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Nitric Oxide: A Potential Mediator of Amino -Induced Renal Hyperemia and Hyperfiltration”2

Andrew J. King,3 Julia L. Troy, Sharon Anderson, Julia P. Neuringer, Mark Gunning, and Barry M. Brenner

4%, LNMMA versus vehicle; P < 0.05). L-NMMA also

A.J. King, J.L. Troy, S. Anderson, JR. Neuringer, M. caused modest blunting of the mixed - Gunning, B.M. Brenner, Renal Division and Department induced hyperfiltration (18 ± 4 versus 30 ± 4%, i- of Medicine, Brigham and Women’s Hospital, and The NMMA versus vehicle; P = 0.056) but failed to curtail Harvard Center for the Study of Kidney Diseases, Har- the renal hyperemia (16 ± 6 versus 20 ± 4%). Dex yard Medical School, Boston, MA trose had no effect on glomerular filtration rate or renal plasma flow. These results with mixed amino (J. Am. Soc. Nephrol. 1991; 1:1271-1277) differed from those with alone, presum- ably because the former was rich in 1-, the metabolic precursor for . For glycine- treated rats, urinary 3’ ,5’-cGMP excretion rates in- creased significantly in both vehicle- and i-NMMA- ABSTRACT treated rats (6.1 ± 1.1 to 15.7 ± 2.4 and 7.9 ± 0.6 to The role of nitric oxide in the modulation of systemic 14.6 ± 0.6 pmol/min, respectively). In summary, nitric and renal hemodynamics was examined by using oxide appears to influence basal systemic, and to a N-monomethyl-i-arginine (i-NMMA, I 10 g/kg/min), lesser extent, basal renal vascular tone. Furthermore, a competitive inhibitor of the conversion of i-arginine the renal effects of mixed amino acids and glycine to nitric oxide. i-NMMA or saline vehicle (9.o L/min) are, at least in part, sensitive to i-NMMA, suggesting was infused intravenously into anesthetized euvo- that nitric oxide formation contributes to the renal lemic Munich-Wistar rats. After 30 mm, i-NMMA re- vasodilation and hyperfiltration responses to acute suited in a uniform increase in mean arterial blood amino acid infusion. pressure (1 1 1 ± I to 128 ± 2 mm Hg; P< 0.05) and a Key Words: EDRF, M-monomefhyk-arginine. glycine. hemo- modest reduction in renal plasma flow rate (4.4 ± dynamics. kidney 0.2 to 4.2 ± 0.1 mL/min; P< 0.05), without change in glomerular filtration rate (1.16 ± 0.03 to 1.15 ± 0.03 mi/mm); vehicle had no effect on these renal param- eters. These rats were then subdivided to receive an O ver the past decade, it has been recognized that endothelial cells produce several vasoactive me- intravenous infusion (37 L/min) of either 10% gly- diators which modulate vascular smooth muscle tone cine, 11.4% mixed amino acids, or equiosmolar dex- in response to a variety of stimuli ( 1 ,2). One such trose. i-NMMA pretreatment markedly attenuated mediator, endothelium-derived relaxing factor (10 ± 6 versus 33 ± glycine-induced hyperfiltration (EDRF), has been identified to be at least In part 5%, i-NMMA versus vehicle; P< 0.05) and obliterated nitric oxide (NO) derived from the guanido the renal hyperemic response (-7 ± 6 versus 16 ± atom(s) of L-arginine (L-Arg) (3). NO is a potent stim- ulator of soluble guanylate cyclase leading to an in- crease in vascular smooth muscle cGMP and subse- ‘Received June 29, 1990. Accepted February 26. 1991.

2 PortIons of these studies were presented at the 21st Annual Meeting of the quent relaxation (1). However, the molecular compo- American Society of Nephrology, December 1989, and were published in sition of EDRF remains controversial, and abstract form (Kidney tnt 1990:37:37 1). nitrosothiols have recently been suggested to be the Correspondence to Dr. A.J. King, Renal Division. Department of Medicine. New England Medical Center, Box 390, 750 Washington Street, Boston, MA primary form of EDRF (4). Studies that used a com- 02111. petitive analog of L-Arg, (N-monomethyl-L-arglnine, 1046-6673/01012-127 I$03.00l0 L-NMMA), have shown in rabbits, guinea pigs, rats, Journal of the American Society of Nephrology Copyright © 1991 by the American Society of Nephrology and humans that the endothelium maintains a

Journal of the American Society of Nephrology 1271 Renal Effects of L-NMMA

steady-state production of NO (5-8). Infusion of L- the following protocol was used to maintain euvole- NMMA results in a sustained pressor effect which Is mia. Isoncotlc rat plasma was infused at 0. 1 mL/min rapidly reversed by subsequent infusion of L-Arg (5). in a total amount equal to 1 % of the body weight, Currently, little is known of the role the endothe- followed by a reduction in infusion rate to 1 .60 mL/ hum plays in the modulation of renal hemodynamics. kg/h. Several endothelium dependent vasodilators, such as The experimental protocols are summarized in Fig- and , are known to induce a ure 1 . After a 1 -h equilibration period, two 1 0-mm marked reduction in renal vascular resistance (9). urine collections with concurrent arterial blood sam- Indeed, a variety of vasoactive hormones influence ples (0.21 mL) were obtained for baseline determi- glomerular filtration, in part by modulation of the nations of hematocrit and plasma concentra- relative resistances of the pre- and postglomerular tion (CA), and for baseline measurements of glomer- microcirculatlon (1 0). Cultured glomerular endothe- ular filtration rate (GFR) (inulin clearance) and hal cells are capable of producing EDRF in response estimated renal plasma flow rate (RPF) (PAH clear- to agents which increase cytosolic free calcium, fur- ance) (baseline). All rats then received a continuous ther supporting a role for NO In the control of renal i.v. infusion of either L-NMMA (3 mg/mL; 9.6 zL/ microcirculatory hemodynamics (1 1). mm), or sodium acetate vehicle (V), which were con- The purpose of the study presented here was to use tinued for the remainder of the experiment. Thirty L-NMMA to examine the role of NO in the mainte- minutes into the infusion, measurements of GFR and nance of basal systemic and renal hemodynamics in RPF were repeated (Period 1 ). Subsequently, with the anesthetized rat. In addition, studies were per- continued infusion of L-NMMA or V. rats were also formed to assess whether NO plays a role in the renal Infused i.v. (37 L/min) with either a 10% glycine hyperemla and hyperfiltration associated with acute solution (N = 16), an 1 1 .4% mixed amino acid solu- amino acid infusion. tion (Novamine,4 KabiVitrum, Alameda, CA; N = 16,) (AA), or an equiosmolar (compared with glycmne) so- lution of dextrose ( 1 ,330 mosmol; N = 1 2). Final METHODS clearance measurements were made after 30 mm Adult male Munich-Wistar rats (240 to 290 g) were (Period 2). At the end of the experiment, samples of studied in accordance with the National Institutes of renal vein blood were drawn for determination of Health Guide for the Care and Use of Laboratory PAH extraction. No difference in PAH extraction was Animals. Rats were anesthetized with mactin (100 found comparing rats which received V or L-NMMA mg/kg l.p.) and placed on a temperature-controlled (88 ± 1 versus 87 ± 2%; not significant [NSJ) or table. The right femoral artery was cannulated and comparing the different experimental infusions (87 a baseline sample of blood was collected for deter- ± 2, 85 ± 2, and 91 ± 1% [glycmne, mixed AA, and minatlon of hematocrit. This arterial catheter was dextrose, respectivelyj; NS). In the groups given gly- used for all subsequent blood sampling and for the cine with and without L-NMMA, urinary cGMP excre- estimation of mean arterial pressure (A) via an elec- tion rates were measured in each urine collection tronic transducer connected to a direct-writing re- period. In a second group, rats were prepared as corder. After tracheostomy, bilateral jugular cathe- described above, given V for L-NMMA and infused ters were Inserted for infusions of rat plasma, 10% with either L-Arg HC1 (37 tL/min of 1 ,330 mM [N = inulin with 0.8% para-aminohippurate (PAH) in 0.9% 4] or 660 mM [N = 51) or D-Arg HC1 (660 mM [N = 4]). NaCl (1.2 mL/h), and experimental Infusions. The left femoral vein was then cannulated for subsequent Analytical Infusion of L-NMMA (Calbiochem, San Diego, CA) or Inulin concentrations in plasma and urine were vehicle. The left ureter was catheterized for urine measured by a macro-anthrone method (13), and collections. PAH concentrations were measured by the method Rats prepared In this fashion have been shown to of Smith et at. (14). Plasma protein concentrations have a 20% reduction in plasma volume (12); thus, were measured by using refractometry. Urinary cGMP levels were measured by radioimmunoassay. Baseline Period 1 Period 2 I . I I . I Statistical Vehicle or L-NMMA Reported values represent means ± SE. Individual baseline and experimental hemodynamic values of Glycine, AA or Dextrose Time 0 Amino acid content of Novamine includes (per deciliter): , 900 mg; (mm) 30 60 9 120 160 , 790 mg; , 790 mg; , 730 mg; . 570 mg: I I . 570 mg; , 570 mg; , 190 mg; . 1.65 g; arginine. 1.12 g; aminoacetic acid, 790 mg: , 680 mg: , 680 mg; Figure 1. Experimental protocol. 570 mg; , 480 mg; , 330 mg: , 30 mg.

1272 Volume I ‘Number 12’ 1991 King et al

TABLE I . Systemic and renal hemodynamic parametersa

x GFR RPF FF (mm Hg) (mL/min) (%)

Glycine + V (N = 8) Baseline 113±2 1.17±0.05 4.99±0.30 23±1 Periodi 119±2” 1.09±0.06 4.93±0.34 24±1 Period 2 123 ± 1b,C 1.44 ± 008b.c 5.67 ± 034b,c 25 ± I

Glycine + i-NMMA (N = 8) Baseline 112±2 1.11±0.04 4.45±0.21 25±1 Period I 128 ± 3b 1.13 ± 0.07 4.26 ± 0.17 26 ± I Period 2 135 ± 5bc 1.21 ± 0.04 3.97 ± 0.28 31 ± 2bc Mixed AA + V (N= 8) Baseline 113±2 1.18±0.04 4.54±0.10 26±1 Periodl 118±Ib 1.10±0.06 4.58±0.23 25±1

Period 2 121 ± 2b 1.46 ± 0.08bc 5.50 ± 0,35b,c 27 ± I Mixed AA + i-NMMA (N =8) Baseline 108±2 1.22±0.05 4.59±0.35 27±2 Periodl 123±2b 1.19±0.05 4.16±0.24 29±2 Period 2 120 ± 2b 1.40 ± 0.06 4.78 ± 0.23 30 ± I

Dextrose + V (N = 6) Baseline 110±2 1.16±0.02 4.43±0.19 26±1 PerlodI 112±2 1.15±0.06 4.75±0.25 24±1 Period2 113±4 ii±OO 446±016 2b±1

DextrQe tNMMA (N = 6) ftceline 116±2 112±OO 415±O2 27±1 Perioi 14±2 1.12±0.06 4.01±0.19 2±1 Prio2 I34±2 121tO,O4 437±019 20±1

. VciIue Q6 meqn :1: SF FF. filtration rqctjon ‘- P K Q 11b yertut baseline. , /2< #{216}#{216} period I ruts gIvtn V or I,NMMA wtre ompiirtd by piIrtd inwnt (120 to 10 mm) tNMMA hiid no effect on test, wlwrt’ group v&i.wh were comprd by un cwR (iRI * 0,0 to 11 ± 003 mL/mln; NS) but piIrd ( tt ftteliiw mind txprImtnti1 Vf1tu inthivtd ft 1Iht but sIgnifInnt Intl In RPF (44 ± 0,2 within groi.ip preht’ntt’l In Thhtti 1 wt’re tuilyd by to 42 ± 0, 1 mL/mln; P 00) (FIgurt 2), Vn1uer for cInt fiwtor insilyI of vrIn with rptiimtd me cwR (117 ± 002 to 112 ± 004 mL/mln; NS) nd ur trttittmtnt mtin wtre compired with tlw RPF (4,7 ± 0,1 to 47 ± 0,2 nuL/nuin, NS) rt’mnlned SIwffe I’ tt1t, Cnmpimrton of ptrnt htrng tmub1e In VInfubed contro1, Ekieimut ptrfuIon pre imong groups were perforrntd by 1ng1e ftwtor trui1y urr’, hut not RPF, Inerni’d with tNMMA, the cnl of vrIm’t with tht Sehtffe I’ tt, Whr vi1ue cuhited renfit vtutnr resIMiunct rose IgnIfIcnt1y wt’re not nornu11y dIstrIbuted i nonpariinwtr$ riink (F1gur 2), testIng with th Krukmi1?Wi1II frt wa ptrfornwd

SttItIciil tgnIfKiin w dtfIned its P ‘ Modulation of R#{149}sponssto Amino Acid Infusion by LNMMA RESULTS Qlyclnti InfuIon It’i;I to mirked lnereneh In HPI’ (1 4%) nn;I fWR (33 %) (both P ‘ 0,0, Tnble 1 Effscts of INMMA on Basal Rsriol and Syitimic nnd Figure 3), In ttw nbtne of ehnnge In remit Himodynamics perfuSIon preure or C (52 0 1 to 2 0 1 g/dt-4 tnfuton of INMMA 1r1 tQ m uniform incrtt In NS), lquIomo1nr dextrose InfuSIon hnd no IgnIfI- Al:1 whKh Wfi IgnIfIiint1y gretter ttuin thit In V cnnt effect on AP, RPF, or OFR In tIm presence or contrn1 ( I 1 1 * 1 to 1 28 2 vtru 1 1 2 1 to 1 1 7 iberwe of I=NMMA (Tnbte 1) However, tNMMA ob I Iui!! Hg, rprt1veIy; P 00) (Flgnre 2) Tht rI Ilternted the hyperenule repone (-7 6 verti 16 In AP gentr1ly whIved s ptiitctu by 1 to 30 nun :t 4%, t=NMMA veru V; P ‘ 0,0) to gtyelne nnd nd remtilned eonsstnnt for the diirmition of tiw txptr thus, miirkedly blunted the hyperflttrntlon repone

Journol ot the American Society or Nephrotoqv 1.73 Renal Effects of L-NMMA

140 40 #{149} Vehicle #{149} L-NMMA a Vehicle 130 30 #{149}L-NMMA

20 120 AP t (mm Hg) 10

110

100 , -10 1.4 50

40 GFR 1.2 30 (mi/mm) * GFR * a) 20 0) 1.0 , (5 10 -C C.) C 40 w RPF 0 (mi/mm) 30 a) : 0. 20 RPF io

0

-10 .20 RVR 11:t 30 (mmHg.mln/rnl) 1 20 FF

In, 10 Pre Post 0 Figure 2. Effects of i-NMMA or vehicle infusion on AP, GFR, Glycine AA RPF, and renal vascular resistance (RVR). Results represent Figure 3. Percent change in AP, GFR, RPF, and filtration baseline values and values after a 30-mm infusion of i- fraction (FF) after a 30-mm infusion of AA or glycine after NMMA (N= 22) or V (N= 22). Values are means ± SE. ‘P< pretreatment with vehicle or i-NMMA. Values are means ± 0.05 versus baseline; tP< 0.05 versus V. SE. ‘P< 0.05, i-NMMA versus V; tP< 0.05, glycine versus AA.

(10 ± 6 versus 33 ± 5%, L-NMMA versus V: P < 0.05). Glycmne had no significant effect on the hypertensive (hence, of dependence on NO), because the mixed AA response to L-NMMA (Table 1 and Figure 3) and did solution contained a substantial quantity of the com- not change CA (5.4 ± 0.1 to 5.3 ± 0.1 g/dL, Period 1 peting substrate L-Arg (53 mM). Indeed, in three of to 2; NS). the eight animals receiving L-NMMA plus mixed AA, Mixed AA Infusion resulted in increases in RPF (20 x had decreased to near baseline values by the end ± 4%) and GFR (30 ± 4%) (both P < 0.05 versus Period of the experiment, whereas none of the rats receiving

1 ; Table 1 , Fig. 3). Mixed AA had no significant L-NMMA plus glycmne or dextrose had a subsequent

Independent effect on X (Table 1 ) or CA (5.5 ± 0. 1 to decrease In . 5.4 ± 0. 1 g/dL). The percent rise in RPF was not The effects of L-NMMA on the renal hyperemic significantly different In the L-NMMA group when responses to glycine and mixed AA were strikingly compared with that in V (16 ± 6 versus 20 ± 4%, different (Figure 3). The mixed AA-induced rise in respectively: NS) nor was the change in CA (-4 ± 1 RPF was minimally blunted by L-NMMA, whereas the versus -4 ± 1%, L-NMMA versus V; NS). However, hyperemic response to glycmne was obliterated. The the hyperfiltration Induced by the amino acids was finding that glycmne-mnduced hyperfiltration, albeit numerically blunted by pretreatment with L-NMMA markedly blunted by L-NMMA, still persisted suggests (18 ± 4 versus 30 ± 4%, L-NMMA versus V; P = that an increased glomerular transcapillary hy- 0.056). This observed effect of L-NMMA may be an draulic pressure difference or a change in the gb- underestimate of the effectiveness of blockade merular capillary surface area may have offset the

1274 Volume I ‘ Number 12 ‘ 1991 King et at

fall in flow, a possibility supported by the finding of Baylis et at. (16), who showed that infusion of high a 1 9% increase in filtration fraction In the presence concentrations of L-Arg alone did not induce hypoten- of glycmne plus L-NMMA (Period 1 to 2: P < 0.05). By sion in the anesthetized guinea pig or in the conscious contrast, with mixed AA plus L-NMMA, GFR and RPF rat, respectively. However, excess L-Arg prolonged fell more or less proportionately, as indicated by the the hypotensive effect of the endothellum-dependent resulting near constancy of filtration fraction (3 ± vasodilator acetylcholmne in the guinea pig. Con- 4%; NS). versely, the hypotensive effects of acetylcholine were To help clarify the role of argmnmne in the response abbreviated by L-NMMA, indicating ready access by to the mixed amino acids, studies of L- and o-Arg both L-NMMA and L-Arg to the active NO synthetase infusions were performed. Doses of L-Arg which were ( 1 5). By contrast, transient hypotenslon in response equlmolar to the glycine group (1,330 mM) failed to to bolus doses of L-Arg (5 to 200 mg/kg) has been Induce hyperfiltratlon (1 .2 1 ± 0. 1 3 to 1 .2 1 ± 0.16 noted in conscious unrestricted rats, an effect which mL/mln; N 4; NS) or hyperemla (4.6 ± 0.3 to 4.7 ± was significantly blunted by N’-nltro-L-argmnmne (17). 0.3 mL/mln; NS). However, this infusion led to ex- L-Arg-induced hypotension has been reported in hu- treme diuresis ( 1 04 ,L/min) potentially superimpos- mans; however, the role of EDRF in this response Ing volume contraction; thus, lower doses were ex- remains controversial (18,19). In the studies pre- amined. L-Arg (660 mM; N = 5) led to no change in sented here, L-Arg failed to induce hypotension, 15 (120 ± 4 to 126 ± 4 mm Hg) and a trend upward whereas a significant rise in systemic pressure was In GFR (11 ± 5%; 1.18 ± 0.07 to 1.30 ± 0.08 mL/ seen with D-Arg infusion. The mechanism of this mm; P = 0.06) and in RPF (13 ± 6%; 0.2 to 5.3 response remains to be determined. ± 0.3 mL/min; P = 0.09). D-Arg (660 mM; N = 4) led The modest effects of L-NMMA on basal renal to an Increase In AP which was comparable to that hemodynamics In this study suggest that in the an- Induced by L-NMMA (1 5 ± 3 mm Hg; 1 22 ± 3 to 137 esthetized rat, the renal vasculature Is less NO-de- ± 5 mm Hg; P < 0.05), though It failed to change GFR pendent in the steady state than other resistance (1.18 ± 0.05 to 1.18 ± 0.06 mL/min; NS) and RPF beds. Confirming these findings, others have dem- (5.2 ± 0.3 to 5. 1 ± 0. 1 mL/min; NS). onstrated no change in GFR or RPF after bolus iv. To assess the activation of soluble guanylate cy- Infusion of L-NMMA ( 1 5 mg/kg) in the anesthetized clase by NO, urinary cGMP excretion rates were rat (20). In the absence of direct measurements, the measured In the rats given glycmne with and without possibility remains that L-NMMA led to offsetting L-NMMA. After glycmne, urinary cGMP rose by 274 ± effects on the determinants of renal perfusion and 32% (6.1 ± 1.1 to 15.7 ± 2.4 pmol/min: P < 0.05) in glomerular ultrafiltration and, thus, resulted In near rats pretreated with V as compared with 1 92 ± 18% constancy of RPF and GFR. Preliminary findings In (7.9 ± 0.6 to 1 4.6 ± 0.6 pmol/mln: P < 0.05) in the L- normal rats indicate that L-NMMA raises the gbomer- NMMA group. Although the percent increase in cGMP ular capillary hydraulic pressure by Increasing effer- excretion was significantly lower in the rats receiving ent arteriolar resistance without changing the single L-NMMA, there was no significant difference in the nephron GFR (2 1). Indeed, the capacity of the renal final absolute excretion rates achieved. vasculature of the rat and dog to respond to known endothelium-dependent vasodilators such as acetyl- and bradykmnmn suggests that this vascular DISCUSSION bed possesses the capability to produce large quan- tities of NO (22,23). Renal hyperfiltration and hyperemia after AA in- The findings in this study suggest that NO produc- fusion and/or protein loading have been recognized tion contributes to the maintenance of basal systemic for over half a century (24,25). However, the mecha- vascular tone In the anesthetized euvolemic normo- nisms involved in this localized response remain un- tensive rat and also contributes to AA-induced renal resolved (26). The study presented here demonstrated hyperemia and hyperfiltratlon. The systemic pressor that similar molar doses of glycmne or mixed AA In- effect Induced by L-NMMA In the basal state confirms duced hyperfiltration and renal hyperemia of approx- previous findings in the rat, rabbit, and guinea pig imately equivalent magnitude, whereas lower doses and suggests that there Is steady-state output of NO of L-Arg Induced lesser changes in GFR and RPF. The (5-7). This evidence, as well as the short half- of vasodilatory effect appears to be specific for the renal the mediator, make this EDRF ideally suited for mo- vasculature, as did not fall with either mixed AA, ment-to-moment modulation of systemic vascular re- glycmne, or L-Arg. In addition, confirming the findings sistance. Infusion of L-Arg alone failed to induce a of others (27,28), equiosmolar amounts of dextrose hypotensive effect in the anesthetized rat, suggesting failed to induce such changes, arguing against a vol- that In the unstimulated state, systemic production ume or osmolar effect as the mediator of the observed of NO Is not limited by the availability of substrate. renal responses. The study presented here indicates This confirms the findings of Aisaka et at. (15) and that the renal effects of glycmne and mixed AA are, at

Journal of the American Society of Nephrology 1275 Penal Effects of L-NMMA

least in part, sensitive to L-NMMA, suggesting that rats. This raises the intriguing possibility that affer- NO formation Is one component of the chain of events ent and efferent arterioles may differ in their ability which leads to postprandial renal hyperemia and to produce NO or respond to stimuli for NO produc- hyperfiltration. tion. Indeed, the vasodilatory response to acetylcho- Glycine is not a nitrogen donor source for the for- line of isolated rabbit afferent arterioles was equiv- matlon of NO (29). The observed renal hyperemic and alent to that of efferent arterioles whereas, with an- hyperfiltration responses must therefore reflect the other NO stimulator, bradykmnmn, the efferent arteri- activation of NO synthetase with the utilization of ole failed to relax (22). endogenous renal vascular NO donor sources, pre- Urinary cGMP (UcGMP) output increased markedly sumably L-Arg. Whether the stimulus for NO produc- In both the V- and L-NMMA-treated rats given gly- tion Indicates a direct effect of glycmne on the endo- cine. Although the percent increase was less in the thellum or vascular smooth muscle and involves the L-NMMA group than in the V rats, the final excretion release of intermediary substances is not yet known. rates were not different. In the absence of plasma However, infusions of glycmne and other metaboliza- levels It is not possible from our studies to determine ble AA into the isolated perfused kidney are known with certainty the source of this cGMP. Mesanglal to Induce vasodilation (30). In the study presented cells in co-culture with either gbomerular or aortic here, L-Arg-mnduced renal effects were modest and endothelial cells have been shown to release cGMP stereospecific. Our high dose failed to induce any (1 1 ,33). Currently, it is not known to what extent change in whole kidney hemodynamics, whereas cGMP produced by the renal microcirculation enters with still higher doses Baylis et at. (1 6) noted promi- the urinary space. It is of interest that UcGMP excre- nent renal hyperemia (+64%) without a significant tion rates are 3 1 % higher in humans after a meat effect on GFR in conscious rats. Several L-Arg salts meal as compared with that after equivalent sodium (methylester > hydroxyamate > ), but not D- and ingestion (34). However, this increase was Arg, lead to renal vasodilation in the isolated per- not contemporaneous with the hemodynamic fused kidney (31). In addition, maneuvers which dis- changes. Others have noted a threefold increase in rupted the endothelium or inhibited EDRF release UcGMP excretion after i.v. acetyicholine infusion significantly impaired this vasodilatory response which was prevented by L-NMMA pretreatment; how- (31). Taken together, these studies suggest a direct ever, plasma levels were not measured (20). Further effect of these AA on the renal vasculature. Whether studies are needed to determine the utility of UcGMP this effect is due to excess substrate for NO synthe- in the assessment of renal vascular tone. tase or occurs by another mechanism remains to be Control of renal vascular smooth muscle tone is determined. Potentially, the renal response to a pro- modulated by an array of circulating and local media- tein load is enhanced by the synergistic effects of the tors. The dynamic of the renal microcircula- stimulated NO synthetase and the relative excess of tion, which permits the glomerulus to adapt to a wide L-Arg substrate. variety of conditions, depends on the availability of L-Arg excess reverses the systemic effects of L- both vasodilating and vasoconstricting mediators, NMMA, due to competitive binding to the NO synthe- the balance of which ultimately determines the renal tase (5). The rate of L-Arg infusion (2.0 tmol/min) vascular resistance. Only recently has the endothe- during the mixed AA experiments in this study ex- hal monolayer been recognized to be an important ceeded that of L-NMMA (0. 1 6 zmol/min) by more than transducer of both chemical and mechanical signals 1 0-fold, which likely accounts for the lesser degree into appropriate changes in vascular smooth muscle of blunting of the renal hyperemia and hyperfiltra- tone. We postulate that AA infusion results in a tion observed in the mixed AA group as compared series of adaptations which alter the balance of vas- with that in the glycmne rats. In support of this con- odilating and vasoconstricting factors leading to a cluslon, preliminary studies have verified that hyper- reduction of renal vascular resistance. The present filtration and hyperemla Induced by a 1 0% mIxed AA finding that this vasodilation is, at least in part, solution were obliterated by Intrarenal L-NMMA (32). sensitive to L-NMMA, suggests that NO plays a role Despite obliteration of the glycmne-mnduced hyper- in this renal hyperemic and hyperfiltration response. emlc response by L-NMMA, there was persistent, al- beit markedly reduced, hyperfiltration. This finding, In conjunction with the observed 19% rise in filtra- tion fraction, suggests that glycmne, in the presence ACKNOWLEDGMENTS of L-NMMA, increased either the glomerular trans- capillary hydraulic pressure gradient or the gbomer- These studies were supported by grant DK35930 from the NIH. A.J. King and JR. Neuringer are recipients of Individual National Re- ular capillary ultrafiltration coefficient. Indeed, Zatz search Service Awards of the NIH 1DK08003 and DK08294. respec- and de Nuccl (21) observed that L-NMMA raises the tively). We are grateful to L.E. Clarey and S.J. Downes for expert gbomerular capillary hydraulic pressure in normal technical assistance.

1276 Volume I ‘Number 12’ 1991 King et al

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