Effect of dietary calcium on in vitro aortic tissue responsiveness to a hypertensive factor

It has been proposed that calcium supplementation in the diet is associated with a reduction in blood pressure. In the present study, we investigated vascular tissue sensitivity to a hypertensive factor (HF) in spontaneously hypertensive rats (SHR) and normotensive Wistar Kyoto rats (WKY) fed a high calcium diet, a low calcium diet and a food restricted diet. HF, which has been isolated from erythrocytes, increases blood pressure when injected into normotensive rats and stimulates calcium uptake by aortic rings in vitro. Five-week-old rats were divided into the following groups: SHR and WKY fed a regular diet (1% calcium), SHR and WKY fed a high calcium diet (4% calcium), SHR and WKY fed a low calcium diet (0.02% calcium) and SHR and WKY fed a regular diet (1% calcium) in which food intake was restricted to 65% of ad libitum intake. Food intake, body weight, urine phosphate excretion and blood pressure development were followed for 8 weeks. At sacrifice, plasma levels of calcium and phosphate were determined. Tissue responsiveness to HF was calculated by incubating aortic rings from the rats in the different groups with HF and measuring lanthanum-resistant calcium uptake. A 4-fold increase in dietary calcium reduced blood pressure and tissue responsiveness to HF in SHR. Neither parameter was affected by the high calcium diet in WKY. The low calcium diet had no effect on either blood pressure or tissue responsiveness to HF in SHR or WKY. Restriction of food intake induced a reduction in blood pressure and in tissue responsiveness to HF in SHR. It did not affect the same parameters in WKY. The results suggest that the increased tissue responsiveness to HF in the SHR may be associated with high blood pressure.