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.


CONCLUSIONS
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+.
Touyz
R M
RM
Experimental Hypertension Laboratory, Institut de recherches cliniques de Montréal and Université de Montréal, Québec, Canada.
Schiffrin
E L
EL
D000804:Angiotensin II / Q000494:pharmacology*; D000818:Animals; D002118:Calcium / Q000172:deficiency / Q000378:metabolism / Q000502:physiology*; D002121:Calcium Channel Blockers / Q000494:pharmacology; D002478:Cells, Cultured; D005110:Extracellular Space / Q000378:metabolism; D006973:Hypertension / Q000473:pathology / Q000503:physiopathology*; D007425:Intracellular Membranes / Q000378:metabolism; D008297:Male; D009131:Muscle, Smooth, Vascular / Q000187:drug effects* / Q000473:pathology / Q000503:physiopathology*; D009994:Osmolar Concentration; D051381:Rats; D011918:Rats, Inbred SHR / Q000502:physiology*; D011921:Rats, Inbred WKY; D017208:Rats, Wistar
D002121:Calcium Channel Blockers; D000804:Angiotensin II; D002118:Calcium