Parathyroid hormone and calcium entry blockade in a vascular tissue.

The mechanism of the vasorelaxing action of a synthetic parathyroid hormone fragment, bPTH-(1-34), was studied. Rat tail artery helical strips were constricted in vitro with potassium chloride (6 x 10(-2) M), arginine vasopressin (2 mU/ml) or BAY-K-8644 (3 x 10(-7) M in the presence of 1.5 x 10(-2) M KCl). bPTH-(1-34) was able to relax the constricted tissue or to inhibit the constriction. All three constricting agents increased calcium uptake by the vascular tissue as determined by the measurement of the low-affinity lanthanum-resistant pool of calcium. Such increases in calcium uptake were significantly reduced by bPTH-(1-34). These data suggest that PTH may be a natural circulating hormone or chemical capable of inhibiting calcium entry in a vascular tissue.