Biomedical subjects

E W Inscho

Publications and source records attributed to E W Inscho.

At least 19 recordsLinked to original sources

PubMed2001

Renal microvascular effects of P2 receptor stimulation.

1. The field of extracellular nucleotides and purinoceptors has undergone a resurgence of interest and enthusiasm in the past decade. More and more investigators are probing the physiological and pathophysiological roles of P2 receptors in virtually every organ system, including the kidney. 2. With this renewed interest has come a new appreciation for the roles extracellular adenine nucleotides can play in regulating or modulating renal function. In the past 5 years, investigators have provided compelling evidence that extracellular nucleotides, working through activation of P2 purinoceptors, have a significant impact on renal microvascular function, mesangial cell function and on renal epithelial transport. 3. Evidence has been uncovered that implicates P2 receptor activation in mediating renal microvascular autoregulatory behaviour. Locally released ATP has a direct paracrine and/or autocrine effect modulating renal epithelial transporters and tubular epithelial channels to influence tubular fluid composition. 4. While the specific roles of extracellular nucleotides and their receptors in the kidney have not been absolutely identified, it now appears clear that endogenously released ATP may play a significant role in regulating kidney function. 5. The purpose of the present review is to update our current understanding of the effect of P2 receptor activation on renal microvascular function and to detail the signal transduction mechanisms known to be involved.

Adenosine Triphosphate↗

PubMed2001

The CYP450 hydroxylase pathway contributes to P2X receptor-mediated afferent arteriolar vasoconstriction.

This study was conducted to test the hypothesis that the cytochrome P-450 (CYP450) metabolite 20-hydroxyeicosatetraenoic acid (20-HETE) contributes to the afferent arteriolar response to P2 receptor activation. Afferent arteriolar responses to ATP, the P2X agonist, alpha,beta-methylene ATP and the P2Y agonist UTP were determined before and after treatment with the selective CYP450 hydroxylase inhibitor, N-methylsulfonyl-12,12-dibromododec-11-enamide (DDMS) or the 20-HETE antagonist, 20-hydroxyeicosa-6(Z),15(Z)-dienoic acid (20-HEDE). Stimulation with 1.0 and 10 microM ATP elicited an initial preglomerular vasoconstriction of 12 +/- 1% and 45 +/- 4% and a sustained vasoconstriction of 11 +/- 1% and 11 +/- 2%, respectively. DDMS or 20-HEDE significantly attenuated the sustained afferent arteriolar constrictor response to ATP. alpha,beta-Methylene ATP (1 microM) induced a rapid initial afferent vasoconstriction of 64 +/- 3%, which partially recovered to a stable diameter 10 +/- 1% smaller than control. Both DDMS and 20-HEDE significantly attenuated the initial vasoconstriction and abolished the sustained vasoconstrictor response to alpha,beta-methylene ATP. UTP decreased afferent diameter by 50 +/- 5% and 20-HEDE did not change this response. In addition, the ATP-induced increase in the intracellular Ca2+ concentration in preglomerular microvascular smooth muscle cells was significantly attenuated by 20-HEDE. Taken together, these results are consistent with the hypothesis that the CYP450 metabolite 20-HETE participates in the afferent arteriolar response to activation of P2X receptors.

Adenosine Triphosphate↗

PubMed2001

Interactions of adenosine A1 and A2a receptors on renal microvascular reactivity.

Adenosine vasoconstricts preglomerular arterioles via adenosine A1 receptors. Because adenosine also activates adenosine A2 receptors, its overall renal vascular actions are complex and not fully understood. The present study was performed to determine the relative contributions of adenosine A1 and A2a receptors to the responsiveness of the renal microvasculature to adenosine. Afferent and efferent arteriolar diameters were monitored in vitro using the blood-perfused rat juxtamedullary nephron preparation. Basal afferent and efferent arteriolar diameters averaged 17.1 +/- 0.5 (n = 35) and 17.8 +/- 0.5 (n = 20) microm, respectively. Superfusion with 0.1 and 1 micromol/l adenosine did not significantly alter afferent and efferent arteriolar diameters; however, 10 micromol/l adenosine significantly reduced afferent and efferent arteriolar diameters (-8.2 +/- 0.8 and -5.7 +/- 0.6%, respectively). The afferent and efferent arteriolar vasoconstrictor responses to adenosine waned at a dose of 100 micromol/l, such that diameters returned to values not significantly different from control within 2 min. During adenosine A1 receptor blockade with 8-noradamantan-3-yl-1,3-dipropylxanthine (KW-3902: 10 micromol/l), 10 and 100 micromol/l adenosine significantly increased afferent diameter by, respectively, 8.1 +/- 1.2 and 13.7 +/- 1.3% (n = 14) and efferent arteriolar diameter by 6.4 +/- 1.3 and 9.3 +/- 1.2% (n = 8). The afferent and efferent arteriolar vasodilatory responses to adenosine in the presence of KW-3902 were significantly attenuated by addition of the adenosine A2a receptor antagonist 1,3-dipropyl-7-methyl-8-(3,4-dimethoxystyryl)xanthine (KF-17837: 15 micromol/l, n = 7 and 6, respectively). The addition of KF-17837 alone significantly enhanced afferent (n = 15) and efferent (n = 6) arteriolar vasoconstrictor responses to 1, 10, and 100 micromol/l adenosine. These results indicate the presence of adenosine A1 and A2a receptors on afferent and efferent arterioles of juxtamedullary nephrons, such that adenosine A2a receptor-mediated vasodilation partially buffers adenosine-induced vasoconstriction in both pre- and postglomerular segments of the renal microvasculature.

Adenosine↗

PubMed2001

Calcium signaling pathways utilized by P2X receptors in freshly isolated preglomerular MVSMC.

This study tested the hypothesis that P2X receptor activation increases intracellular Ca(2+) concentration ([Ca(2+)](i)) in preglomerular microvascular smooth muscle cells (MVSMC) by evoking voltage-dependent calcium influx. MVSMC were obtained and loaded with the calcium-sensitive dye fura 2 and studied by using single-cell fluorescence microscopy. The effect of P2X receptor activation on [Ca(2+)](i) was assessed by using the P2X receptor-selective agonist alpha,beta-methylene-ATP and was compared with responses elicited by the endogenous P2 receptor agonist ATP. alpha,beta-Methylene-ATP increased [Ca(2+)](i) dose dependently. Peak increases in [Ca(2+)](i) averaged 37 +/- 11, 73 +/- 15, and 103 +/- 21 nM at agonist concentrations of 0.1, 1, and 10 microM, respectively. The average peak response elicited by 10 microM alpha,beta-methylene-ATP was approximately 34% of the response obtained with 10 microM ATP. alpha,beta-Methylene-ATP induced a transient increase in [Ca(2+)](i) before [Ca(2+)](i) returned to baseline, whereas ATP induced a biphasic response including a peak response followed by a sustained plateau. In Ca(2+)-free medium, ATP induced a sharp transient increase in [Ca(2+)](i), whereas the response to alpha,beta-methylene-ATP was abolished. Ca(2+) channel blockade with 10 microM diltiazem or nifedipine attenuated the response to alpha,beta-methylene-ATP, whereas nonspecific blockade of Ca(2+) influx pathways with 5 mM Ni(2+) abolished the response. Blockade of P2X receptors with the novel P2X receptor antagonist NF-279 completely but reversibly abolished the response to alpha,beta-methylene-ATP. These results indicate that P2X receptor activation by alpha,beta-methylene-ATP increases [Ca(2+)](i) in preglomerular MVSMC, in part, by stimulating voltage-dependent Ca(2+) influx through L-type Ca(2+) channels.

Adenosine Triphosphate↗

PubMed2001

P2 receptors in regulation of renal microvascular function.

In the last 10-15 years, interest in the physiological role of P2 receptors has grown rapidly. Cellular, tissue, and organ responses to P2 receptor activation have been described in numerous in vivo and in vitro models. The purpose of this review is to provide an update of the recent advances made in determining the involvement of P2 receptors in the control of renal hemodynamics and the renal microcirculation. Special attention will be paid to work published in the last 5-6 years directed at understanding the role of P2 receptors in the physiological control of renal microvascular function. Several investigators have begun to evaluate the effects of P2 receptor activation on renal microvascular function across several species. In vivo and in vitro evidence consistently supports the hypothesis that P2 receptor activation by locally released extracellular nucleotides influences microvascular function. Extracellular nucleotides selectively influence preglomerular resistance without having an effect on postglomerular tone. P2 receptor inactivation blocks autoregulatory behavior whereas responsiveness to other vasoconstrictor agonists is retained. P2 receptor stimulation activates multiple intracellular signal transduction pathways in preglomerular smooth muscle cells and mesangial cells. Renal microvascular cells and mesangial cells express multiple subtypes of P2 receptors; however, the specific role each plays in regulating vascular and mesangial cell function remains unclear. Accordingly, the results of studies performed to date provide strong support for the hypothesis that P2 receptors are important contributors to the physiological regulation of renal microvascular and/or glomerular function.

Animals↗

PubMed2000

Postglomerular vasoconstriction to angiotensin II and norepinephrine depends on intracellular calcium release.

The current study was performed to determine the effect of calcium store depletion with cyclopiazonic acid (CPA) on the pre- and postglomerular vasoconstrictor responses to angiotensin II (ANG II) and norepinephrine (NE). CPA treatment significantly attenuated the afferent arteriolar response to 10 nM ANG II by 51% and to 1000 nM NE by 19%. Efferent arteriolar responses to ANG II and NE were also greatly attenuated in the presence of CPA. These data demonstrate that afferent and efferent arteriolar responses to ANG II and NE depend on release of calcium from CPA-sensitive intracellular stores. Furthermore, the postglomerular response to these agents exhibits a greater dependency on calcium release from intracellular stores.

Angiotensin II↗

PubMed2000

Endothelin-mediated calcium signaling in preglomerular smooth muscle cells.

This study was performed to test the hypothesis that endothelin peptides differentially influence intracellular calcium concentration ([Ca(2+)](i)) in preglomerular microvascular smooth muscle cells (MVSMC), in part through activation of endothelin (ET)(A) receptors. Experiments were performed in vitro with the use of single MVSMC freshly isolated from rat preglomerular microvessels. The effect of ET-1, ET-2, and ET-3 on [Ca(2+)](i) was measured with the use of the calcium-sensitive dye, fura 2, and standard fluorescence microscopy techniques. Baseline [Ca(2+)](i) averaged 84+/-3 nmol/L (n=141 cells from 23 dispersions). ET-1 concentrations of 1, 10, and 100 nmol/L evoked peak increases in [Ca(2+)](i) of 48+/-16, 930+/-125, and 810+/-130 nmol/L, respectively. The time course of the [Ca(2+)](i) response was biphasic, beginning with a rapid initial increase followed by a sustained plateau phase or a period during which [Ca(2+)](i) oscillated sharply. Similar responses were observed after ET-2 administration. In contrast, ET-3 stimulated monophasic increases in [Ca(2+)](i) of only 14+/-5, 33+/-16, and 44+/-19 nmol/L at peptide concentrations of 1, 10, and 100 nmol/L, respectively. These responses are significantly smaller than responses to ET-1 or ET-2, respectively. The relative contributions of calcium mobilization and calcium influx in the response to ET-1 were also evaluated. Removal of calcium from the bathing medium did not significantly alter the peak response to 10 nmol/L ET-1 but abolished the late phase elevation of [Ca(2+)](i). These data demonstrate that endothelin peptides increase [Ca(2+)](i) in preglomerular MVSMC. The concentration-response profiles are consistent with the response involving activation of ET(A) receptors. Furthermore, these results suggest that ET-1 increases [Ca(2+)](i) by stimulating both the release of intracellular calcium and the influx of calcium from the extracellular medium.

Animals↗

PubMed2000

Cytochrome P450 and cyclooxygenase metabolites contribute to the endothelin-1 afferent arteriolar vasoconstrictor and calcium responses.

Arachidonic acid metabolites contribute to the endothelin-1 (ET-1)-induced decrease in renal blood flow, but the vascular sites of action are unknown. Experiments performed in vitro used the rat juxtamedullary nephron preparation combined with videomicroscopy. The response of afferent arterioles to ET-1 was determined before and after cytochrome P450 (CYP450) or cyclooxygenase (COX) inhibition. Afferent arteriolar diameter averaged 20+/-1 microm (n=17) at a renal perfusion pressure of 100 mm Hg. Superfusion with 0.001 to 10 nmol/L ET-1 caused a graded decrease in diameter of the afferent arteriole. Vessel diameter decreased by 30+/-2% and 41+/-2% in response to 1 and 10 nmol/L ET-1, respectively. The afferent arteriolar response to ET-1 was significantly attenuated during administration of the CYP450 hydroxylase inhibitor N-methylsulfonyl-12,12-dibromododec-11-enamide (DDMS), such that afferent arteriolar diameter decreased by 19+/-3% and 22+/-3% in response to 1 and 10 nmol/L ET-1, respectively. COX inhibition also greatly attenuated the vasoconstriction elicited by ET-1, whereas the CYP450 epoxygenase inhibitor N-methylsulfonyl-6-(2-proparglyoxyphenyl) hexanamide enhanced the ET-1-mediated vascular response. Additional studies were performed using freshly isolated smooth muscle cells prepared from preglomerular microvessels. Renal microvascular smooth muscle cells were loaded with the calcium-sensitive dye fura 2 and studied by use of single-cell fluorescence microscopy. Basal renal microvascular smooth muscle cell [Ca(2+)](i) averaged 95+/-3 nmol/L (n=42). ET-1 (10 nmol/L) increased microvascular smooth muscle cell [Ca(2+)](i) to a peak value of 731+/-75 nmol/L before stabilizing at 136+/-8 nmol/L. Administration of DDMS or the COX inhibitor indomethacin significantly attenuated the renal microvascular smooth muscle cell calcium response to ET-1. These data demonstrate that CYP450 hydroxylase and COX arachidonic acid metabolites contribute importantly to the afferent arteriolar diameter and renal microvascular smooth muscle cell calcium responses elicited by ET-1.

Amides↗

PubMed1999

Extracellular uridine nucleotides initiate cytokine production by murine dendritic cells.

While it is recognized that activated dendritic cells perform their immune functions with greater efficacy, it is not altogether clear what factors are responsible for such activation. Recent evidence points to an important role for extracellular nucleotides in the modulation of leukocyte function. In the present study we investigated the ability of extracellular nucleotides to activate CD11c(+) murine dendritic cells. Mobilization of intracellular calcium was observed following treatment of these cells with UTP or UDP, but not ATP. Furthermore, this nucleotide receptor was pertussis toxin-sensitive, suggesting the presence of a P2Y nucleotide receptor. Such receptors were not present on murine peritoneal macrophages or on CD11c-negative leukocyte populations. Importantly, activation of these P2Y nucleotide receptors on dendritic cells provided a potent stimulus for cytokine mRNA expression and secretion. Thus, expression of a P2Y nucleotide receptor on CD11c(+) dendritic cells functions to mobilize intracellular calcium and to induce cytokine production.

Adenine Nucleotides↗

PubMed1999

Contribution of cytochrome P450 epoxygenase and hydroxylase pathways to afferent arteriolar autoregulatory responsiveness.

Previous studies have demonstrated an important role for the cytochrome P450 (CYT-P450) pathway in afferent arteriole autoregulatory responses but the involvement of specific pathways remains unknown. Experiments were performed to determine the role of CYT-P450 epoxygenase and hydroxylase pathways in pressure mediated preglomerular autoregulatory responses. Afferent arteriolar diameter was measured as renal perfusion pressure was increased from 80-160 mmHg. Afferent arteriolar diameter averaged 19+/-2 microm at a renal perfusion pressure of 80 mmHg and decreased by 15+/-2% when pressure was increased to 160 mmHg. Inhibition of the epoxygenase pathway with 6-(2-proparglyloxyphenyl)hexanoic acid (PPOH), enhanced the microvascular response to increasing renal perfusion pressure. In the presence of 50 microM PPOH, afferent arteriolar diameter decreased by 29+/-4% when pressure was increased from 80-160 mmHg. Likewise, the sulphonimide derivative of PPOH, N-methylsulphonyl-6-(2-proparglyloxyphenyl) hexanamide (MS-PPOH, 50 microM), enhanced the afferent arteriolar response to increasing renal perfusion pressure. In contrast, the selective CYT-P450 hydroxylase inhibitor, N-methylsulphonyl-12,12-dibromododec-11-enamide (DDMS) attenuated the vascular response to increasing renal perfusion pressure. In the pressure of 25 microM DDMS, afferent arteriolar diameter decreased by 4+/-2% when pressure was increased from 80-160 mmHg. These results suggest that CYT-P450 metabolites of the epoxygenase pathway alter afferent arteriolar responsiveness and thereby modify the ability of the preglomerular vasculature to autoregulate renal blood flow. Additionally, these results provide further support to the concept that a metabolite of the hydroxylase pathway is an integral component of the afferent arteriolar response to elevations in perfusion pressure.

Amides↗

PubMed1999

ATP-mediated Ca2+ signaling in preglomerular smooth muscle cells.

We performed studies to determine the effect of extracellular ATP on the intracellular Ca2+ concentration ([Ca2+]i) in freshly isolated microvascular smooth muscle cells (MVSMC). Suspensions of preglomerular MVSMC were prepared by enzymatic digestion and loaded with fura 2. Single cells were studied using a microscope-based fluorescence spectrophotometer during superfusion of a physiological salt solution with 1.8 mM Ca2+ and during exposure to similar solutions containing ATP. Under control conditions, baseline [Ca2+]i averaged 107 +/- 6 nM (n = 86 cells from 34 animals). ATP administration elicited concentration-dependent increases in [Ca2+]i. Exposure to ATP concentrations of 1, 10, and 100 microM increased intracellular Ca2+ to peak concentrations of 133 +/- 20, 338 +/- 37, and 367 +/- 35 nM, respectively (P < 0.05 vs. respective baseline). Steady-state [Ca2+]i increased to 113 +/- 15, 150 +/- 16 (P < 0.05 vs. baseline), and 180 +/- 12 nM (P < 0.05 vs. baseline) for the same groups. The [Ca2+]i response to ATP was also assessed in the absence of extracellular Ca2+ and during blockade of L-type Ca2+ channels with diltiazem. In these studies, exposure to 100 microM ATP induced a transient peak increase in [Ca2+]i with the plateau phase being totally abolished under Ca2+-free conditions and markedly attenuated during Ca2+ channel blockade, respectively. These data indicate that ATP-mediated P2-receptor activation increases [Ca2+]i in freshly isolated preglomerular MVSMC by stimulating Ca2+ release from intracellular stores, in addition to stimulating the influx of extracellular Ca2+ through voltage-gated L-type Ca2+ channels.

Adenosine Triphosphate↗

PubMed1999

Purinoceptor-mediated calcium signaling in preglomerular smooth muscle cells.

-The current studies were performed to determine the contribution of calcium mobilization and voltage-dependent calcium influx to the increase in [Ca2+]i elicited by ATP and UTP. Suspensions of freshly isolated smooth muscle cells were prepared from preglomerular microvessels by enzymatic digestion and loaded with the Ca2+-sensitive dye fura 2. The effect of ATP and UTP on [Ca2+]i was studied on single cells with standard microscope-based fluorescence photometry techniques. Resting [Ca2+]i averaged 80+/-3 nmol/L (n=219 single cells from 58 dispersions). ATP (100 micromol/L) increased [Ca2+]i to a peak value of 845+/-55 nmol/L (n=70 single cells from 38 dispersions) before stabilizing at 124+/-81 nmol/L. Similarly, 100 micromol/L UTP (n=39 single cells from 26 dispersions) stimulated a peak increase in [Ca2+]i of 1426+/-584 nmol/L before reaching a stable plateau of 123+/-10 nmol/L. The [Ca2+]i response to ATP and UTP was also assessed in the absence of extracellular calcium. In these studies, exposure to 100 micromol/L ATP induced a transient peak increase in [Ca2+]i, with the plateau phase being totally abolished. In contrast, exposure to 100 micromol/L UTP under calcium-free conditions resulted in no detectable change in the UTP-mediated increase in [Ca2+]i. The role of L-type calcium channels in the response was assessed with the calcium channel antagonist diltiazem. Incubation with diltiazem (10 micromol/L) markedly reduced the response to ATP, whereas the response to UTP was only slightly reduced. These data demonstrate that both ATP and UTP directly stimulate a biphasic increase in [Ca2+]i in renal microvascular smooth muscle cells. Furthermore, the data suggest that the elevation of [Ca2+]i elicited by ATP is largely dependent on calcium influx through L-type calcium channels, whereas the response to UTP appears to derive primarily from mobilization of calcium from intracellular stores.

Adenosine Triphosphate↗

PubMed1999

Afferent arteriolar vasodilation to the sulfonimide analog of 11, 12-epoxyeicosatrienoic acid involves protein kinase A.

The current study determined the contribution of protein kinase-A (PKA) and protein kinase-G (PKG) to the vasodilation elicited by the N-methylsulfonimide analog of 11,12-epoxyeicosatrienoic acid (11, 12-EET). Experiments were performed, in vitro, using the juxtamedullary nephron preparation combined with videomicroscopy. The response of afferent arterioles to the sulfonimide analog of 11, 12-EET, was determined before and after inhibition of PKA, PKG, or guanylyl cyclase. Afferent arterioles, preconstricted with 0.5 micromol/L norepinephrine, averaged 18+/-1 microm (n=25) at a renal perfusion pressure of 100 mm Hg. Superfusion with 0.01 to 100 nmol/L of the 11,12-EET analog caused a graded increase in diameter of the afferent arteriole. Vessel diameter increased by 11+/-1% and 15+/-1%, respectively, in response to 10 and 100 nmol/L of the 11,12-EET analog. The afferent arteriolar response to 10 and 100 nmol/L of the 11,12-EET analog was significantly attenuated during inhibition of PKA with 10 micromol/L H-89 (n=7) or 5 micromol/L myristolated PKI (n=6), such that afferent arteriolar diameter increased by only 5+/-2% and 2+/-1%, respectively, in response to 100 nmol/L of the 11, 12-EET analog. In contrast, the afferent arteriolar vasodilatory response to the 11,12-EET analog was unaffected by PKG or guanylyl cyclase inhibition. In the presence of 200 micromol/L histone H2B (n=5) or 10 micromol/L ODQ (n=7), the afferent arteriolar diameter increased by 16+/-3% and 12+/-2%, respectively, in response to 100 nmol/L of the 11,12-EET analog. These results demonstrate that activation of PKA is an important mechanism responsible for the afferent arteriolar vasodilation elicited by the sulfonimide analog of 11,12-EET.

8,11,14-Eicosatrienoic Acid↗

PubMed1999

P2 purinoceptor saturation by adenosine triphosphate impairs renal autoregulation in dogs.

Recent studies have suggested a role for P2 purinoceptors on vascular smooth muscle cells in the mechanism of renal autoregulation. Experiments were performed in anesthetized dogs (n = 9) to examine renal blood flow (RBF) autoregulatory efficiency before and after saturation of P2 purinoceptors with acute intra-arterial administration of ATP (1 mg/kg per min). Dogs were pretreated with the nitric oxide synthase inhibitor nitro-L-arginine (NLA) (50 microg/kg per min), to avoid endothelial P2 receptor-mediated effects on nitric oxide release caused by the intra-arterial ATP infusions. NLA treatment decreased RBF (5.3+/-0.3 to 3.6+/-0.2 ml/min per g) and sodium excretion (3.6+/-0.4 to 0.9+/-0.2 ml/min per g) without producing significant changes in GFR (0.92+/-0.04 to 0.90+/-0.06 ml/min per g) or RBF autoregulatory efficiency. ATP administration to NLA-treated dogs resulted in further decreases in RBF (2.8+/-0.2 ml/min per g), GFR (0.58+/-0.05 ml/min per g), and sodium excretion (0.6+/-0.2 micromol/min per g). In addition, there was marked impairment of RBF autoregulatory efficiency during ATP infusion. The slopes of the arterial pressure-blood flow relationships at renal arterial pressures of >75 mmHg were significantly altered, from 0.003+/-0.001 to 0.2+/-0.002 ml/min per g per mmHg. Discontinuation of ATP infusion restored RBF autoregulatory efficiency. Norepinephrine (5 microg/kg per min) administration in these NLA-treated dogs decreased RBF (2.5+/-0.3 ml/min per g; n = 4) to a similar extent, compared with ATP, but did not impair RBF autoregulation. These results support the hypothesis that P2 purinoceptors may be involved in mediating autoregulatory adjustments in renal vascular resistance.

Adenosine Triphosphate↗

PubMed1999

Candesartan cilexetil protects against loss of autoregulatory efficiency in angiotensin II-infused rats.

Renal autoregulatory efficiency is compromised in angiotensin-II (AngII)-dependent Goldblatt hypertension. The current studies were performed to assess renal autoregulatory capability in AngII-infused hypertensive rats and to determine the effect of chronic candesartan cilexetil treatment on autoregulatory behavior. Rats received chronic infusion of AngII (60 ng/min) or vehicle via an osmotic minipump implanted subcutaneously in the dorsum of the neck. Selected rats received the novel AT1 receptor blocker candesartan cilexetil (1.0 mg/kg per d) in the drinking water. Systolic BP averaged 118+/-1 mmHg (n=34) before pump implantation. Chronic AngII infusion for 6 d increased arterial pressure to 151+/-4 mmHg. Candesartan cilexetil administration prevented the AngII-dependent increase in systolic BP. Microvascular autoregulation experiments were performed in vitro using the blood-perfused juxtamedullary nephron technique combined with videomicroscopy. Renal perfusion pressure was set at 100 mmHg during the control period before being decreased to 65 mmHg. Afferent arteriolar diameter was measured continuously as the perfusion pressure was increased from 65 mmHg to 170 mmHg in 15-mmHg increments. Afferent arteriolar diameter in sham-treated rats was 120% of control at a perfusion pressure of 65 mmHg and decreased to 76% of the control diameter at 170 mmHg (n=6). This behavior is consistent with normal autoregulatory behavior. Arterioles from rats receiving chronic infusion of AngII exhibited compromised renal microvascular autoregulatory efficiency. Afferent arteriolar diameter in AngII-treated kidneys varied from 103 to 100% (n=6) of the control diameter over the same pressure range of 65 to 170 mmHg. This blunting of autoregulatory behavior was prevented by AT1 receptor blockade. In animals receiving AngII + candesartan cilexetil, stepwise changes in perfusion pressure elicited changes in afferent arteriolar diameter between 120 and 84% after 6 d of treatment (n=6). These data suggest that chronic elevations in circulating AngII and/or the associated increase in arterial pressure impairs renal autoregulatory capability. Furthermore, inhibition of AT1 receptors with candesartan cilexetil provides protection against AngII-mediated increases in arterial pressure and prevents the associated deterioration of renal autoregulatory responsiveness.

Angiotensin II↗

PubMed1998

Heterogeneous activation mechanisms in the renal microvasculature.

Vascular smooth muscle cells in different renal microvascular segments utilize different activation mechanisms to respond to mechanical and vasoactive stimuli. L-type Ca2+ channel blockers vasodilate primarily the preglomerular vascular resistance component responsible for autoregulation. Local interstitial infiltration of Ca2+ channel blockers increases glomerular pressure and markedly reduces vascular responsiveness of the tubuloglomerular feedback mechanism. Ca2+ channel blockers selectively attenuate the afferent vasoconstrictor responses to increases in perfusion pressure. Although both afferent and efferent arterioles constrict in response to angiotensin II (Ang II), afferent but not efferent constriction requires Ca2+ influx through L-type Ca2+ channels. Sensitivity of the preglomerular arterioles to Ang II is also heterogeneous with the greatest sensitivity in glomerulus-near, terminal segments. Adenosine triphosphate (ATP) is a vasoconstrictor agonist that selectively activates Ca2+ entry pathways in afferent arterioles but has no effect on efferent arterioles. In isolated preglomerular smooth muscle cells, increasing extracellular [KCl] increases intracellular Ca2+ by stimulating voltage-dependent Ca2+ influx. Ang II, norepinephrine, and ATP also elicit similar increases in intracellular Ca2+. Mechanical and agonist-induced voltage-dependent Ca2+ influx is thus a primary pathway in the control of cytosolic Ca2+ in afferent arterioles. Efferent arterioles, however, rely primarily on intracellular Ca2+ mobilization and other Ca2+ influx pathways.

Animals↗

PubMed1998

Neuronal nitric oxide synthase modulates rat renal microvascular function.

This study was performed to determine the influence of neuronal nitric oxide synthase (nNOS) on renal arteriolar tone under conditions of normal, interrupted, and increased volume delivery to the macula densa segment and on the microvascular responses to angiotensin II (ANG II). Experiments were performed in vitro on afferent (21.2 +/- 0.2 microns) and efferent (18.5 +/- 0.2 microns) arterioles of kidneys harvested from male Sprague-Dawley rats, using the blood-perfused juxtamedullary nephron technique. Superfusion with the specific nNOS inhibitor, S-methyl-L-thiocitrulline (L-SMTC), decreased afferent and efferent arteriolar diameters, and these decreases in arteriolar diameters were prevented by interruption of distal volume delivery by papillectomy. When 10 mM acetazolamide was added to the blood perfusate to increase volume delivery to the macula densa segment, afferent arteriolar vasoconstrictor responses to L-SMTC were enhanced, but this effect was again completely prevented after papillectomy. In contrast, the arteriolar diameter responses to the nonselective NOS inhibitor, N omega-nitro-L-arginine (L-NNA) were only attenuated by papillectomy. L-SMTC (10 microM) enhanced the efferent arteriolar vasoconstrictor response to ANG II but did not alter the afferent arteriolar vasoconstrictor responsiveness to ANG II. In contrast, L-NNA (100 microM) enhanced both afferent and efferent arteriolar vasoconstrictor responses to ANG II. These results indicate that the modulating influence of nNOS on afferent arteriolar tone of juxtamedullary nephrons is dependent on distal tubular fluid flow. Furthermore, nNOS exerts a differential modulatory action on the juxtamedullary micro-vasculature by enhancing efferent, but not afferent, arteriolar responsiveness to ANG II.

Acetazolamide↗

PubMed1998

Direct assessment of renal microvascular responses to P2-purinoceptor agonists.

Studies were performed to determine the responsiveness of rat juxtamedullary afferent arterioles to receptor-selective P2-purinoceptor agonists. Experiments were performed in vitro using the blood perfused juxtamedullary nephron technique, combined with videomicroscopy. Renal perfusion pressure was set at 110 mmHg and held constant. Basal afferent arteriolar diameter averaged 22.0 +/- 0.6 microns (n = 69). Stimulation with 0.1, 1.0, 10, and 100 microM ATP (n = 10) elicited a concentration-dependent vasoconstriction averaging 8 +/- 2, 17 +/- 2, 21 +/- 4, and 23 +/- 5%, respectively. A nearly identical afferent arteriolar vasoconstriction was observed in response to the P2X-selective agonist beta,gamma-methylene ATP (n = 10); however, another P2X agonist, alpha,beta-methylene ATP, evoked marked receptor desensitization (n = 10). Vessel diameter decreased by approximately 7 +/- 2, 16 +/- 2, 23 +/- 3, and 22 +/- 3%, respectively, over the same concentration range. The P2Y-selective agonist, 2-methylthio-ATP, evoked only a modest vasoconstriction, whereas UTP and adenosine 5'-O-(3-thiotriphosphate) (ATP gamma S) reduced afferent diameter markedly at concentrations > 1.0 microM. Afferent arteriolar diameter decreased by 5 +/- 4, 31 +/- 8, and 72 +/- 8% during UTP administration (n = 7) at concentrations of 1.0, 10, and 100 microM, respectively. Similarly, ATP gamma S (n = 6) decreased afferent diameter by 16 +/- 2, 58 +/- 8, and 98 +/- 3%, respectively, over the same concentration range. Nitric oxide synthesis inhibition with N omega-nitro-L-arginine did not significantly alter the afferent arteriolar response to ATP but did potentiate ATP-mediated arcuate artery vasoconstriction. The following data suggest the presence of multiple P2 receptors on juxtamedullary afferent arterioles and are consistent with classification of those receptors as members of the P2X- and P2Y2 (P2U)-receptor subtypes.

Adenosine Monophosphate↗