The diltiazem analog TA-3090 mimics the actions of high extracellular Ca2+ on parathyroid function in dispersed bovine parathyroid cells

Abstract

We previously showed that the calcium channel blocker diltiazem raises cytosolic Ca2+ and inhibits PTH release in bovine parathyroid cells. To investigate further possible mechanisms underlying these effects, we examined the effects of the more potent diltiazem analog TA-3090, which is a Ca2+ channel antagonist in vascular smooth muscle, on several aspects of the function of dispersed bovine parathyroid cells. Like diltiazem, TA-3090 (10(-6)-10(-4] produced a dose-dependent inhibition of immunoreactive PTH release at 0.5 mM Ca2+ and raised the cytosolic Ca2+ concentration by 25-50% in fura-2-loaded parathyroid cells in the presence but not in the absence of extracellular Ca2+, suggesting that it activated rather than inhibited Ca2+ channels. To determine whether this compound affects other aspects of parathyroid function, we examined its effects on the inhibition of cAMP accumulation by Ca2+, a process we recently found to involve inhibition of cAMP generation by Gi through a receptorlike mechanism, which is independent of changes in cytosolic Ca2+. TA-3090 (10(-4) M) inhibited dopamine-stimulated cAMP accumulation by up to 75% (from 663 to 166 fmol per 10(5) cells), with a higher apparent potency at greater extracellular Ca2+ concentrations. Moreover, the addition of 10(-4) M TA-3090 potentiated the inhibitory effects of both Ca2+ and Mg2+, decreasing the concentration of the divalent cation necessary to produce half-maximal inhibition of cAMP accumulation by about twofold. In the absence of extracellular Ca2+, however, TA-3090 had no effect on the stimulation of cAMP by dopamine or on the inhibition of dopamine-stimulated cAMP accumulation by PGF2 alpha, which also regulates cAMP via Gi.(ABSTRACT TRUNCATED AT 250 WORDS)