to activatio of the rubuloglomerular feedback mechaism. I the preset study, real perfusio pressure was cotrolled at the level of 80 mm Hg by a servo-cotrol device, which could atteuate the atypically icreased GH. The estimated GH i the experimet was 68.1 ±1.4 mm Hg, which is close to the value of 65.8 ±2.9 mm Hg obtaied by Ott et al 22 with a micropucture techique i dogs. Durig the cotrol experimet, GH was stable throughout the 60-miute experimet (see Figure 1), suggestig that the experimetal time course was ot a factor affectig GH i this preparatio. ET-1 ifusio produced dose-depedet chages i RBF, but it did ot affect GH util the dose reached 5.0 g/kg per miute, at which poit RBF was already reduced by 45%. Therefore, low doses of ET-1 did ot alter GH despite the fall i RBF. This discrepat effect o RBF ad GH by ET-1 reflects the fact that ET-1 may affect the preglomerular ad postglomerular resistaces i parallel, thereby tedig to maitai GH while decreasig RBF. With higher doses, ET-1 icreased preglomerular resistace % more tha postglomerular resistace. Therefore, i the upper dose rage, GH decreased cocomitatly with RBF. Such chages i RBF ad GH ca result i severe decreases i GFR, such as those reported by Miura et al. 8 I their study, GFR fell by as much as 75% with a dose of 5.0 g/kg per miute. The physiological importace of edotheli i regulatig real hemodyamics is still ucertai, but it appears that the effect of ET-1 is differet from that of other vasocostrictors, such as agiotesio II ad orepiephrie. Several studies have idicated that agiotesi II selectively costricted the efferet arterioles I a preparatio similar to the oe i the preset study, Hall ad Grager 21 reported that agiotesi II ifusio raised GH despite a fall i RBF, suggestig that agiotesi II maily affected the postglomerular vessels. I cotrast, we foud (upublished data) that orepiephrie ifusio could cocomitatly reduce RBF ad GH. This fidig suggests that orepiephrie may preferetially affect preglomerular vessels. A similar fidig was also reported by others with orepiephrie ifusio. 21 Ulike agiotesi II ad orepiephrie, ET-1 has a parallel effect o the preglomerular ad postglomerular vessels at a low dose. There has bee some cotroversy regardig the effect of edotheli o rei release, particularly i vivo. Miller et al 12 foud that itraveously ifused ET-1 (50.0 g/kg per miute) i aesthetized dogs produced a icrease i plasma rei activity, ad they believed the icrease was due to activatio of the itrareal baroreceptor ad macula desa pathway. However, i the study by Cavero et al, 13 the itraveously ifused ET-1 (5.0 g/kg per miute) suppressed plasma rei activity i dogs. Such discrepat effects o rei secretio i vivo might be ascribed to dose depedece. Because a low dose of ET-1 had little effect o afferet arteriolar pressure ad a relatively small effect o GFR, the direct ihibitory effect of ET-1 o rei release was revealed. With a higher dose, ET-1 may severely reduce GFR ad perfusio pressure at the level of the juxtaglomerular cells, thereby stimulatig rei release ad overcomig the direct ihibitory effect by the ET-1. We foud that ET-1 ifusio produced a ihibitory effect o rei release i vivo i the ofilterig kidey. This fidig is cosistet with those from i vitro studies, 9-11 which idicated that ET-1 had a direct ihibitory effect o rei secretio. The preset study also showed that ET-1 ihibited rei release ot oly at the low dose but also at higher doses. This result does ot agree with the previous i vivo study by Miller et al, 12 probably because the macula desa-mediated pathway was blocked i our experimet, so rei release could be affected oly by the itrareal baroreceptor mechaism ad by direct effects o the juxtaglomerular cells. Data from a previous study i this laboratory usig ofilterig kidey 24 suggested that the itrareal baroreceptor mechaism still played a active role i cotrollig rei release. At the low dose, ET-1 did ot alter GH ad, presumptively, did ot chage the perfusio pressure at the juxtaglomerular cells; therefore, the itrareal baroreceptor mechaism was ot activated to stimulate rei release. However, with higher doses of ET-1, GH was also reduced by as much as 25%, suggestig that the itrareal baroreceptor mechaism may have participated i stimulatig rei release. But the results of the preset study idicate that this stimulatio was ot strog eough to offset the direct ihibitory effect of ET-1 o the juxtaglomerular cells. This fidig suggests that if a higher dose of ET-1 could cause a icrease i rei release i the itact kidey uder ormal coditios, its effect is most likely depedet o idirect effects mediated by the macula desa-mediated pathway. I summary, the preset study demostrated that i the ofilterig kidey ET-1 decreased RBF i a dosedepedet fashio. The effects of ET-1 o preglomerular ad postglomerular resistaces were quatitatively similar at low doses, whereas they were much greater o preglomerular resistace at a high dose. As a result, GH remaied early uchaged at low doses eve though RBF fell, whereas at higher doses GH decreased sharply. These fidigs suggest that edotheli may play a importat role i regulatig GFR ad real hemodyamics as a potetial local factor. ET-1 sigificatly ihibited rei release at both low ad high ifusio rates. This ihibitio was idepedet of chages i GH, suggestig that the direct effect of ET-1 o rei release may have bee stroger tha the itrareal baroreceptor mechaism whe tubuloglomerular feedback was elimiated. Refereces 1. Marsde A, Dorfma DM, Breer BM, Orki BJ, Ballerma BJ: Edotheli: Gee expressio, release ad actio i cultured cells of the real glomerulus. (abstract) Am JHypertes 1989;2:49A

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