The diltiazem analog TA‐3090 mimics the actions of high extracellular Ca 2+ on parathyroid function in dispersed bovine parathyroid cells

We previously showed that the calcium channel blocker diltiazem raises cytosolic Ca 2+ 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 Ca 2+ 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 Ca 2+ and raised the cytosolic Ca 2+ concentration by 25‐50% in fura‐2‐loaded parathyroid cells in the presence but not in the absence of extracellular Ca 2 , suggesting that it activated rather than inhibited Ca 2+ channels. To determine whether this compound affects other aspects of parathyroid function, we examined its effects on the inhibition of cAMP accumulation by Ca 2+ , a process we recently found to involve inhibition of cAMP generation by G 1 through a receptorlike mechanism, which is independent of changes in cytosolic Ca 2+ , 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 Ca 2+ concentrations. Moreover, the addition of 10 −4 M TA‐3090 potentiated the inhibitory effects of both Ca 2+ and Mg 2+ , decreasing the concentration of the divalent cation necessary to produce half‐maximal inhibition of cAMP accumulation by about twofold. In the absence of extracellular Ca 2+ , however, TA‐3090 had no effect on the stimulation of cAMP by dopamine or on the inhibition of dopamine‐stimulated cAMP accumulation by PGF 2a , which also regulates cAMP via G 1 . Finally, the effects of TA‐3090 on the inhibition of cAMP by Ca 2+ were totally abolished following preincubation with pertussis toxin for 20‐24 h. These data suggest that TA‐3090 not only modulates the function of bovine parathyroid cells at the level of Ca 2+ channels per se but also may affect cAMP metabolism by potentiating the effects of high extracellular Ca 2+ concentrations at or near the putative Ca 2+ “receptor” or “sensor.”