In a recent series of studies, the hypotensive action of parathyroid hormone (PTH) was demonstrated in several vertebrate species. In mammals, the mechanism of this direct action on specific vascular beds was studied. An increase in intracellular cyclic AMP and an inhibition of entry of extracellular calcium into the vascular tissue was correlated with the vasorelaxing action of PTH. In the present report, PTH was found to relax chicken mesenteric vessels in vitro in the presence or absence of constriction with concentrated potassium chloride (KC1). PTH produced dose-related inhibition of the chicken vessel constricted with 10, 20, 40, or 60 mM KC1. Extracellular calcium-dependent constriction of the chicken mesenteric artery was also decreased by PTH. An inhibitory effect was also observed with D600, a known calcium-entry inhibitor, in the above in vitro test systems. The low affinity lanthanum-resistant pool of calcium, which supposedly reflects the rate of calcium entry into cells, was also decreased by PTH in the control or KC1stimulated chicken mesenteric artery. These data suggest that PTH may produce vasorelaxation in chicken mesenteric artery by inhibition of calcium entry into cells. This vessel from the chicken seems to have a high turnover rate of calcium and may be a good model for studying the effects of substances on calcium entry in vascular smooth muscle.
The hypotensive action of parathyroid hor-shown in all vertebrates thus far studied. mone has a long history. When Collip and These include the South American lungfish, Clark first extracted the active principle from bullfrog, water snake, chicken, rat, and dog the gland, they ran a number of physiologi- (Pang et al., '80a,b). In the bullfrog, we cal tests, one of them being the blood pres-showed that PTH was effective in producing