Angiotensin II-induced Changes Research Articles

Effects of a nonpeptide angiotensin II receptor antagonist (CV-11974) on [Ca 2+] i and cell motion in cultured ventricular myocytes

We studied the effects of angiotensin II and CV-11974 (a newly synthesized angiotensin II receptor antagonist) on cell contraction and [Ca 2+] i in cultured neonatal rat ventricular myocytes. Exposure of these cells to 10 nM angiotensin II significantly decreased peak systolic cell position (60.1±3.3% of the control, P < 0.01) and peak systolic [Ca 2+] i (from 1111 ± 250 to 572 ± 143 nM, P < 0.05) within 60 s. Pretreatment of ventricular myocytes with CV-11974 (10–100 nM) completely suppressed the angiotensin II-induced changes in peak systolic cell position and [Ca 2+] i. These results suggest that CV-11974 inhibits cardiac angiotensin II receptors.

Effects of chronic treatment with caffeine on kidney responses to angiotensin II

Previous studies demonstrate that chronic administration of caffeine causes glomerular filtration to deteriorate in rats with high-renin renovascular hypertension. A partial explanation for these findings could be that chronic administration of caffeine alters the effects of angiotensin II on the kidney. As an initial test of this hypothesis, we compared the acute effects of intrarenal infusions of angiotensin II (3 ng/min) on renal function in control rats versus rats treated with 0.1% caffeine in their drinking water for 1 week. The renal responses to angiotensin II in a group of animals receiving acute intrarenal infusions of adenosine (10 μg/min) were also measured to determine whether caffeine and adenosine modulated renal responses to angiotensin II in opposite directions. All studies were performed in the in situ blood perfused rat kidney. Neither caffeine nor adenosine significantly altered angiotensin II-induced changes in renal blood flow, urinary excretory function or renin release. However, caffeine augmented and adenosine attenuated the increase in filtration fraction caused by angiotensin II. The fact that caffeine potentiates angiotensin II-induced increases in filtration fraction without affecting angiotensin II-induced reductions in renal blood flow is consistent with, but does not prove, the hypothesis that chronic administration of caffeine modifies the effects of angiotensin II on the renal microvasculature. If this inference is correct, caffeine could facilitate renal damage in high-renin hypertension by exacerbating angiotensin II-induced increases in glomerular capillary hydrostatic pressure.

Effect of angiotensin II-induced changes in perfusion flow rate on chlorothiazide transport in the isolated perfused rat kidney.

Angiotensin II was used as a probe to study the effect of changes in perfusate flow rate on the renal clearance parameters of chlorothiazide in the isolated perfused rat kidney. Perfusion studies were performed in five rats with no angiotensin II present in the perfusate and in five rats with a 1-4 ng/min infusion of angiotensin II into the perfusate. Angiotensin II had a dramatic effect on the renal hemodynamics, resulting in a 43% decrease in perfusate flow, a 16% decrease in glomerular filtration rate (GFR), and a 45% increase in filtration fraction. Values for the fractional excretion of glucose were low and consistent, with or without angiotensin II. Although the unbound fraction (fu) of chlorothiazide was unchanged between treatments, the renal (CLr) and the secretion clearances were reduced by about 50% in the presence of angiotensin II; the excretion ratio [ER = CLr/(fu.GFR)] was reduced by 38% with angiotensin II present in the perfusate. Analysis of the data was complicated by the presence of a capacity-limited transport for renal tubular secretion. Transport parameters (+/- SD) were obtained and the corrected intrinsic secretory clearance [(Vmax/GFR)/Km] of chlorothiazide was 123 +/- 18 without angiotensin II vs. 72.8 +/- 30.0 with angiotensin II. These results demonstrate that alterations in organ perfusion can significantly reduce the clearance parameters of chlorothiazide in the rat IPK. These flow-induced changes in intrinsic secretory transport may reflect perturbations other than that of perfusion flow rate alone.

Angiotensin II-induced changes in guanine nucleotide binding and regulatory proteins.

Systemic infusion of angiotensin II, a potent agonist, using doses that are initially subpressor, eventually produces sustained blood pressure elevation and reductions in glomerular capillary ultrafiltration coefficient characterized by enhanced signal transduction to angiotensin II and other agonists. In this setting, there is a significant increased affinity of angiotensin II binding to smooth muscle and glomerular mesangial receptors and enhanced sensitivity and magnitude of angiotensin II-induced decrements in cyclic AMP. Since G proteins are important modulators of binding and signal transduction, the present studies were designed to test the hypothesis that differences in the relative amounts of G proteins may be present and have accounted for differences observed. G proteins were identified and quantitated by isoelectric focusing/sodium dodecyl sulfate-polyacrylamide gel electrophoresis, radiolabeling in the presence of activated toxins with [gamma-32P]NAD+, immunoprecipitation, and immunoblotting. A 168% and 465% increase in pertussis toxin-catalyzed ADP ribosylation of alpha 40-41 was found in angiotensin II-treated groups over control groups for glomerular and mesenteric membranes, respectively. Immunoblotting revealed a 250% and 35% increase in the levels of the Gi isoforms alpha i-2 and alpha i-3, respectively, and a decrease of 53% in alpha i-1 from the angiotensin II-treated group. No differences were observed in cholera toxin labeling or immunoblotting of Gs. These results demonstrate multiple mechanisms whereby angiotensin-induced signal transduction can be modulated involving both the receptors and G proteins. These observed differences in G proteins in systemic and renal vasculature accompanying angiotensin II infusion suggest the possibility of a regulatory role in the pathophysiology of angiotensin II-induced hypertension and renal disease.

Angiotensin II-induced changes in noradrenaline overflow in the rat tail artery: E. Thorin, D. Henrion and J. Atkinson, Facultéde Pharmacie, Laboratoire de Pharmacologie, 5 rue Albert Lebrun, 54000 Nancy, France

Angiotensin II-induced changes in noradrenaline overflow in the rat tail artery: E. Thorin, D. Henrion and J. Atkinson, Facultéde Pharmacie, Laboratoire de Pharmacologie, 5 rue Albert Lebrun, 54000 Nancy, France

Different Patterns of Agonist‐Stimulated Increases of 3H‐Inositol Phosphate Isomers and Cytosolic Ca2+ in Bovine Adrenal Chromaffin Cells: Comparison of the Effects of Histamine and Angiotensin II

Bovine adrenal chromaffin cells (BCC) were used to compare histamine- and angiotensin II-induced changes of inositol mono-, bis-, and trisphosphate (InsP1, InsP2, and InsP3, respectively) isomers, intracellular free Ca2+ ([Ca2+]i), and the pathways of inositol phosphate metabolism. Both agonists elevated [Ca2+]i by 200 nM 3-4 s after addition, but afterwards the histamine response was much more prolonged. Histamine and angiotensin II also produced similar four- to fivefold increases of Ins(1,4,5)P3 that peaked within 5 s. Over the first minute of stimulation, however, Ins(1,4,5)P3 formation was monophasic after angiotensin II, but biphasic after histamine, evidence supporting differential regulation of angiotensin II- and histamine-stimulated signal transduction. The metabolism of Ins(1,4,5)P3 by BCC homogenates was found to proceed via (a) sequential dephosphorylation to Ins(1,4)P2 and Ins(4)P, and (b) phosphorylation to inositol 1,3,4,5-tetrakisphosphate, followed by dephosphorylation to Ins(1,3,4)P3, Ins(1,3)P2, and Ins(3,4)P2, and finally to Ins(1 or 3)P. In whole cells, Ins(1 or 3)P only increased after histamine treatment. Additionally, Ins(1,3)P2 was the only other InsP2 besides Ins(1,4)P2 to accumulate within 1 min of agonist treatment [Ins(3,4)P2 did not increase]. These results support a correlation between the time course of Ins(1,4,5)P3 formation and the time course of [Ca2+]i transients and illustrate that Ca2(+)-mobilizing agonists can produce distinguishable patterns of inositol phosphate formation and [Ca2+]i changes in BCC. Different patterns of second-messenger formation are likely to be important in signal recognition and may encode agonist-specific information.

Angiotensin II-induced changes in cochlear blood flow and blood pressure in normotensive and spontaneously hypertensive rats

Previous investigations in our laboratory have measured significant increases in the circulating levels of the potent vasoconstric-tive hormone, angiotensin II (AII; 26 and 64 pg/100 μl plasma, normal and noise exposed, respectively), during and following noise exposure in the alert rat (Wright et al., 1981). In the present study, these levels were approximated through intra-arterial infusion in the anesthetized spontaneously hypertensive rat (SHR) and normotensive Wistar-Kyoto (WKY) rat. Laser Doppler flowmeter measurements of cochlear blood flow (CBF) indicated that despite equivalent All-induced elevations in systemic blood pressure, CBF in the SHR did not increase to the levels measured in the WKY. Pretreatment with the specific angiotensin receptor antagonist sarile, (Sar 1,Ile 8-AII), reduced All-induced elevations in systemic blood pressure in members of both strains, but did not change the overall pattern of CBF. These results indicate that SHRs may have a compromised cochlear circulation that is refractory to increases in systemic blood pressure.

Role of calcium fluxes in the sustained phase of angiotensin II-mediated aldosterone secretion from adrenal glomerulosa cells.

When angiotensin II stimulates aldosterone secretion, it causes a rapid but transient mobilization of calcium from an intracellular pool and a sustained increase in the influx of calcium in adrenal glomerulosa cells. The present studies were undertaken to determine the respective roles of the two angiotensin II-induced changes in cellular calcium metabolism in modulating events during the sustained phase of cellular response which is thought to be mediated by the C-kinase branch of the calcium messenger system. The sustained response to angiotensin II is only 50% of maximal in cells pretreated with dantrolene in a concentration sufficient to inhibit the angiotensin II-induced mobilization of intracellular calcium. Also, if A23187 is added to cells simultaneously with 1-oleoyl-2-acetylglycerol (OAG), the aldosterone secretory response is similar to that seen after angiotensin II. However, if A23187 is added first and the transient aldosterone secretory response allowed to decay, and OAG then added, the sustained aldosterone secretory response is only 45-50% of maximal. Addition of the calcium channel agonist, BAY K 8644, with OAG leads to an aldosterone secretory response which is only 50% of maximal, but if upon addition of OAG and BAY K 8644 the cells are also exposed for 5 min to media containing 8 mM K+, then the sustained secretory response is maximal. These data imply that the initial transient rise in the [Ca2+] of the cell cytosol plays a role in determining the extent to which C-kinase is shifted from its calcium-insensitive to its calcium-sensitive form. The second group of experiments examined the relationship between the sustained angiotensin II-induced increase in plasma membrane calcium influx and the sustained aldosterone secretory response. The results show that in the presence of 1 microM nitrendipine or 2 mM extracellular K+, angiotensin II causes no increase in calcium influx and only a transient rather than a sustained increase in the rate of aldosterone secretion indicating that the sustained phase of the response is dependent upon a continued high rate of Ca2+ influx which regulates the rate of turnover of the activated C-kinase.

Angiotensin increases inositol trisphosphate and calcium in vascular smooth muscle.

Angiotensin II stimulated the breakdown of phosphatidylinositol-4,5-bisphosphate (PIP2) and the generation of inositol trisphosphate (IP3) in cultured rat aortic smooth muscle cells. The decrease in PIP2 and increase in IP3 levels were rapid (measurable at 5 seconds; maximum IP3 levels at 15 seconds). The time course of these changes was comparable to that of angiotensin II-induced increases in cytosolic free calcium, as measured by the calcium-sensitive fluorescent indicator quin 2. The IP3 formation was not stimulated by the calcium ionophore A23187 (5 microM), nor were angiotensin II-induced changes in IP3 formation inhibited by the removal of extracellular calcium with EGTA. Angiotensin II appears to be capable of generating more IP3 than is required for maximal release of intracellular calcium. These data are consistent with the hypothesis that generation of IP3 plays a role in the angiotensin II-induced mobilization of calcium from intracellular storage sites in vascular smooth muscle cells.

Angiotensin increases cytosolic free calcium in cultured vascular smooth muscle cells.

We used the calcium-sensitive fluorescent indicator quin 2 to monitor changes in cytosolic free calcium concentration ([Ca2+]i) associated with angiotensin II receptor activation in cultured vascular smooth muscle cells isolated from rat aorta. Resting [Ca2+]i in unstimulated vascular smooth muscle cells was 198 +/- 7 nM. Angiotensin II induced concentration-dependent rapid increases in [Ca2+]i (threshold congruent to 10(-11) M; effective concentration, 50% congruent to 5 X 10(-10) M; maximum congruent to 10(-8) M); the rate of increase in [Ca2+]i also appeared to be concentration dependent. The angiotensin II-induced changes were completely blocked by the angiotensin II receptor antagonist [Sar1, Ile8]-angiotensin II. In the presence of extracellular calcium, 10(-8) M angiotensin II induced an increase in [Ca2+]i that reached peak values of five to six times the resting levels within 15 seconds, followed by a gradual decline to a plateau at two to three times the resting level. When EGTA was added to chelate external calcium, the angiotensin II-induced increases in peak [Ca2+]i were attenuated and the plateau phase was abolished. These data show that (1) quin 2 can be used in cultured vascular smooth muscle cells to study changes in calcium homeostasis induced by angiotensin II, and (2) angiotensin II acts on cultured vascular smooth muscle cells to cause a rapid increase in [Ca2+]i that appears to depend on both the mobilization of intracellular calcium and the influx of extracellular calcium.