EFFECTS OF DIETARY CALCIUM ON BLOOD-PRESSURE, VASCULAR REACTIVITY AND VASCULAR SMOOTH-MUSCLE CALCIUM EFFLUX RATE IN ZUCKER RATS

Previous data from this laboratory indicate that hypertension in insulin resistant Zucker obese rats is accompanied by an impairment in vascular smooth muscle Ca2+ efflux. Since insulin resistant states are also generally salt-sensitive and dietary Ca2+ reduces blood pressure in some salt-sensitive states, we evaluated the effects of dietary Ca2+ on blood pressure and vascular reactivity and examined whether these effects are due to increased vascular smooth muscle Ca2+ efflux. We assigned 16 obese and 16 lean rats to a normal (0.5%) or high (1.5%) Ca2+ diet for 28 days, following which intraarterial blood pressure and in vitro vascular smooth muscle Ca-45 efflux and vascular reactivity responses to phenylephrine and serotonin were measured. Blood pressure was elevated in the obese rats on both diets (P < 0.2), and the high calcium diet lowered both systolic and diastolic pressure in both the lean and obese rats (P < 0.5). Vascular reactivity was higher in the obese rats (P < 0.2), but dietary Ca2+ exerted opposite effects on vascular reactivity to the agonists. High Ca2+ reduced sensitivity to serotonin in the obese rats by 54% (P < 0.5) without affecting sensitivity in the lean rats. In contrast, the high Ca2+ diet increased sensitivity to phenylephrine by 31% in both groups (P < .01). Ca-45 efflux was lower in the obese rats compared to the lean rats (P < .05), and the high Ca2+ diet increased this rate by 23% in the lean, but not the obese, rats (P < .05). Thus, the high Ca2+ diet caused a reduction in blood pressure in both lean and obese rats, but this effect may not be attributed solely to stimulation of vascular smooth muscle Ca-45 efflux, as this effect was observed only in the lean animals.