Endothelin-induced increases in Ca2+ entry mechanisms of vascular contraction are enhanced during high-salt diet

High-salt diet is often associated with increases in arterial pressure, and a role for endothelin (ET)-1 in salt-sensitive hypertension has been suggested; however, the vascular mechanisms involved are unclear. We investigated whether ET increases the sensitivity of the mechanisms of vascular contraction to changes in dietary salt intake. Active stress and 45Ca2+ influx were measured in endothelium-denuded aortic strips of male Sprague-Dawley rats not treated or chronically infused intravenously with ET (5 pmol/kg per minute) and fed either normal-sodium diet (NS, 1%) or high-sodium diet (HS, 8%) for 9 days. Phenylephrine (Phe) caused increases in active stress that were similar in NS and HS, but were greater in NS/ET (maximum, 10.5+/-0.7) than in NS (maximum, 7.4+/-0.9) rats, and further enhanced in HS/ET (maximum, 14.4+/-1.1) compared with HS rats (maximum, 8.0+/-0.8 x 10(4)N/m2). Phe was more potent in causing contraction in NS/ET than in NS rats and in HS/ET than in HS rats. In Ca2+-free (2 mmol/L EGTA) Krebs, stimulation of intracellular Ca2+ release by Phe (10(-5) mol/L) or caffeine (25 mmol/L) caused a transient contraction that was not significantly different in all groups of rats. In contrast, membrane depolarization by high-KCl solution, which stimulates Ca2+ entry from the extracellular space, caused greater contraction in ET-infused rats, particularly those on HS diet. Phe (10(-5) mol/L) caused an increase in 45Ca2+ influx that was greater in NS/ET (27.9+/-1.7) than in NS (20.1+/-1.8) rats and further enhanced in HS/ET (35.2+/-1.8) compared with HS rats (21.8+/-1.9 micromol/kg/min). The Phe-induced 45Ca2+ influx-stress relation was not different between NS and HS rats, but was enhanced in ET-infused rats particularly those on HS. The enhancement of the 45Ca2+ influx-active stress relation in ET-infused rats was not observed in vascular strips treated with the protein kinase C inhibitor GF109203X or calphostin C (10(-6) mol/L). Thus, low-dose infusion of ET, particularly during HS, is associated with increased vascular reactivity that involves Ca2+ entry from the extracellular space, but not Ca2+ release from the intracellular stores. The ET-induced enhancement of the Ca2+ influx-stress relation particularly during HS suggests activation of other mechanisms in addition to Ca2+ entry, possibly involving protein kinase C. The results suggest that ET increases the sensitivity of the mechanisms of vascular smooth muscle contraction to high dietary salt intake and may, in part, explain the possible role of ET in salt-sensitive hypertension.