Action of parathyroid extract on arterial blood pressure and on contraction and 45Ca exchange in isolated aorta of the rat.

The acute effects of parathyroid extract (PTE) on the cardiovascular system were the object of the study. PTE injected at concentrations of 10--100 U USP/kg into conscious rats induced a rapid, transitory fall in systolic blood pressure and a decrease in the total peripheral resistance. Since the cardiac output was unchanged, PTE must act on vascular smooth muscle. In vitro, PTE (2--16 U USP/ml) inhibited the tonic phase (which requires the presence of extracellular calcium) of the contraction of the thoracic aorta in response to phenylephrine or noradrenaline. 45Ca fluxes in the isolated aorta, measured by the lanthanum method, changed in the presence of a high concentration of PTE (1.0 U USP/ml). After 2 min, the influx was lowered and the efflux was enhanced. These effects may have been specific, since they did not appear when PTE had been previously inactivated. Our results suggest that the vasodilator effect of PTE in the rat is due to an action on Ca exchange in vascular smooth muscle.