Abstract

This study investigated the modulatory effect of magnesium (Mg⁺⁺) on basal and agonist-stimulated intracellular free calcium (Ca⁺⁺) concentration ([Ca⁺⁺]_i) in vascular smooth muscle cells from spontaneously hypertensive rats (SHR). Effects of increasing extracellular Mg⁺⁺ concentration ([Mg⁺⁺]_e) on vasopressin (AVP)-induced [Ca⁺⁺]_i responses were determined in primary cultured unpassaged vascular smooth muscle cells from mesenteric and aortic vessels (representing resistance and conduit arteries, respectively) of Wistar Kyoto rats (WKY) and SHR. [Ca⁺⁺]_i was measured by fura-2 methodology. Underlying mechanisms for Mg⁺⁺ actions were determined in Ca⁺⁺-free buffer and in the presence of diltiazem (10⁻⁶ M), an l-type Ca⁺⁺ channel blocker. Basal and AVP-stimulated [Ca⁺⁺]_iresponses were significantly increased (p < .05) in SHR (pD₂ = 8.3 ± 0.1, E_max = 532 ± 14 nM for SHR; pD₂ = 8.0 ± 0.04,E_max = 480 ± 15 nM for WKY). [Mg⁺⁺]_e dose-dependently reduced basal and agonist-induced [Ca⁺⁺]_i responses. High [Mg⁺⁺]_e (4.8 mM) attenuated [Ca⁺⁺]_i responses to AVP in WKY (E_max = 328 ± 30 nM) and SHR (E_max = 265 ± 27 nM) and normalized AVP-elicited hyper-responsiveness in SHR (pD₂ in high [Mg⁺⁺]_e, 8.1 ± 0.3 for SHR, 7.8 ± 0.6 for WKY). Extracellular Ca⁺⁺ withdrawal and diltiazem abolished the attenuating effects of high [Mg⁺⁺]_e in WKY but not in SHR. These findings demonstrate that Mg⁺⁺ dose-dependently reduces [Ca⁺⁺]_i and that high [Mg⁺⁺]_e attenuates AVP-stimulated [Ca⁺⁺]_i responses and normalizes sensitivity to AVP in SHR. In WKY, Mg⁺⁺ actions are dependent primarily on Ca⁺⁺ influx through l-type Ca⁺⁺channels, whereas in SHR, the modulatory effects of [Mg⁺⁺]_e are mediated both by Ca⁺⁺influx through Ca⁺⁺ channels and by intracellular Ca⁺⁺ release.