Vol. 284, Issue 3, 998-1005, March 1998

Effect of Magnesium on Calcium Responses to Vasopressin in Vascular Smooth Muscle Cells of Spontaneously Hypertensive Rats¹

Rhian M. Touyz, Pascal Laurant and Ernesto L. Schiffrin

MRC Multidisciplinary Research Group on Hypertension (R.M.T., E.L.S.), Clinical Research Institute of Montreal, University of Montreal, Montreal (Quebec) Canada, Laboratoire Physiologie (P.L.), Pharmacologie et Nutrition Préventive Expérimentale, UFR Médecine et Pharmacie, Université de Franche-Comté, Besançon, France

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 (10⁶ M), an L-type Ca⁺⁺ channel blocker. Basal and AVP-stimulated [Ca⁺⁺]_i responses 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.