Effects of a high calcium diet and deoxycorticosterone on vascular smooth muscle responses in spontaneously hypertensive rats.

The effects of calcium and deoxycorticosterone (DOC) were studied in four groups of spontaneously hypertensive rats (SHR): control, calcium, DOC and DOC + calcium. Calcium was administered in drinking fluid as 1.5% calcium chloride, and DOC was injected weekly (25 mg/kg subcutaneously). During the 9-week study the increase in systolic blood pressure was enhanced in the DOC and attenuated in the calcium group, but did not differ from control values in the DOC + calcium group. DOC augmented in vitro contractions of aortic and mesenteric arterial rings induced by noradrenaline and impaired relaxations in response to nitroprusside and acetylcholine. Calcium alone enhanced the relaxation in response to nitroprusside in the mesenteric artery. In the DOC + calcium group vascular contractions did not differ from control values, but the relaxations caused by nitroprusside and acetylcholine were augmented in the mesenteric artery. The activity of erythrocyte Ca2(+)-ATPase increased in both calcium groups. The Na+:K+ ratio of tail artery tissue was reduced in the calcium group. In conclusion, calcium supplementation attenuates the development of hypertension, and prevents DOC-induced blood pressure increases in SHR by altering vascular reactivity. Changes in smooth muscle electrolyte ratios and Ca2(+)-ATPase activity may account for these alterations.