Calcium signaling mechanisms in renal vascular responses to vasopressin in genetic hypertension

Impaired P2X signalling pathways in renal microvascular myocytes in genetic hypertension

AIMS

P2X receptors (P2XRs) mediate sympathetic control and autoregulation of renal circulation triggering preglomerular vasoconstriction, which protects glomeruli from elevated pressures. Although previous studies established a casual link between glomerular susceptibility to hypertensive injury and decreased preglomerular vascular reactivity to P2XR activation, the mechanisms of attenuation of the P2XR signalling in hypertension remained unknown. We aimed to analyse molecular mechanisms of the impairment of P2XR signalling in renal vascular smooth muscle cells (RVSMCs) in genetic hypertension.

METHODS AND RESULTS

We compared the expression of pertinent genes and P2XR-linked Ca(2+) entry and Ca(2+) release mechanisms in RVSMCs of spontaneously hypertensive rats (SHRs) and their normotensive controls, Wistar Kyoto (WKY) rats. We found that, in SHR RVSMCs, P2XR-linked Ca(2+) entry and Ca(2+) release from the sarcoplasmic reticulum (SR) are both significantly reduced. The former is due to down-regulation of the P2X1 subunit. The latter is caused by a decrease of the SR Ca(2+) load. The SR Ca(2+) load reduction is caused by attenuated Ca(2+) uptake via down-regulated sarco-/endoplasmic reticulum Ca(2+)-ATPase 2b and elevated Ca(2+) leak from the SR via ryanodine receptors (RyRs) and inositol 1,4,5-trisphosphate receptors. Spontaneous activity of these Ca(2+)-release channels is augmented due to up-regulation of RyR type 2 and elevated IP3 production by up-regulated phospholipase C-β1.

CONCLUSIONS

Our study unravels the cellular and molecular mechanisms of attenuation of P2XR-mediated preglomerular vasoconstriction that elevates glomerular susceptibility to harmful hypertensive pressures. This provides an important impetus towards understanding of the pathology of hypertensive renal injury.

Renal vasoconstriction by vasopressin V1a receptors is modulated by nitric oxide, prostanoids, and superoxide but not the ADP ribosyl cyclase CD38

Renal blood flow (RBF) responses to arginine vasopressin (AVP) were tested in anesthetized wild-type (WT) and CD38(-/-) mice that lack the major calcium-mobilizing second messenger cyclic ADP ribose. AVP (3-25 ng) injected intravenously produced dose-dependent decreases in RBF, reaching a maximum of 25 ± 2% below basal RBF in WT and 27 ± 2% in CD38(-/-) mice with 25 ng of AVP. Renal vascular resistance (RVR) increased 75 ± 6% and 78 ± 6% in WT and CD38(-/-) mice. Inhibition of nitric oxide (NO) synthase with nitro-L-arginine methyl ester (L-NAME) increased the maximum RVR response to AVP to 308 ± 76% in WT and 388 ± 81% in CD38(-/-) (P < 0.001 for both). Cyclooxygenase inhibition with indomethacin increased the maximum RVR response to 125 ± 15% in WT and 120 ± 14% in CD38(-/-) mice (P < 0.001, <0.05). Superoxide suppression with tempol inhibited the maximum RVR response to AVP by 38% in both strains (P < 0.005) but was ineffective when administered after L-NAME. The rate of RBF recovery (relaxation) after AVP was slowed by L-NAME and indomethacin (P < 0.001, <0.005) but was unchanged by tempol. All vascular responses to AVP were abolished by an AVP V1a receptor antagonist. A V2 receptor agonist or antagonist had no effect on AVP-induced renal vasoconstriction. Taken together, the results indicate that renal vasoconstriction by AVP in the mouse is strongly buffered by vasodilatory actions of NO and prostanoids. The vasoconstriction depends on V1a receptor activation without involvement of CD38 or concomitant vasodilatation by V2 receptors. The role of superoxide is to enhance the contractile response to AVP, most likely by reducing the availability of NO rather than directly stimulating intracellular contraction signaling pathways.

Spaceflight regulates ryanodine receptor subtype 1 in portal vein myocytes in the opposite way of hypertension

Gravity has a structural role for living systems. Tissue development, architecture, and organization are modified when the gravity vector is changed. In particular, microgravity induces a redistribution of blood volume and thus pressure in the astronaut body, abolishing an upright blood pressure gradient, inducing orthostatic hypotension. The present study was designed to investigate whether isolated vascular smooth muscle cells are directly sensitive to altered gravitational forces and, second, whether sustained blood pressure changes act on the same molecular target. Exposure to microgravity during 8 days in the International Space Station induced the decrease of ryanodine receptor subtype 1 expression in primary cultured myocytes from rat hepatic portal vein. Identical results were found in portal vein from mice exposed to microgravity during an 8-day shuttle spaceflight. To evaluate the functional consequences of this physiological adaptation, we have compared evoked calcium signals obtained in myocytes from hindlimb unloaded rats, in which the shift of blood pressure mimics the one produced by the microgravity, with those obtained in myocytes from rats injected with antisense oligonucleotide directed against ryanodine receptor subtype 1. In both conditions, calcium signals implicating calcium-induced calcium release were significantly decreased. In contrast, in spontaneous hypertensive rat, an increase in ryanodine receptor subtype 1 expression was observed as well as the calcium-induced calcium release mechanism. Taken together, our results shown that myocytes were directly sensitive to gravity level and that they adapt their calcium signaling pathways to pressure by the regulation of the ryanodine receptor subtype 1 expression.

Ca2+ entry following P2X receptor activation induces IP3 receptor-mediated Ca2+ release in myocytes from small renal arteries

BACKGROUND AND PURPOSE

P2X receptors mediate sympathetic control and autoregulation of the renal circulation triggering contraction of renal vascular smooth muscle cells (RVSMCs) via an elevation of intracellular Ca(2+) concentration ([Ca(2+) ](i) ). Although it is well-appreciated that the myocyte Ca(2+) signalling system is composed of microdomains, little is known about the structure of the [Ca(2+) ](i) responses induced by P2X receptor stimulation in vascular myocytes.

EXPERIMENTAL APPROACHES

Using confocal microscopy, perforated-patch electrical recordings, immuno-/organelle-specific staining, flash photolysis and RT-PCR analysis we explored, at the subcellular level, the Ca(2+) signalling system engaged in RVSMCs on stimulation of P2X receptors with the selective agonist αβ-methylene ATP (αβ-meATP).

KEY RESULTS

RT-PCR analysis of single RVSMCs showed the presence of genes encoding inositol 1,4,5-trisphosphate receptor type 1(IP(3) R1) and ryanodine receptor type 2 (RyR2). The amplitude of the [Ca(2+) ](i) transients depended on αβ-meATP concentration. Depolarization induced by 10 µmol·L(-1) αβ-meATP triggered an abrupt Ca(2+) release from sub-plasmalemmal ('junctional') sarcoplasmic reticulum enriched with IP(3) Rs but poor in RyRs. Depletion of calcium stores, block of voltage-gated Ca(2+) channels (VGCCs) or IP(3) Rs suppressed the sub-plasmalemmal [Ca(2+) ](i) upstroke significantly more than block of RyRs. The effect of calcium store depletion or IP(3) R inhibition on the sub-plasmalemmal [Ca(2+) ](i) upstroke was attenuated following block of VGCCs.

CONCLUSIONS AND IMPLICATIONS

Depolarization of RVSMCs following P2X receptor activation induces IP(3) R-mediated Ca(2+) release from sub-plasmalemmal ('junctional') sarcoplasmic reticulum, which is activated mainly by Ca(2+) influx through VGCCs. This mechanism provides convergence of signalling pathways engaged in electromechanical and pharmacomechanical coupling in renal vascular myocytes.

NO-independent mechanism mediates tempol-induced renal vasodilation in SHR

We investigated whether tempol, a superoxide dismutase mimetic, affected renal hemodynamics and arterial pressure in spontaneously hypertensive rats (SHR) and Sprague-Dawley (SD) rats. We also examined whether tempol affected exaggerated renal vasoconstrictor responses to ANG II in SHR. To test whether the effects of tempol were due to a restored NO system, we used the NOS inhibitor Nw-nitro-l-arginine methyl ester (l-NAME). Renal blood flow (RBF) and mean arterial pressure (MAP) were measured in vivo by electromagnetic flowmetry and arterial catheterization in 10- to 12-wk-old anesthetized SHR and SD rats. Systolic arterial pressure (SAP) was measured in conscious rats using the tail cuff method. Tempol (1 mM) was given in the drinking water to SD (SD-T) and SHR (SHR-T) for 5–7 days for RBF measurements and for 15 days for SAP measurements. Age-matched SD (SD-C) and SHR (SHR-C) were used as controls. ANG II (1–4 ng) was administered as a bolus via a renal artery catheter. l-NAME was administered intravenously for 15–20 min. Renal vascular resistance (RVR) was elevated in SHR-C compared with SD-C. In SHR-T, baseline RVR was not different from SD-C and SD-T rats. Tempol had no effect on RVR in SD. l-NAME elevated RVR to the same extent in all four groups. Arterial pressure was not affected by tempol. The RVR responses to ANG II were higher in SHR-C than in the SD-C group. ANG II responses were not different between SHR-T and SD-T. Overall, tempol reduced the renovascular responses to ANG II in SHR. l-NAME elevated the effects of ANG II in SD-C rats but had no effect on the ANG II responses in the other groups. Thus l-NAME treatment did not influence tempol’s effects on baseline RVR or ANG II responses. We conclude that in SHR, tempol has a significant renal vasodilator effect and that it normalizes the increased renovascular ANG II sensitivity. As the effects of l-NAME are not greater in SHR-T rats, it is not likely that the elevated renal resistance and ANG II sensitivity in SHR are due to reactive oxygen species-induced quenching of nitric oxide.

L-type calcium channels in the renal microcirculatory response to endothelin

The signaling pathways of endothelin (ET)-1-mediated vasoconstriction in the renal circulation have not been elucidated but appear to be distinct between ETAand ETBreceptors. The purpose of this study was to determine the role of L-type Ca2+channels in the vasoconstrictor response to ET-1 and the ETBreceptor agonist sarafotoxin 6c (S6c) in the rat kidney. Renal blood flow (RBF) was measured with an ultrasonic flow probe in anesthetized rats, and a microcatheter was inserted into the renal artery for drug infusion. All rats were given vehicle (0.9% NaCl) or three successive bolus injections (1, 10, and 100 pmol) of ET-1 or S6c at 30-min intervals ( n = 6 in each group). ET-1 and S6c produced dose-dependent decreases in RBF. The Ca2+channel blocker nifedipine (1.5 μg) significantly attenuated the RBF response only at the highest doses of ET-1 and S6c. In the isolated blood-perfused juxtamedullary nephron preparation, Ca2+channel blockade with diltiazem had a very small inhibitory effect on ET-1-induced decreases in afferent arteriolar diameter only at the lowest concentrations of ET-1. In vascular smooth muscle cells isolated from preglomerular vessels, ET-1 produced a typical biphasic Ca2+response, whereas S6c had no effect on cytosolic Ca2+. Furthermore, Ca2+channel blockade (diltiazem or Ni2+) had no effect on the peak or sustained increase in cytosolic Ca2+produced by ET-1. These results support the hypothesis that L-type Ca2+channels play only a minor role in the constrictor responses to ET-1 in the renal microcirculation.

Enhanced Ca2+ response to AVP in preglomerular vessels from rats with genetic hypertension during different hydration states

Exaggerated arginine vasopressin (AVP)-induced calcium signaling and renal vasoconstriction, characteristic in young spontaneously hypertensive rats (SHR) during euvolemia, are related to greater amounts of V1a receptor mRNA and V1a protein in preglomerular resistance arterioles. The present study determined whether V1a receptor density and calcium signal transduction in the renal vasculature of young SHR is regulated appropriately during physiological changes in hydration state. [3H]AVP ligand binding documented two- to threefold greater density of V1a receptors in euvolemic SHR vs. Wistar-Kyoto (WKY) rats. Parallel changes in V1a receptor density were observed in both strains during chronic water loading (plus approximately 50 fmol/mg) and during dehydration (minus approximately 50 fmol/mg). Affinity was unchanged. Real-time RT-PCR demonstrated that V1a mRNA in preglomerular arterioles was three times greater in euvolemic SHR. Dehydration decreased expression approximately 50% in renal vessels independent of rat strain; water loading increased V1a mRNA. Thus V1a receptor regulation correlated with changes in mRNA in a normal manner in response to chronic changes in AVP concentration, albeit set at a higher level in SHR. In dehydrated animals, AVP increased the cytosolic Ca2+ concentration ([Ca2+]i) by 60 +/- 5 and 112 +/- 13 nM cytosolic Ca2+ in WKY and SHR, respectively (P < 0.01), whereas in hydrated animals the [Ca2+]i increase was 168 +/- 10 and 220 +/- 18 nM, respectively (P < 0.05). In all hydration states, calcium signaling was greater in SHR compared with WKY (P < 0.05). Calcium signaling paralleled changes in the receptor density and mRNA. Mechanisms other than hydration state per se are likely to be responsible for the two- to threefold difference in the V1a receptor density between WKY and SHR in the renal vasculature at the critical age of 6 wk.

Enhanced response to AVP in the interlobular artery from the spontaneously hypertensive rat

Arginine vasopressin (AVP) induces exaggerated intracellular free calcium (Cai2+) responses in preglomerular smooth muscle cells from young spontaneously hypertensive rats (SHR) due to increased density of the AVP V1a receptor. The intention of the present paper was to examine the relative contribution of afferent arterioles (AA) and interlobular artery (ILA) in AVP- and norepinephrine-induced calcium signaling. The kidneys were perfused with agar solution in vivo, and thin cortical slices were enzyme digested to produce isolated agar-filled vascular fragments. Calcium responses were recorded in fura 2-loaded cells by Ca2+ imaging. Diameter changes were measured after AVP stimulation and mRNA for V1a was measured on isolated vessel fragments. SHR had a significantly higher baseline calcium ratio and lower resting diameter compared with normotensive Wistar-Kyoto rats (WKY). Stimulation with AVP (10(-7) M) in ILA fragments from SHR induced a ratio increase of 0.49 +/- 0.09, significantly higher than the ratio increase in AA from SHR (0.20 +/- 0.03, P < 0.01) and in ILA from WKY (0.24 +/- 0.03, P < 0.01). Stimulation with norepinephrine (10(-7) M) induced responses homogeneously distributed between the segments and strains. Nifedipine treatment or removal of external calcium (Cao2+) reduced the norepinephrine-induced peak response. Both norepinephrine- and AVP-induced sustained responses were abolished after Cao2+ removal in SHR and WKY (P < 0.01). Measurements of V1a receptor mRNA on isolated segments showed a threefold increase in ILA from SHR. The present findings indicate that the exaggerated Ca2+ and contractile response to AVP in SHR is mainly mediated through ILA vasoconstriction.

Relative contributions of Ca2+ mobilization and influx in renal arteriolar contractile responses to arginine vasopressin

Experiments addressed the hypothesis that afferent and efferent arterioles differentially rely on Ca2+ influx and/or release from intracellular stores in generating contractile responses to AVP. The effect of Ca2+ store depletion or voltage-gated Ca2+ channel (VGCC) blockade on contractile responsiveness to AVP (0.01-1.0 nM) was assessed in blood-perfused juxtamedullary nephrons from rat kidney. Depletion of intracellular Ca2+ stores by 100 microM cyclopiazonic acid (CPA) or 1 microM thapsigargin treatment increased afferent arteriolar baseline diameter by 14 and 21%, respectively, but did not significantly alter efferent arteriolar diameter. CPA attenuated the contractile response to 1.0 nM AVP by 34 and 55% in afferent and efferent arterioles, respectively (P = 0.013). The impact of thapsigargin on AVP-induced afferent arteriolar contraction (52% inhibition) was also less than its effect on the efferent arteriolar response (88% inhibition; P = 0.046). In experiments probing the role of the Ca2+ influx through VGCCs, 10 microM diltiazem evoked a 34% increase in baseline afferent arteriolar diameter and attenuated the contractile response to 1.0 nM AVP by 45%, without significantly altering efferent arteriolar baseline diameter or responsiveness to AVP. Combined treatment with both diltiazem and thapsigargin prevented AVP-induced contraction of both vascular segments. We conclude that Ca2+ release from the intracellular stores contributes to the contractile response to AVP in both afferent and efferent arterioles but is more prominent in the efferent arteriole. Moreover, the VGCC contribution to AVP-induced renal arteriolar contraction resides primarily in the afferent arteriole.

Calcium handling in afferent arterioles

The cytosolic intracellular calcium concentration ([Ca(2+)](i)) is a major determining factor in the vascular smooth muscle tone. In the afferent arteriole it has been shown that agonists utilizing G-protein coupled receptors recruit Ca(2+) via release from intracellular stores and entry via pathways in the plasma membrane. The relative importances of entry vs. mobilization seem to differ between different agonists, species and preparations. The entry pathway might include different types of voltage sensitive Ca(2+) channels located in the plasmalemma such as dihydropyridine sensitive L-type channels, T-type channels and P/Q channels. A role for non-voltage sensitive entry pathways has also been suggested. The importance of voltage sensitive Ca(2+) channels in the control of the tone of the afferent arteriole (and thus in the control of renal function and whole body control of extracellular fluid volume and blood pressure) sheds light on the control of the membrane potential of afferent arteriolar smooth muscle cells. Thus, K(+) and Cl(-) channels are of importance in their role as major determinants of membrane potential. Some studies suggest a role for calcium-activated chloride (Cl(Ca)) channels in the renal vasoconstriction elicited by agonists. Other investigators have found evidence for several types of K(+) channels in the regulation of the afferent arteriolar tone. The available literature in this field regarding afferent arterioles is, however, relatively sparse and not conclusive. This review is an attempt to summarize the results obtained by others and ourselves in the field of agonist induced afferent arteriolar Ca(2+) recruitment, with special emphasis on the control of voltage sensitive Ca(2+) entry. Outline of the Manuscript: This manuscript is structured as follows: it begins with an introduction where the general role for [Ca(2+)](i) as a key factor in the regulation of the tone of vascular smooth muscles (VSMC) is detailed. In this section there is an emphasis is on observations that could be attributed to afferent arteriolar function. We then investigate the literature and describe our results regarding the relative roles for Ca(2+) entry and intracellular release in afferent arterioles in response to vasoactive agents, with the focus on noradrenalin (NA) and angiotensin II (Ang II). Finally, we examine the role of ion channels (i.e. K(+) and Cl(-) channels) for the membrane potential, and thus activation of voltage sensitive Ca(2+) channels.

Age-dependent regulation of vasopressin V1areceptors in preglomerular vessels from the spontaneously hypertensive rat

Experiments were performed to get insight into the role of AVP receptor V1aregulation with age, i.e., during development and maintenance of high blood pressure. Previous studies showed an increased gene expression and renal vascular response to AVP in young spontaneously hypertensive rats (SHR). The age regulation of the V1areceptor was examined in preglomerular vessels from 5-, 10-, 20-, and 70-wk-old SHR using normotensive Wistar-Kyoto rats (WKY) as controls. Real-time PCR and ligand binding were used for analysis of receptor expression, and the change in cytosolic calcium concentration during stimulation of isolated preglomerular vessels with AVP was studied. Studies showed an increase of the V1areceptor protein and mRNA from 5-and 10-wk-old SHR compared with vessels from 20- and 70-wk-old SHR. In 5-wk-old SHR receptor density was 84 ± 13 fmol/mg protein, and 38 ± 11 fmol/mg protein in 70-wk-old SHR ( P < 0.05). mRNA in the 5- and 70-wk-old SHR was 15,854 ± 629 and 3,181 ± 224 V1amRNA/108 18S ribosomal RNA, respectively ( P < 0.001). Values from WKY at all ages were similar to 20- and 70-wk-old SHR. During stimulation with AVP, the change in cytosolic calcium in vessels from 5-wk-old SHR increased 234 ± 59 nM, whereas the increase was 89 ± 9 nM in 70-wk-old SHR ( P = 0.03). These results indicate that the V1areceptor is increased at protein and mRNA level during development of hypertension in SHR but is normalized when hypertension is established.

Variations in cell signaling pathways for different vasoconstrictor agonists in renal circulation of the rat

BACKGROUND

Major cell signaling pathways involved in agonist-induced vasoconstriction are recognized to be Ca2+ mobilization via inositol-1,4,5 triphosphate (IP3), Ca2+ influx through l-type channels, activation of protein kinase C (PKC), and of Rho-associated kinase (ROK). However, their contribution for renal vasoconstriction induced by different agonists is not well characterized.

METHODS

Increasing doses of angiotensin II (Ang II), norepinephrine, and arginine vasopressin (AVP) were infused into the left renal artery of anesthetized rats to reduce renal blood flow from a threshold value to about 50%. Rightward shift of the dose-response curves due to coinfusion of inhibitors served to assess contribution of different pathways: trimethoxybenzoate (TMB-8) against Ca2+ mobilization, nifedipine against Ca2+ influx, staurosporine and Ro-318220 against PKC, and Y-27632 and HA-1077 against ROK. Effects of inhibitors were also determined for renal response to a single dose of U-46619, a thromboxane A2 agonist. Composite response to U-46619 consisting of a fast and slow component did not permit determination of dose-response curves.

RESULTS

Inhibition of ROK by Y-27632 or HA-1077 had the largest effect on renal responses to agonists. They shifted dose-response curves of Ang II, norepinephrine, and AVP to sevenfold and higher values. Staurosporine, nifedipine, and TMB-8 had variable effect on agonist responses. They attenuated effects of Ang II and norepinephrine in an additive manner, and each of them increased effective dose values about fourfold. TMB-8 did not attenuate response to AVP and U-46619. Staurosporine and nifedipine diminished effects of AVP in a nonadditive manner, and attenuated additively the fast component of U-46619 response.

CONCLUSION

In contrast to other cell signaling pathways, ROK plays a common role for all vasoconstrictor agonistsis in renal circulation.

alpha 2-Adrenoceptors potentiate angiotensin II- and vasopressin-induced renal vasoconstriction in spontaneously hypertensive rats

Hypertension in spontaneously hypertensive rats (SHRs) is due in part to enhanced effects of vasoactive peptides on the renal vasculature. We hypothesize that the G(i) signal transduction pathway enhances renovascular responses to vasoactive peptides in SHRs more so than in normotensive Wistar-Kyoto (WKY) rats. To test this hypothesis, we examined in isolated perfused kidneys from SHRs and WKY rats the renovascular responses (assessed as changes in renal perfusion pressure in mm Hg) to angiotensin II (10 nM) and vasopressin (3 nM) in the presence and absence of UK-14,304 [5-bromo-N-(4,5-dihydro-1H-imidazol-2-yl)-6-quinoxalinamine; an agonist that selectively activates the G(i) pathway by stimulating alpha(2)-adrenoceptors]. In SHR, but not WKY, kidneys, UK-14,304 (10 nM) enhanced (P < 0.05) renovascular responses to angiotensin II (control WKY, 43 +/- 6; UK-14,304-treated WKY, 52 +/- 19; control SHR, 66 +/- 17; UK-14,304-treated SHR, 125 +/- 16) and vasopressin (control WKY, 42 +/- 17; UK-14,304-treated WKY, 36 +/- 11; control SHR, 16 +/- 8; UK-14,304-treated SHR, 83 +/- 17). Pretreatment of SHRs with pertussis toxin (30 microg/kg, intravenously, 3-4 days before study) to inactivate G(i) blocked the effects of UK-14,304. Western blot analysis of receptor expression in whole kidney and preglomerular microvessels revealed similar levels of expression of AT(1), V(1a), and alpha(2A) receptors in SHRs compared with WKY rats. We conclude that activation of alpha(2)-adrenoceptors selectively enhances renovascular responses to angiotensin II and vasopressin in SHRs via an enhanced cross talk between the G(i) signal transduction pathway and signal transduction pathways activated by angiotensin II and vasopressin.

Increased contraction to noradrenaline by vasopressin in human renal arteries

OBJECTIVE

Arginine vasopressin (AVP) not only acts directly on blood vessels through vasopressin V1 receptor stimulation but also may modulate adrenergic-mediated responses in animal experiments. The aim of the present study was to assess whether subpressor concentrations of AVP could contribute to an abnormal adrenergic contractile response of human renal arteries.

METHODS

Renal artery rings were obtained from 27 patients undergoing nephrectomy. The rings were suspended in organ bath chambers for isometric recording of tension.

RESULTS

AVP (10(-10) mol/l) and the vasopressin V1 receptor agonist [Phe2, Orn8]-vasotocin (10(-10) mol/l) produced a leftward shift of the concentration-response curve to noradrenaline (half-maximal effective concentration decreased from 1.1 x 10(-6) mol/l to 3.1 x 10(-7) mol/l). The enhancement of noradrenaline-induced contractions was inhibited by the vasopressin V1 receptor antagonist d(CH2)5Tyr(Me)AVP (10-8 mol/l) and unaffected by endothelium removal or pretreatment with the inhibitor of nitric oxide (NO) synthase NG-monomethyl-l-arginine (l-NMMA). The vasopressin V2 receptor agonist 1-desamino-8-D-arginine vasopressin (dDAVP) (10(-10)-10(-8) mol/l) did not modify contractile responses to noradrenaline. In the presence of the dihydropyridine calcium antagonist nifedipine (10(-6) mol/l), vasopressin failed to enhance the contractile response to noradrenaline.

CONCLUSIONS

The results demonstrate that subpressor concentrations of vasopressin potentiate the contractile effects of noradrenaline without intervention of the NO system. The effects appear to be mediated by vasopressin V1 receptor stimulation, which brings about an increase in calcium entry through dihydropyridine-sensitive calcium channels.

Store-operated Ca2+ entry is exaggerated in fresh preglomerular vascular smooth muscle cells of SHR

BACKGROUND

Regulation of preglomerular vasomotor tone vessels ultimately control glomerular filtration rate, sodium reabsorption and systemic blood pressure. To gain insight into the complex renal hemodynamic factors that may result in hypertension, we studied calcium signaling pathways.

METHODS

Fresh, single, preglomerular vascular smooth muscle cells (VSMC) were isolated from 5- to 6-week-old SHR and WKY utilizing a magnetized microsphere/sieving technique. Cytosolic Ca2+ ([Ca2+]i) was measured with fura-2 ratiometric fluorescence. To examine store-operated calcium entry (SOC), VSMC were activated in calcium-free buffer containing nifedipine. To deplete the sarcoplasmic reticulum (SR) of Ca2+, vasopressin-1 receptor agonist [V1R; inositol trisphosphate (IP3)-mediated mobilization], ryanodine (non-IP3 induced mobilization), and cyclopiazonic acid (CPA; Ca2+-ATPase inhibition) were utilized. Addition of external calcium followed by quenching of the fura/Ca2+ signal with Mn2+ permitted assessment of divalent cation entry via SOC.

RESULTS

V1R caused greater mobilization in SHR than WKY (P < 0.01) as well as greater calcium entry (P < 0.001). Ryanodine and CPA both caused SR calcium depletion that was not statistically different between strains, but absolute calcium entry through SOC was more than double in SHR following either maneuver (P < 0.001). 2-Amino-ethoxybiphenyl borane (2-APB), an inhibitor not only of IP3 receptors, but also of SOC, blocked calcium entry in the ryanodine and CPA experiments independent of IP3. As well, Gd3+, a selective inhibitor of SOC, inhibited the Ca2+ response. We also studied L-channel calcium entry stimulated by V1R. The total calcium response was greater in SHR as was the absolute inhibition by nifedipine. As a percent of the total response, participation of L-type channels sensitive to nifedipine was about 45% in both strains of rat.

CONCLUSION

Utilizing three separate mechanisms to deplete the SR of Ca2+ in order to activate SOC, we show for the first time, that SOC is exaggerated in preglomerular VSMC of young SHR.

Norepinephrine-induced calcium signaling pathways in afferent arterioles of genetically hypertensive rats

This study provides new information about the relative importance of calcium mobilization and entry in the renal vascular response to adrenoceptor activation in afferent arterioles isolated from 7- to 8-wk-old Wistar-Kyoto (WKY) and spontaneously hypertensive rats (SHR). Intracellular free calcium concentration ([Ca(2+)](i)) was measured in microdissected arterioles utilizing ratiometric photometry of fura 2 fluorescence. There was no significant strain difference in baseline [Ca(2+)](i). Norepinephrine (NE; 10(-6) and 10(-7) M) elicited immediate, sustained increases in [Ca(2+)](i). The general temporal pattern of response to 10(-6) M NE consisted of an initial peak and a maintained plateau phase. The response to NE was partially blocked by nifedipine (10(-6) M) or 8-(N,N-diethylamino) octyl-3,4,5-trimetoxybenzoate (TMB-8; 10(-5) M). A calcium-free external solution abolished the sustained [Ca(2+)](i) plateau response to NE, with less influence on the peak response. In the absence of calcium entry, TMB-8 (10(-5) M) completely blocked the calcium response to NE in WKY but not SHR, suggesting strain differences in mobilization. A higher concentration of TMB-8 (10(-4) M), however, blocked all discernible mobilization in both strains. We conclude that there are differences in Ca(2+) handling in renal resistance vessels between young WKY and SHR with respect to mobilization stimulated by alpha-adrenoceptors. Afferent arterioles of young SHR appear to have a larger inositol-1,4,5-trisphosphate-sensitive pool or release from a site less accessible to TMB-8.

Ryanodine receptor and capacitative Ca2+ entry in fresh preglomerular vascular smooth muscle cells

BACKGROUND

A multiplicity of hormonal, neural, and paracrine factors regulates preglomerular arterial tone by stimulating calcium entry or mobilization. We have previously provided evidence for capacitative (store-operated) Ca2+ entry in fresh renal vascular smooth muscle cells (VSMCs). Ryanodine-sensitive receptors (RyRs) have recently been identified in a variety of nonrenal vascular beds.

METHODS

We isolated fresh rat preglomerular VSMCs with a magnetized microsphere/sieving technique; cytosolic Ca2+ ([Ca2+]i) was measured with fura-2 ratiometric fluorescence.

RESULTS

Ryanodine (3 micromol/L) increased [Ca2+]i from 79 to 138 nmol/L (P = 0.01). Nifedipine (Nif), given before or after ryanodine, was without effect. The addition of calcium (1 mmol/L) to VSMCs in calcium-free buffer did not alter resting [Ca2+]i. In Ca-free buffer containing Nif, [Ca2+]i rose from 61 to 88 nmol/L after the addition of the Ca2+-ATPase inhibitor cyclopiazonic acid and to 159 nmol/L after the addition of Ca2+ (1 mmol/L). Mn2+ quenched the Ca/fura signal, confirming divalent cation entry. In Ca-free buffer with Nif, [Ca2+]i increased from 80 to 94 nmol/L with the addition of ryanodine and further to 166 nmol/L after the addition of Ca2+ (1 mmol/L). Mn2+ quenching was again shown. Thus, emptying of the sarcoplasmic reticulum (SR) with ryanodine stimulated capacitative Ca2+ entry.

CONCLUSION

Preglomerular VSMCs have functional RyR, and a capacitative (store-operated) entry mechanism is activated by the depletion of SR Ca2+ with ryanodine, as is the case with inhibitors of SR Ca2+-ATPase.

Upregulation of V(1) receptors in renal resistance vessels of rats developing genetic hypertension

Previous studies have demonstrated that arginine vasopressin (AVP) produces exaggerated renal vasoconstriction in young spontaneously hypertensive rats (SHR) relative to normotensive rats. The exaggerated renal vascular reactivity does not appear to be due to a primary defect in postreceptor calcium signal transduction. Although the magnitudes of vascular responses differ, the relative proportions of calcium entry and mobilization pathways evoked by AVP in renal resistance vessels are similar in these rat strains. The purpose of the present study was to evaluate possible differences in V(1) mRNA and receptor density and affinity in preglomerular resistance vessels (<50 microm) obtained from young Wistar-Kyoto (WKY) and SHR. Quantitative RT-PCR analysis revealed twofold greater expression of the V(1a) receptor gene in preglomerular arterioles of 7-wk-old SHR compared with WKY. In vitro radiolabeled ligand binding studies were performed under equilibrium conditions on preglomerular resistance arterioles freshly isolated from kidneys of 7-wk-old rats. The results indicate that AVP receptor density (B(max)) is two to three times greater in SHR than in WKY (248 +/- 24 vs. 91 +/- 11 fmol/mg protein, P < 0.001). The affinity does not differ between strains (K(d) = 0.5 nM). Displacement studies yielded similar results for SHR and WKY. Unlabeled AVP completely displaced [(3)H]AVP binding, with an IC(50) of 2.5 x 10(-10) M. Expression of AVP receptor types in afferent arterioles was evaluated using the V(1) receptor agonist, [Phe(2), Ile(3),Org(8)]vasopressin, the V(1) receptor antagonist, [d(CH(2))(5), Tyr(Me)(2), Tyr(NH(2))(9)]Arg(8)-vasopressin, and the V(2) receptor agonist, desamino-[D-Arg(8)]vasopressin. Both the V(1) agonist and antagonist displaced up to 90% of the AVP binding with IC(50) values of 4 x 10(-8) and 8 x 10(-7) M, respectively. The V(2) receptor agonist was a weak inhibitor, displacing less than 15% of AVP binding at a high concentration of 10(-4) M. These results demonstrate that virtually all AVP receptors in the preglomerular arterioles are of the V(1) type. Collectively, our results provide evidence that the enhanced renal reactivity to AVP is mediated by a higher density of V(1) receptors associated with increased gene expression in renal resistance vessels of SHR developing genetic hypertension.

alpha(1)-adrenoceptor subtypes in rat renal resistance vessels: in vivo and in vitro studies

This study provides new information about the relative importance of different alpha(1)-adrenoceptors during norepinephrine (NE) activation in rat renal resistance vessels. In Sprague-Dawley rats, we measured renal blood flow (RBF) using electromagnetic flowmetry in vivo and the intracellular free calcium concentration ([Ca(2+)](i)) utilizing ratiometric photometry of fura 2 fluorescence in isolated afferent arterioles. Renal arterial bolus injection of NE produced a transient 46% decrease in RBF. In microdissected afferent arterioles, NE (1 microM) elicited an immediate square-shaped increase in [Ca(2+)](i), from 90 to 175 nM (P < 0.001). Chloroethylclonidine (CEC) (50 microM) had no chronic irreversible alkylating effect in vitro but exerted acute reversible blockade on norepinephrine (NE) responses both on [Ca(2+)](i) in vitro and on RBF in vivo. The RBF response was attenuated by approximately 50% by the putative alpha(1A)-adrenoceptor and alpha(1D)-adrenoceptor antagonists 5-methylurapidil (5-MU), and 8-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-8-azaspiro[4. 5]decane-7,9-dione dihydrochloride (BMY-7378) (12.5 and 62.5 microg/h), respectively. The in vitro [Ca(2+)](i) response to NE was blocked approximately 25% and 50% by 5-MU (100 nM and 1 microM). BMY-7378 (100 nM and 1 microM) attenuated the NE-induced response by approximately 40% and 100%. The degree of inhibition in vitro was similar to the in vivo experiments. In conclusion, 5-MU and BMY-7378 attenuated the NE-induced responses, although relatively high concentrations were required, suggesting involvement of both the alpha(1A)-adrenoceptor and alpha(1D)-adrenoceptor. Participation of the alpha(1B)-adrenoceptor is less likely, as we found no evidence for CEC-induced alkylation.

Capacitative calcium entry in smooth muscle cells from preglomerular vessels

Calcium entry via voltage-gated L-type channels is responsible for at least half of the increase in cytosolic calcium ([Ca(2+)](i)) in afferent arterioles following agonist stimulation. We sought the presence of capacitative calcium entry in fresh vascular smooth muscle cells (VSMC) derived from rat preglomerular vessels. [Ca(2+)](i) was measured using fura-2 ratiometric fluorescence. Vasopressin V1 receptor agonist (V1R) (10(-7) M) increased [Ca(2+)](i) by approximately 100 nM. A calcium channel blocker (CCB), nifedipine or verapamil (10(-7) M), inhibited the response by approximately 50%. V1R in the presence of CCB increased [Ca(2+)](i) from 106 to 176 nM, confirming that calcium mobilization and/or entry may occur independent of voltage-gated channels. In nominally Ca(2+)-free buffer, V1R increased [Ca(2+)](i) from 94 to 129 nM, denoting mobilization; addition of CaCl(2) (1 mM) further elevated [Ca(2+)](i) to 176 nM, indicating a secondary phase of Ca(2+) entry. Similar responses were obtained when CCB was present in calcium-free buffer or when EGTA was present. In nominally Ca(2+)-free medium, the sarcoplasmic reticulum Ca(2+)-ATPase inhibitors (SRCAI), thapsigargin and cyclopiazonic acid (CPA), increased [Ca(2+)](i) from 97 to 128 and 143 nM, respectively, and to 214 and 220 nM, respectively, when 1 mM extracellular Ca(2+) was added. In the presence of verapamil, the results with CPA acid were nearly identical. In Ca(2+)-free buffer, the stimulatory effect of V1R or SRCAI on the Ca(2+)/fura signal was quenched by the addition of Mn(2+) (1 mM), demonstrating divalent cation entry. These studies provide evidence for capacitative (store- operated) calcium entry in VSMC freshly isolated from rat preglomerular arterioles.

Calcium recruitment in renal vasculature: NE effects on blood flow and cytosolic calcium concentration

This study provides new information about the relative importance of Ca2+ mobilization and entry in the renal vascular response to adrenoceptor activation. We measured renal blood flow (RBF) in Sprague-Dawley rats in vivo using electromagnetic flowmetry. We measured intracellular free Ca2+ concentration ([Ca2+]i) in isolated afferent arterioles utilizing ratiometric photometry of fura-2 fluorescence. Renal arterial injection of NE produced a transient decrease in RBF. The response was attenuated, in a dose-dependent manner, up to approximately 50% by nifedipine, an antagonist of L-type Ca2+ entry channels. Inhibition of Ca2+ mobilization by 3,4, 5-trimethoxybenzoic acid-8-(diethylamino)octyl ester (TMB-8) inhibited the renal vascular effects of NE in a dose-dependent manner, with maximal blockade of approximately 80%. No additional attenuation was observed when nifedipine and TMB-8 were administered together. In microdissected afferent arterioles, norepinephrine (NE; 10(-6) M) elicited an immediate square-shaped increase in [Ca2+]i, from 110 to 240 nM. This in vitro response was blocked by nifedipine (10(-6) M) and TMB-8 (10(-5) M) to a degree similar to that of the in vivo experiments. A nominally calcium-free solution blocked 80-90% of the [Ca2+]i response to NE. The increased [Ca2+]i elicited by depolarization with medium containing 50 mM KCl was totally blocked by nifedipine. In contrast, TMB-8 had no effect. Our results indicate that both Ca2+ entry and mobilization play important roles in the renal vascular Ca2+ and contractile response to adrenoceptor activation. The entry and mobilization mechanisms activated by NE may interact. That a calcium-free solution caused a larger inhibition of the NE effects on afferent arterioles than nifedipine suggests more than one Ca2+ entry pathway.

Exaggerated Ca2+ signaling in preglomerular arteriolar smooth muscle cells of genetically hypertensive rats

Experiments were conducted to gain insight into mechanisms responsible for exaggerated renal vascular reactivity to ANG II and vasopressin (AVP) in spontaneously hypertensive rats (SHR) during the development of hypertension. Cytosolic calcium concentration ([Ca2+]i) was measured by ratiometric fura 2 fluorescence and a microscope-based photometer. Vascular smooth muscle cells (SMC) from preglomerular arterioles were isolated and dispersed using an iron oxide-sieving method plus collagenase treatment. ANG II and AVP produced rapid and sustained increases in [Ca2+]i. ANG II elicited similar dose-dependent increases in [Ca2+]i in SMC from SHR and Wistar-Kyoto rats (WKY). In contrast, AVP caused almost twofold larger responses in afferent arteriolar SMC from SHR. ANG II effects were inhibited by the AT1 receptor antagonist losartan. AVP action was blocked by the V1 receptor antagonist [d(CH2)5,Tyr(NH2)9]AVP. In SMC pretreated with nifedipine, neither ANG II nor AVP elicited [Ca2+]i responses. Poststimulation nifedipine reversed elevated [Ca2+]i to basal levels. Short-term reductions in external [Ca2+]i (EGTA) mimicked the nifedipine effects. Our study shows that AT1 and V1 receptors stimulate [Ca2+]i by a common mechanism characterized by preferential action on voltage-gated L-type channels sensitive to dihydropyridines. Calcium signaling elicited by AT1 receptors does not differ between SHR and WKY; thus the in vivo exaggerated reactivity may be dependent on interactions with other cell types, e. g., endothelium. In contrast, AVP produced larger changes in [Ca2+]i in arteriolar SMC from SHR, and such direct effects can account for the exaggerated renal blood flow responses.

ANG II and vasopressin stimulate calcium entry in dispersed smooth muscle cells of preglomerular arterioles

Calcium signaling mechanisms were examined in vessel segments and dispersed single smooth muscle cells (SMC) of interlobular arteries and afferent arterioles (< 50 microns diameter) from the rat kidney. These resistance vessels were isolated from rat kidneys, using an iron oxide-sieving technique with subsequent collagenase digestion. Individual cells were identified by their characteristic oval appearance and positive staining for smooth muscle-specific alpha-actin and heavy chain myosin SM-1 and SM-2. Cytosolic calcium concentration ([Ca2+]i) was measured using fura 2 ratiometric fluorescence at 340 and 380 nm wavelength with a microscope-based photometer. Angiotensin II (ANG II) and arginine vasopressin (AVP), at concentrations of 10(-10)-10(-6) M, produced dose-dependent increases in [Ca2+]i; maximum increases were 221 +/- 49 nM for ANG II and 237 +/- 49 nM for AVP. The temporal response patterns for both agonists were characterized by a square-shaped, immediate step increase in [Ca2+]i to a near maximum level that was maintained through the recording period of 150-200 s. Responses of individual dispersed SMC and short vessel segments were similar. Losartan antagonized the action of ANG II, indicating mediation by AT1 receptors on preglomerular arteriolar SMC. The V1-selective antagonist [d(CH2)5Tyr(Me)2Tyr(NH2)9]AVP completely inhibited AVP-induced [Ca2+]i changes. The importance of calcium entry in hormone-induced changes in [Ca2+]i was demonstrated by the finding that neither ANG II nor AVP elicited a [Ca2+]i response in media rendered nominally calcium free by addition of ethylene glycol-bis(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid. Calcium entry occurred primarily through L-type, voltage-gated calcium channels as the dihydropyridine, nifedipine, completely prevented or reversed [Ca2+]i changes normally elicited by either hormone. Our results provide new information about the similarity of calcium signaling in single SMC and short segments freshly isolated from renal interlobular arteries and afferent arterioles. The observations indicate that AT1 and V1 receptors are coupled to signal transduction pathways leading to rapid changes in [Ca2+]i. Calcium mobilization appears to play a minor to nonexistent role under the experimental conditions. The predominant mechanism involves calcium entry through dihydropyridine-sensitive, voltage-gated calcium channels in single SMC from these resistance vessels.