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The afferent arteriolar myogenic response contributes to the autoregulation of renal blood flow (RBF) and glomerular filtration rate (GFR), and plays an essential role in protecting the kidney against hypertensive injury. Systolic blood pressure (SBP) is most closely linked to renal injury, and a myogenic response coupled to this signal would facilitate renal protection, whereas mean blood pressure (MBP) influences RBF and GFR. The relative role of SBP vs. MBP as the primary determinant of myogenic tone is an area of current controversy. Here, we describe two mathematical models, Model-Avg and Model-Sys, that replicate the different delays and time constants of vasoconstrictor and vasodilator phases of the myogenic responses of the afferent arteriole. When oscillating pressures are applied, the MBP determines the magnitude of the myogenic response of Model-Avg, and the SBP determines the response of Model-Sys. Simulations evaluating the responses of both models to square-wave pressure oscillations and to narrow pressure pulses show decidedly better agreement between Model-Sys and afferent arteriolar responses observed in cortical nephrons in the in vitro hydronephrotic kidney model. Analysis showing that the difference in delay times of the vasoconstrictor and vasodilator phases determines the frequency range over which SBP triggers Model-Sys's response was confirmed with simulations using authentic blood pressure waveforms. These observations support the postulate that SBP is the primary determinant of the afferent arteriole's myogenic response and indicate that differences in the delays in initiation vs. termination of the response, rather than in time constants, are integral to this phenomenon. autoregulation; hypertension; kidney

Recent studies have implicated epithelial [Na.sup.+] channels (ENaC) in myogenic signaling. The present study was undertaken to determine if ENaC and/or [Na.sup.+] entry are involved in the myogenic response of the rat afferent arteriole. Myogenic responses were assessed in the in vitro hydronephrotic kidney model. ENaC expression and membrane potential responses were evaluated with afferent arterioles isolated from normal rat kidneys. Our findings do not support a role of ENaC, in that ENaC channel blockers did not reduce myogenic responses and ENaC expression could not be demonstrated in this vessel. Reducing extracellular [Na.sup.+] concentration ([[Na.sup.+]]o; 100 mmol/1) did not attenuate myogenic responses, and amiloride had no effect on membrane potential. Benzamil, an inhibitor of ENaC that also blocks [Na.sup.+]/[Ca.sup.2+] exchange (NCX), potentiated myogenic vasoconstriction. Benzarnil and low [[Na.sup.+]]o elicited vasoconstriction; however, these responses were attenuated by diltiazem and were associated with significant membrane depolarization, suggesting a contribution of mechanisms other than a reduction in NCX. [Na.sup.+] repletion induced a vasodilation in pressurized afferent arterioles preequilibrated in low [[Na.sup.+]]o, a hallmark of NCX, and this response was reduced by 10 [micro]mol/1 benzamil. The dilation was eliminated, however, by a combination of benzamil plus ouabain, suggesting an involvement of the electrogenic [Na.sup.+]-[K.sup.+]-ATPase. In concert, these findings refute the premise that ENaC plays a significant role in the rat afferent arteriole and instead suggest that reducing [[Na.sup.+]]o and/or [Na.sup.+] entry is coupled to membrane depolarization. The mechanisms underlying these unexpected and paradoxical effects of [Na.sup.+] are not resolved at the present time. renal autoregulation; vascular smooth muscle; sodium ion channels; sodium/calcium exchange; benzamil; amiloride; ouabain; sodiumpotassium-adenosinetriphosphatase; epithelial sodium channel

Phosphorylation of the 20-kDa myosin regulatory light chains ([LC.sub.20]) plays a key role in the regulation of smooth muscle contraction. The level of [LC.sub.20] phosphorylation is governed by the relative activities of myosin light chain kinase and phosphatase pathways. The regulation of these two pathways differs in different smooth muscle types and in the actions of different vasoactive stimuli. Little is known concerning the regulation of [LC.sub.20] phosphorylation in the renal microcirculation. The available pharmacological probes are often nonspecific, and current techniques to directly measure [LC.sub.20] phosphorylation are not sensitive enough for quantification in small arterioles. We describe here a novel approach to address this important issue. Using SDS-PAGE with polyacrylamide-bound [Mn.sup.2+]-phosphate-binding tag and enhanced Western blot analysis, we were able to detect [LC.sub.20] phosphorylation using as little as 5 pg (250 amol) of isolated [LC.sub.20]. Phosphorylated and unphosphorylated [LC.sub.20] were detected in single isolated afferent arterioles, and [LC.sub.20] phosphorylation levels could be accurately quantified in pooled samples of three arterioles ( afferent arteriole; Phos-tag SDS-PAGE

Recent studies of renal autoregulation have shown modulation of the faster myogenic mechanism by the slower tubuloglomerular feedback and that the modulation can be detected in the dynamics of the myogenic mechanism. Conceptual and empirical considerations suggest that perfusion pressure may modulate the myogenic mechanism, although this has not been tested to date. Here we present data showing that the myogenic operating frequency, assessed by transfer-function analysis, varied directly as a function of perfusion pressure in the hydronephrotic kidney perfused in vitro over the range from 80 to 140 mmHg. A similar result was obtained in intact kidneys in vivo when renal perfusion pressure was altered by systemic injection of [N.sup.G]-nitro-L-arginine methyl ester (L-NAME). When perfusion pressure was not allowed to increase, L-NAME did not affect the myogenic operating frequency despite equivalent reduction of renal vascular conductance. Blood-flow dynamics were assessed in the superior mesenteric artery before and after L-NAME. In this vascular bed, the operating frequency of the myogenic mechanism was not affected by perfusion pressure. Thus the operating frequency of the renal myogenic mechanism is modulated by perfusion pressure independently of tubuloglomerular feedback, and the data suggest some degree of renal specificity of this response. dynamics; tubuloglomerular feedback; interaction

The [Na.sup.+]/[K.sup.+]/2[Cl.sup.-] cotransporter (NKCC) plays diverse roles in the kidney, contributing sodium reabsorption and tubuloglomerular feedback (TGF). However, NKCC is also expressed in smooth muscle and inhibitors of this transporter affect contractility in both vascular and nonvascular smooth muscle. In the present study, we investigated the effects of NKCC inhibitors on vasoconstrictor responses of the renal afferent arteriole using the in vitro perfused hydronephrotic rat kidney. This preparation has no tubules and no TGF, eliminating this potential complication. Furosemide and bumetanide inhibited myogenic responses in a concentration-dependent manner. Bumetanide was ~20-fold more potent (I[C.sub.50] 1.0 vs. 20 [micro]mo1/l). At 100 and 10 [micro]mol/1, furosemide and bumetanide inhibited myogenic responses by 72 [+ or -] 4 and 68 [+ or -] 5%, respectively. The maximal level of inhibition by bumetanide was not affected by nitric oxide synthase inhibition (100 [micro]mol/1 [N.sup.G]-nitro-L-arginine methyl ester). However, the time course for the dilation was slowed (from [t.sub.1/2] = 4.0 [+ or -] 0.5 to 8.3 [+ or -] 1.7 min, P = 0.04), suggesting either a partial involvement of NO or a permissive effect of NO on relaxation kinetics. Bumetanide also inhibited ANG II-induced afferent arteriolar vasconstriction at similar concentrations. Finally, NKCC1, but not NKCC2, expression was demonstrated in the afferent arteriole by RT-PCR and the presence of NKCC1 in afferent arteriolar myocytes was confirmed by immunohistochemistry. In concert, these results indicate that NKCC modulation is capable of altering myogenic responses by a mechanism that does not involve TGF and suggest a potential role of NKCC1 in the regulation of vasomotor function in the renal microvasculature. renal autoregulation; tubuloglomerular feedback; vascular smooth muscle; chloride

When the kidney is subjected to acute increases in blood pressure (BP), renal blood flow (RBF) and glomerular filtration rate (GFR) are observed to remain relatively constant. Two mechanisms, tubuloglomerular feedback (TGF) and the myogenic response, are thought to act in concert to achieve a precise moment-by-moment regulation of GFR and distal salt delivery. The current view is that this mechanism insulates renal excretory function from fluctuations in BP. Indeed, the concept that renal autoregulation is necessary for normal renal function and volume homeostasis has long been a cornerstone of renal physiology. This article presents a very different view, at least regarding the myogenic component of this response. We suggest that its primary purpose is to protect the kidney against the damaging effects of hypertension. The arguments advanced take into consideration the unique properties of the afferent arteriolar myogenic response that allow it to protect against the oscillating systolic pressure and the accruing evidence that when this response is impaired, the primary consequence is not a disturbed volume homeostasis but rather an increased susceptibility to hypertensive injury. It is suggested that redundant and compensatory mechanisms achieve volume regulation, despite considerable fluctuations in distal delivery, and the assumed moment-by-moment regulation of renal hemodynamics is questioned. Evidence is presented suggesting that additional mechanisms exist to maintain ambient levels of RBF and GFR within normal range, despite chronic alterations in BP and severely impaired acute responses to pressure. Finally, the implications of this new perspective on the divergent roles of the myogenic response to pressure vs. the TGF response to changes in distal delivery are considered, and it is proposed that in addition to TGF-induced vasoconstriction, vasodepressor responses to reduced distal delivery may play a critical role in modulating afferent arteriolar reactivity to integrate the regulatory and protective functions of the renal microvasculature. renal microcirculation; afferent arteriole; myogenic; tubuloglomerular feedback

Redundant signaling mechanisms contribute to the vasodilatory response of the afferent arteriole to proteinase-activated receptor-2. Am J Physiol Renal Physiol 288: F65-F75, 2005. First published August 24, 2004; doi: 10.1152/ajprenal.00194.2004.--We previously demonstrated that stimulation of proteinase-activated receptor-2 (PAR-2) by SLIGRL-N[H.sub.2] elicits afferent arteriolar vasodilation, in part, by elaborating nitric oxide (NO), suggesting an endothelium-dependent mechanism (Trottier G, Hollenberg M, Wang X, Gui Y, Loutzenhiser K, and Loutzenhiser R. Am J Physiol Renal Physiol 282: F891-F897, 2002). In the present study, we characterized the NO-independent component of this response, using the in vitro perfused hydronephrotic rat kidney. SLIGRL-N[H.sub.2] (10 [micro]mol/l) dilated afferent arterioles preconstricted with ANG II, and the initial transient component of this response was resistant to NO synthase (NOS) and cyclooxygenase inhibition. This NO-independent response was not prevented by treatment with 10 nmol/l charybdotoxin and 1 [micro]mol/l apamin, a manipulation that prevents the endothelium-derived hyperpolarizing factor (EDHF)-like response of the afferent arteriole to acetylcholine, nor was it blocked by the addition of 1 mmol/l tetraethylammonium (TEA) or 50 [micro]mol/l 17-octadecynoic acid, treatments that block the EDHF-like response to bradykinin. To determine whether the PAR-2 response additionally involves the electrogenic [Na.sup.+]-[K.sup.+]-ATPase, responses were evaluated in the presence of 3 mmol/l ouabain. In this setting, SLIGRL-N[H.sub.2] induced a biphasic dilation in control and a transient response after NOS inhibition. The latter was not prevented by charybdotoxin plus apamin or by TEA alone but was abolished by combined treatment with charybdotoxin, apamin, and TEA. This treatment did not prevent the NO-dependent dilation evoked in the absence of NOS inhibition. Our findings indicate a remarkable redundancy in the signaling cascade mediating PAR-2-induced afferent arteriolar vasodilation, suggesting an importance in settings such as inflamation or ischemia, in which vascular mechanisms might be impaired and the PAR system is thought to be activated. nitric oxide; SLIGRL-N[H.sub.2]; potassium channels; ouabain-sensitive [Na.sub.+]-[K.sub.+]-ATPase; tetrathylammonium; apamin; charybdotoxin; 17-octadecynoic acid; endothelium-dependent hyperpolarizing factor

Effects of calcium channel blockers on 'dynamic' and 'steady-state step' renal autoregulation. Am J Physiol Renal Physiol 286:F1136-F1143, 2004. First published March 2, 2004; 10.1152/ajprenal.00401.2003.--Renal autoregulation (AR) mechanisms provide the primary protection against transmission of systemic pressures and hypertensive renal damage. However, the relative merits of the 'step' change vs. 'dynamic' methods for the assessment of AR capacity remain controversial. The effects of 48-72 h of orally administered amlodipine (L-type) and mibefradil (T-type) calcium channel blockers (CCBs) on step and dynamic AR in Sprague-Dawley rats were compared. Both CCBs significantly impaired 'steady-state step' AR (autoregulatory indexes = ~0.5 vs. ~0.1 in controls, P < 0.05; n = 9-10/group). By contrast, dynamic AR compensation in separate conscious rats (n = 12) was not significantly altered by either amlodipine (n = 10) or mibefradil (n = 6; fractional gain in admittance ~0.4-0.5 in all groups at frequencies in the range of 0.0025-0.025 Hz). However, both CCBs tended to attenuate the myogenic resonance peak along with shifting it to a significantly slower frequency (P < 0.001) during dynamic AR, but no consistent effects were observed on the tubuloglomerular feedback resonance peak. While the reasons for the insensitivity of dynamic vs. steady-state step AR capacity estimates to CCBs remain to be established, the present data indicate that dynamic AR methods may have a limited utility for assessing AR capacity but may provide potentially important insights into the operational characteristics of AR control mechanisms. A strong correlation was also observed between the average conductance and the admittance gain at the heart beat frequency (r = 0.77, P < 0.001), suggesting that such parameters may provide additional and possibly more meaningful indexes of BP transmission in conscious animals during dynamic AR. renal hemodynamics; hypertension; myogenic response: tubuloglomerular feedback

The determinants of bradykinin (BK)-induced afferent arteriolar vasodilation were investigated in the in vitro perfused hydronephrotic rat kidney. BK elicited a concentration-dependent vasodilation of afferent arterioles that had been preconstricted with ANG II (0.1 nmol/l), but this dilation was transient in character. Pretreatment with the nitric oxide synthase inhibitor N[omega]-nitro-L-arginine methyl ester (100 [micro]mol/l) and the cyclooxygenase inhibitor ibuprofen (10 [micro]mol/l) did not prevent this dilation when tone was established by ANG II but fully blocked the response when tone was established by elevated extracellular KCl, which suggests roles for both NO and endotheliumderived hyperpolarizing factor (EDHF). We had previously shown that the EDHF-like response of the afferent arteriole evoked by ACh was fully abolished by a combination of charybdotoxin (ChTX;10 nmol/l) and apamin (AP; 1 [micro]mol/l). However, in the current study, treatment with ChTX plus AP only reduced the EDHF-like component of the BK response from 98 [+ or -] 5 to 53 [+ or -] 6% dilation. Tetraethylammonium (TEA; 1 mmol/l), which had no effect on the EDHF-induced vasodilation associated with ACh, reduced the EDHF-like response to BK to 88 [+ or -] 3% dilation. However, the combination of TEA plus ChTX plus AP abolished the response (0.3 [+ or -] 1% dilation). Similarly, 17-octadecynoic acid (17-ODYA) did not prevent the dilation when it was administered alone (77 [+ or -] 9% dilation) but fully abolished the EDHF-like response when added in combination with ChTX plus AP (-0.5 [+ or -] 4% dilation). These findings suggest that BK acts via multiple EDHFs: one that is similar to that evoked by ACh in that it is blocked by ChTX plus AP, and a second that is blocked by either TEA or 17-ODYA. Our finding that a component of the BK response is sensitive to TEA and 17-ODYA is consistent with previous suggestions that the EDHF released by BK is an epoxyeicosatrienoic acid. arteriole; endothelium-derived hyperpolarizing factor; acetylcholine; 17-octadecynoic; epoxyeicosatrienoic acids; tetraethylammonium; charybdotoxin; apamin; potassium channels

Renal autoregulatory (AR) mechanisms provide the primary protection against transmission of systemic pressures, and their impairment is believed to be responsible for the enhanced susceptibility to hypertensive renal damage in renal mass reduction (RMR) models. Assessment of AR capacity by the 'step' change methodology under anesthesia was compared with that by 'dynamic' methods in separate conscious control Sprague-Dawley rats and after uninephrectomy (UNX) and 3/4 RMR (RK-NX) (n = 7-10/group). Substantially less AR capacity was seen by the dynamic vs. the step methodology in control rats. Moreover, dynamic AR capacity did not differ among controls, UNX, and RK-NX rats (fractional gain in admittance ~0.4-0.5 in all groups at frequencies in the range of 0.0025-0.025 Hz). By contrast, significant impairment of step AR was seen in RK-NX vs. control or UNX rats (AR indexes 0.7 [+ or -] 0.1 vs. 0.1 [+ or -] 0.02 and 0.2 [+ or -] 0.04, respectively, P < 0.01). We propose that the step and dynamic methods evaluate the renal AR responses to different components of blood pressure (BP) power with the step AR assessing the ability to buffer large changes in average BP (DC power), whereas the present 'dynamic' methods assess the AR ability to buffer slow BP fluctuations ( renal hemodynamics; hypertension; nephrosclerosis; myogenic response; tubuloglomerular feedback

Proteinase-activated receptors (PARs) are activated by either serine proteinases or synthetic peptides corresponding to the N[H.sub.2]-terminal tethered ligand sequences that are unmasked by proteolytic cleavage. Although PARs are highly expressed in the kidney, their roles in regulating renal function are not known. In the present study, we evaluated the impact of PAR activation on renal hemodynamics using PA[R.sub.1]- and PA[R.sub.2]-activating peptides (TFLLR-N[H.sub.2] and SLIGRL-N[H.sub.2]) and proteinases (thrombin and trypsin) as PAR agonists in the isolated perfused rat kidney preparation. PA[R.sub.1] activation resulted in renal vasoconstriction and a marked reduction in the glomerular filtration rate (GFR). In contrast, PA[R.sub.2] activation caused vasodilation, partially reversing the vasoconstriction induced by TFLLR-N[H.sub.2] and ANG II and increasing GFR that had been prereduced by ANG II. The vasoconstrictor actions of PA[R.sub.1] activation were abolished by protein kinase C inhibition. The PA[R.sub.2]-induced vasodilation was only partially blocked by [N.sup.G]-nitro-L-arginine methyl ester, suggesting both nitric oxide-dependent and -independent mechanisms. Although PA[R.sub.4] mRNA was detected in renal parenchyma, the PA[R.sub.4]-activating peptide AYPGKF-N[H.sub.2] had no effect on renal perfusion flow rate. We conclude that PA[R.sub.1] and PA[R.sub.2] play bidirectional roles in the regulation of renal hemodynamics. nitric oxide; trypsin; thrombin; endothelium-derived relaxing factor; glomerular filtration rate

Proteinase-activated receptors (PARs) are a novel class of G protein-coupled receptors that respond to signals through endogenous proteinases. PAR activation involves enzymatic cleavage of the extracellular N[H.sub.2]-terminal domain and unmasking of a new N[H.sub.2] terminus, which serves as an anchored ligand to activate the receptor. At least four PAR subtypes have been identified. In the present study, we used the in vitro perfused hydronephrotic rat kidney to examine the effects of activating PAR-2 on the afferent arteriole. The synthetic peptide SLIGRL-N[H.sub.2], which corresponds to the exposed ligand sequence and selectively activates PAR-2, did not alter basal afferent arteriolar diameter but caused a concentration-dependent vasodilation (3-30 [micro]M) of arterioles preconstricted by angiotensin II (0.1 nM). A modified peptide sequence (LSIGRL-N[H.sub.2], inactive at PAR-2) had no effect. This vasodilation was characterized by an initial transient component followed by a smaller sustained response. A similar pattern of vasodilation was seen when SLIGRL-N[H.sub.2] was administered to isolated perfused normal rat kidney. The sustained component of the PAR-2-induced afferent arteriolar vasodilation was eliminated by nitric oxide (NO) synthase inhibition (100 [micro]M nitro-L-arginine methyl ester). In contrast, the transient vasodilation persisted under these conditions. This transient response was not observed when afferent arterioles were preconstricted with elevated KCl, suggesting involvement of an endothelium-derived hyperpolarizing factor. Finally, RT-PCR revealed the presence of PAR-2 mRNA in isolated afferent arterioles. These findings indicate that PAR-2 is expressed in the afferent arteriole and that its activation elicits afferent arteriolar vasodilation by NO-dependent and NO-independent mechanisms. microcirculation; endothelium; proteinase; angiotensin II; potassium chloride; nitric oxide

Determinants of the renal microvascular response to ACh: afferent and efferent arteriolar actions of EDHF. Am J Physiol Renal Physiol 282: F124-F132, 2002. First published August 15, 2001; 10.1152/ajprenal.00157.2002.--The renal microvascular actions of ACh were investigated using the in vitro perfused hydronephrotic rat kidney. ACh reversed ANG II-induced vasoconstriction in the afferent and efferent arteriole by 106 [+ or -] 2 and 75 [+ or -] 5%, respectively. Inhibition of nitric oxide synthase [NOS; 100 [micro]mol/l [N.sup.G]-nitro-L-arginine methyl ester (L-NAME)] and cyclooxygenase (COX; 10 [micro]mol/l ibuprofen) prevented the sustained response of the afferent arteriole but did not reduce the magnitude of the initial dilation (97 [+ or -] 7%). However, NOS/COX inhibition abolished the response of the efferent arteriole. The underlying mechanisms mediating this endothelium-derived hyperpolarizing factor (EDHF)-like response were characterized using K channel blockers. Ba (100 [micro]mol/l), tetraethylammonium (1 mmol/l), and ouabain (3 mmol/l) had no effect, arguing against a role of an inward rectifier K channel, large-conductance Ca-activated K channel, or Na,K-ATPase. Charybdotoxin (10 nmol/l) and apamin (1.0 [micro]mol/l) attenuated the response when administered alone (63 [+ or -] 7% and 37 [+ or -] 5%, respectively) and abolished the response when coadministered (0.1 [+ or -] 1.0%). These findings indicate that, as in other vascular beds, the renal EDHF-like response to ACh involves K channels that are sensitive to a combination of apamin and charybdotoxin. Our finding that EDHF modulates preglomerular, but not postglomerular, tone is consistent with the evolving concept that vasomotor mechanisms in cortical efferent arterioles do not involve voltage-gated Ca entry. calcium-activated potassium channels; voltage-gated potassium channels; inward rectitier potassium channels; tetraethylammonium; apamin; charybdotoxin; 4-aminopyridine; barium

Research was conducted to examine the hypothesis that alterations in basal protein kinase C activity modulate renal afferent arteriolar myogenic reactivity. The role of protein kinase C (PKC) in the regulation of myogenic response was investigated using the in vitro perfused hydronephrotic rat kidney. Results indicate that basal PKC myogenic activity regulates voltage-gated K channel activity which has an indirect effect on myogenic reactivity.

Isolated, perfused hydronephrotic rat kidneys were used in the investigation of the dynamic autoregulation of hemodynamic parameters. These kidneys still exhibit afferent arteriolar vasoconstriction despite the absence of glomerulo-tubular feedback. Results reveal that myogenic autoregulation is responsible for the hemodynamic responses observed in hydronephrotic kidneys which lack the glomerulo-tubular feedback mechanism of autoregulation.

An experiment was undertaken to determine the vasodilatory effect of calcitonin gene-related peptide (CGRP) through the activation of the ATP-sensitive potassium channels (KATP) in the renal microcirculation mechanism. The mechanism of CGRP was compared during pressure- and angiotensin II-induced vasoconstriction. Results revealed that glibenclamide weakens the effects of CGRP during myogenic constriction. The activation of KATP complemented the vasodilatory actions of CGRP during pressure-produced vasoconstriction.

The effect of angiotensin (ANG) II on the membrane potential measurements in renal afferent and efferent arterioles was examined using the in vitro perfused hydronephrotic rat kidney model. Results of the analysis revealed that ANG II-induced vasoconstriction was closely coupled with membrane depolarization in the afferent arteriole. While electrical mechanical responses were apparently dissociated in the efferent arteriole.

Tang, Lilong, Michael Parker, Qing Fei, and Rodger Loutzenhiser. Afferent arteriolar adenosine [A.sub.2a] receptors are coupled to [K.sub.ATP] in in vitro perfused hydronephrotic rat kidney. Am. J. Physiol. 277 (Renal Physiol. 46): F926-F933, 1999.--Adenosine is known to exert dual actions on the afferent arteriole, eliciting vasoconstriction, by activating A1 receptors, and vasodilation at higher concentrations, by activating lower-affinity [A.sub.2] receptors. We could demonstrate both of these known adenosine responses in the in vitro perfused hydronephrotic rat kidney. Thus, 1.0 [micro]M adenosine elicited a transient vasoconstriction blocked by 8-cyclopentyl-1,3-dipropylxanthine (DPCPX), and 10-30 [micro]M adenosine reversed KCl-induced vasoconstriction. However, when we examined the effects of adenosine on pressure-induced afferent arteriolar vasoconstriction, we observed a third action. In this setting, a high-affinity adenosine vasodilatory response was observed at concentrations of 10-300 [micro]M. This response was blocked by both 4-(2-[7-amino-2-(2-furyl)[1,2,4]triazolo[2,3-a] [1,3,5]triazin-5-yl-amino]ethyl)phenol (ZM-241385) and glibenclamide and was mimicked by 2-phenylaminoadenosine (CV-1808) ([IC.sub.50] of 100 nM), implicating adenosine [A.sub.2a] receptors coupled to ATP-sensitive K channels ([K.sub.ATP]). Like adenosine, 5'-N-ethylcarboxamidoadenosine (NECA) elicited both glibenclamide-sensitive and glibenclamide-insensitive vasodilatory responses. The order of potency for the glibenclamidesensitive component was NECA [is greater than] adenosine = CV-1808. Our findings suggest that, in addition to the previously described adenosine [A.sub.1] and low-affinity [A.sub.2b] receptors, the renal microvasculature is also capable of expressing high-affinity adenosine [A.sub.2a] receptors. This renal adenosine receptor elicits afferent arteriolar vasodilation at submicromolar adenosine levels by activating [K.sub.ATP]. ATP-sensitive potassium channels; glibenclamide; CV-1808; 5'-N-ethylcarboxamidoadenosine; ZM-241385; renal microcirculation; afferent arteriole; hydronephrosis; myogenic vasoconstriction; adenosine 3',5'-cyclic monophosphate; Ro-20-1724