Activation mechanisms of human renal artery: effects of KCl, norepinephrine and nitrendipine upon tension development and 45Ca influx

Superoxide enhances Ca2+ entry through L-type channels in the renal afferent arteriole

Reactive oxygen species regulate cardiovascular and renal function in health and disease. Superoxide participates in acute calcium signaling in afferent arterioles and renal vasoconstriction produced by angiotensin II, endothelin, thromboxane, and pressure-induced myogenic tone. Known mechanisms by which superoxide acts include quenching of nitric oxide and increased ADP ribosyl cyclase/ryanodine-mediated calcium mobilization. The effect(s) of superoxide on other calcium signaling pathways in the renal microcirculation is poorly understood. The present experiments examined the acute effect of superoxide generated by paraquat on calcium entry pathways in isolated rat afferent arterioles. The peak increase in cytosolic calcium concentration caused by KCl (40 mmol/L) was 99±14 nmol/L. The response to this membrane depolarization was mediated exclusively by L-type channels because it was abolished by nifedipine but was unaffected by the T-type channel blocker mibefradil. Paraquat increased superoxide production (dihydroethidium fluorescence), tripled the peak response to KCl to 314±68 nmol/L (P<0.001) and doubled the plateau response. These effects were abolished by tempol and nitroblue tetrazolium, but not by catalase, confirming actions of superoxide and not of hydrogen peroxide. Unaffected by paraquat and superoxide was calcium entry through store-operated calcium channels activated by thapsigargin-induced calcium depletion of sarcoplasmic reticular stores. Also unresponsive to paraquat was ryanodine receptor-mediated calcium-induced calcium release from the sarcoplasmic reticulum. Our results provide new evidence that superoxide enhances calcium entry through L-type channels activated by membrane depolarization in rat cortical afferent arterioles, without affecting calcium entry through store-operated entry or ryanodine receptor-mediated calcium mobilization.

Prostaglandin I 2 and Prostaglandin E 2 Modulate Human Intrarenal Artery Contractility Through Prostaglandin E2-EP4, Prostacyclin-IP, and Thromboxane A2-TP Receptors

Cyclooxygenase inhibitors decrease renal blood flow in settings with decreased effective circulating volume. The present study examined the hypothesis that prostaglandins, prostaglandin E 2 (PGE 2 ) and prostacyclin (PGI 2 ), induce relaxation of human intrarenal arteries through PGE 2 -EP and PGI 2 -IP receptors. Intrarenal arteries were microdissected from human nephrectomy samples (n=53, median diameter ≈362 μm, 88% viable, 76% relaxed in response to acetylcholine). Rings were suspended in myographs to record force development. In vessels with K + -induced tension (EC 70 : –log [mol/L]=1.36±0.03), PGE 2 and PGI 2 induced concentration-dependent relaxation (–log EC 50 : PGE 2 =7.1±0.3 and PGI 2 =7.7). The response to PGE 2 displayed endothelium dependence and desensitization. Relaxation by PGE 2 was mimicked by an EP4 receptor agonist (CAY10598, EC 50 =6.7±0.2). The relaxation after PGI 2 was abolished by an IP receptor antagonist (BR5064, 10 –8 mol/L). Pretreatment of quiescent arteries with PGE 2 for 5 minutes (10 –6 mol/L) led to a significant right shift of the concentration–response to norepinephrine (EC 50 from 6.6±0.1–5.9±0.1). In intrarenal arteries with K + -induced tone, PGE 2 and PGI 2 at 10 –5 mol/L elicited increased tension. This was abolished by thromboxane receptor (TP) antagonist (S18886, 10 –6 mol/L). A TP agonist (U46619, n=6) evoked tension (EC 50 =8.1±0.2) that was inhibited by S18886. Polymerase chain reaction and immunoblotting showed EP4, IP, and TP receptors in intrarenal arteries. In conclusion, PGE 2 and PGI 2 may protect renal perfusion by activating cognate IP and EP4 receptors associated with smooth muscle cells and endothelium in human intrarenal arteries and contribute to increased renal vascular resistance at high pathological concentrations mediated by noncognate TP receptor.

Chronic exposure to cyclosporine affects endothelial and smooth muscle reactivity in the rat aorta

Chronic exposure to cyclosporine affects vascular reactivity. Experiments were designed to characterize the endothelium-dependent and endothelium-independent vascular reactivity of rats exposed to oral cyclosporin A (CyA). Two subsets of rats (n = 6) were treated with CyA (20 mg/kg/day) and olive oil (cyclosporine vehicle), respectively, for a period of 8 weeks. Aortic rings (4-5 mm) were suspended for isometric force measurement in organ chambers containing Krebs Ringer solution (37 degrees C, 95% O2, 5% CO2). The maximal endothelium-dependent relaxation to cumulative doses of acetylcholine was significantly decreased in the CyA-treated aortic rings compared to olive oil-treated ones (data expressed as percent of initial contraction; CyA, 50% +/- 3% versus olive oil, 37% +/- 7%; p < 0.05). However, endothelium-dependent relaxations to histamine and adenosine diphosphate and endothelium-independent relaxation to sodium nitroprusside were not affected in both groups. An endothelium-dependent contraction to serotonin and aggregating platelets were observed in the CyA group, but not in the control group. The endothelium-independent contraction to norepinephrine was enhanced in the CyA group (CyA ED50, log -7.66 +/- 0.18 mol/L versus olive oil ED50, log -7.01 +/- 0.11 mol/L; p < 0.01). These experiments suggest that chronic exposure to cyclosporine A could contribute to augmenting vascular tone by (1) decreased release of endothelial relaxing factor mediated by muscarinic receptors, (2) increased production of endothelium-related constricting factor mediated by serotoninergic receptors, and (3) greater vascular smooth muscle sensitivity to circulating catecholamine.

Effect of elevated extracellular glucose concentrations on transmembrane calcium ion fluxes in cultured rat VSMC

Blood flow autoregulation is impaired in early diabetes mellitus, predisposing the renal microcirculation to injury. These hemodynamic changes have been strongly implicated in the development and progression of diabetic glomerulopathy. Blood flow autoregulation is predominantly a myogenic reflex which is strongly dependent on Ca2+ uptake by vascular smooth muscle cells (VSMC). Because impaired blood flow autoregulation may be responsive to glycemic control, the present study examined the effects of elevated extracellular glucose concentrations on basal, voltage sensitive and receptor operated Ca2+ uptake by VSMC. Confluent cultured rat VSMC were exposed to: (1) control medium (CM; 5 mM glucose); (2) high glucose medium (HGM; 10 to 30 mM glucose); or (3) osmotic control medium (OCM; glucose 5 mM + L-glucose 25 mM or mannitol 25 mM). A threshold glucose concentration of 15 mM markedly and maximally depressed basal Ca2+ uptake by VSMC (HGM 52% vs. CM). In addition, HGM significantly depressed voltage sensitive Ca2+ uptake by VSMC as determined by responses to BAY K 8644 (10(-7) M) or high extracellular [K+] (65 mM, HGM 50% vs. CM). HGM similarly depressed pressor hormone-stimulated Ca2+ uptake (AVP or Ang II 10(-7) M) by VSMC. The effects of HGM on Ca2+ uptake were time exposure dependent and reversible. Ca2+ uptake by VSMC in the presence of OCM did not differ from CM. Elevated extracellular glucose concentrations thus exert a direct and profound effect on basal, voltage sensitive and receptor operated Ca2+ uptake by VSMC. These observations may provide a biochemical basis for glucose-induced dysregulation of regional blood flow autoregulation in early diabetes mellitus.

Vasodilator drug effects on internal mammary artery and saphenous vein grafts

The internal mammary artery is a dynamic conduit used for myocardial revascularization in which potential exists for spasm as well as for vasodilation. This study investigated vasodilator drug effects on the mammary artery using nitroprusside and nitroglycerin in vitro to measure the inhibition of contraction of human internal mammary artery and in vivo to examine blood flow through a canine mammary artery. In the in vitro study, ring segments of human internal mammary arteries were suspended on strain gauges in muscle baths containing 37 degrees C Krebs solution for measurement of isometric tension in vitro. Arterial contraction was stimulated with 70 mM potassium and 10 microM norepinephrine, and inhibition of contraction by vasodilators was measured. Nitroprusside was more potent and effective than was nitroglycerin in inhibiting potassium and norepinephrine contraction. The in vivo study utilized a canine (n = 8) right heart bypass preparation that allowed precise control of cardiac output, blood pressure and heart rate, which were maintained constant. The internal mammary artery graft and the saphenous vein graft perfused the same coronary artery bed. Electromagnetic flow probes measured graft flow (with the other graft occluded) before and after 15 min of drug infusion (1 microgram/kg per min). Nitroglycerin significantly increased mammary artery flow 36 +/- 13%, whereas nitroprusside significantly decreased it 12 +/- 2%. Saphenous vein grafts responded differently; graft blood flow decreased with nitroglycerin and increased with nitroprusside. Thus, although nitroprusside was more effective than nitroglycerin in inhibiting mammary artery contraction in vitro, it decreased internal mammary artery graft flow measured in vivo. Nitroglycerin had the opposite effect, increasing mammary graft flow.(ABSTRACT TRUNCATED AT 250 WORDS)

Calcium antagonists and the renal hemodynamic response to vasoconstrictors

These findings demonstrate that calcium antagonists reverse renal vasoconstriction in a variety of settings. The ability of calcium antagonists to augment GFR of the vasoconstricted kidney is striking and has also been demonstrated in a number of in vivo settings. These observations and others raise the possibility that calcium antagonists have potential utility in the treatment of a number of disorders characterized by renal ischemia and consequent renal insufficiency. Further studies to evaluate this possibility are required. The unique effects of calcium antagonists on GFR reflect a regional heterogeneity within the renal microcirculation and a preferential action of calcium antagonists on the afferent arteriole. Final resolution of the pharmacological basis for the renal hemodynamic actions of calcium antagonists will require a more complete understanding of the divergent activating mechanisms within the renal microcirculation.

Multiple effects of calcium entry blockers on renal function in hypertension

Characterization of the renal effects of calcium entry blockers has not been easy because the inhibition of Ca2+ cellular influx alters several regulatory functions. The ability of calcium blockers to dilate renal vasculature and to increase glomerular filtration rate is largely determined by the preexisting vascular tone. However, the increments in sodium excretion could occur without alterations in renal hemodynamics. Calcium blockers could increase sodium excretion by inducing a redistribution of renal blood flow toward juxtamedullary nephrons, by inhibiting tubuloglomerular feedback responses, or by a direct action on the tubular transport of sodium. These effects are poorly understood at present. In vitro studies show that the blockade of calcium entry enhances renin secretion and decreases prostaglandin synthesis. This dissociation has not been found during long-term administration, which has been proved to be effective for the treatment of essential hypertension with normal maintenance of renal function. In this respect, there are reports indicating that calcium blockers are particularly effective in a subgroup of patients with essential hypertension who exhibit subtle but detectable alterations in calcium metabolism. Further studies are needed to determine whether this significant response to calcium blockers is due to correction of an early defect of calcium cellular kinetics that initiated the increase in blood pressure.