Effect of phosphate depletion on blood pressure and vascular reactivity to norepinephrine and angiotensin II in the rat.

Phosphate depletion (PD) adversely affects myocardial function but its influence on blood pressure is not well elucidated. In this study we evaluated whether PD influences blood pressure and/or affects its hormonal regulation or the peripheral vascular response to pressor agonists. Mean arterial pressure (MAP) in PD rats was lower than in normal animals, whereas heart rate was not significantly different between the two groups. Cardiac index (CI) in PD rats was lower and systemic vascular resistance (SVR) was higher than in controls. Plasma norepinephrine (NE) in the resting state and during the stress of immobilization was significantly greater in PD rats than in controls. Base-line plasma renin activity was also significantly higher in PD rats and increased similarly in the two groups of rats after administration of isoproterenol. Bolus injections of NE or angiotensin II produced a smaller rise in MAP in PD rats than in controls. Reduced responsiveness to NE was also demonstrated in isolated hind-limb preparation from PD rats. When NE was infused to achieve a rise in MAP of 30 mmHg, the dose required in PD was higher than in controls. Treatment with indomethacin did not affect the response in MAP to NE. The content and affinity of both alpha- and beta-receptors in PD hearts were not different from those of control hearts. The contents of PiATP, and AMP in the mesenteric vessels of PD rats were significantly lower than in control animals. These data show that PD leads to reductions in MAP, arteriolar response to pressor agents, and CI with appropriate compensatory rise in NE but with inadequate compensatory increase in SVR.(ABSTRACT TRUNCATED AT 250 WORDS)