Role of calcium influx and intracellular calcium stores in angiotensin II-mediated calcium hyper-responsiveness in smooth muscle from spontaneously hypertensive rats

Objective To investigate post-receptor mechanisms that underlie enhanced angiotensin II (Ang II)-stimulated cytosolic free Ca2+ concentration ([Ca2+](i)) responses in vascular smooth muscle cells from small arteries of SHR. Methods To determine whether Ca2+ influx is altered in SHR, effects of Ca2+ channel antagonists (nitrendipine and diltiazem) and depletion of extracellular Ca2+ on Ang II-stimulated [Ca2+](i) responses in primary cultured unpassaged vascular smooth muscle cells from mesenteric arteries of spontaneously hypertensive rats (SHR), Wistar and Wistar-Kyoto (WKY) rats aged 17 weeks were studied. To assess whether Ca2+ stores contribute to increases in Ang II-stimulated Ca2+ mobilization and [Ca2+](i) in SHR, cells were exposed to thapsigargin, a selective reticular Ca2+-ATPase inhibitor. [Ca2+](i) was measured by fura-2 methodology. Results Basal and 1 nmol/l Ang II-stimulated [Ca2+](i) were significantly greater in SHR cells (123 +/- 7.1 nmol/l basal; 268 +/- 7.0 nmol/l stimulated) than they were in those from WKY rats (88 +/- 4.8 nmol/l basal; 221 +/- 8.6 nmol/l stimulated) and Wistar rats (85 +/- 3.0 nmol/l basal; 216 +/- 8.3 nmol/l stimulated). In Ca2+-free medium, basal and Ang II-induced [Ca2+](i) were reduced in all groups, but Ang II-stimulated [Ca2+](i) responses were still significantly enhanced in SHR cells compared with those in Wistar and WKY rat cells (205 +/- 11.2 versus 173 +/- 8.0 and 161 +/- 2.6 nmol/l, respectively). Administrations of 10(-6) mol/l diltiazem and 10(-7) mol/l nitrendipine decreased Ang II-elicited [Ca2+](i) responses and normalized basal [Ca2+](i) in SHR cells. The inhibition induced by Ca2+ channel antagonists was greater (P < 0.05) in WKY and Wistar rat cells than it was in those from SHR. Administration of thapsigargin, in Ca2+-free buffer, induced a greater (P < 0.05) dose-dependent [Ca2+](i) increase in SHR cells than it did in WKY rat cells. Administration of 1 nmol/l Ang II increased [Ca2+](i) in thapsigargin-pretreated cells of SHR but not in those of WKY rats. Conclusion Different mechanisms contribute to increases in basal and Ang II-stimulated [Ca2+](i) responses in vascular smooth muscle cells from small arteries of SHR, which contribute to elevated peripheral resistance in hypertension. Increases in basal [Ca2+](i) may be partly due to augmentation of Ca2+ influx, whereas Ang II-induced [Ca2+](i) hyper-responsiveness might depend primarily on Ca2+ mobilization rather than on influx of extracellular Ca2+.