NEOMYCIN MIMICS THE EFFECTS OF HIGH EXTRACELLULAR CALCIUM CONCENTRATIONS ON PARATHYROID FUNCTION IN DISPERSED BOVINE PARATHYROID CELLS

We examined the effects of the polycationic antibiotic, neomycin, on the function of dispersed bovine parathyroid cells. Neomycin caused a reversible, dose-dependent inhibition of low calcium (Ca++)-stimulated PTH release, with half-maximal inhibition at 30-mu-M. Maximal inhibition (with 200-mu-M neomycin) was not additive with the suppressive effects of high (2 mM) Ca++. Neomycin also inhibited dopamine-stimulated cAMP accumulation by 90-98% at 100-200-mu-M, with a half-maximal effect at 40-50-mu-M. This action was reversible and was blocked by preincubating the cells overnight with 0.5-mu-g/ml pertussis toxin. In addition to its suppressive effects on cAMP metabolism and PTH release, neomycin stimulated the accumulation of inositol phosphates and produced a transient increase in the cytosolic Ca++ concentration (Ca(i)) in fura-2-loaded parathyroid cells. The neomycin-evoked spike in Ca(i) persisted despite removal of extracellular Ca++, indicating that it arises from intracellular Ca++ stores. Exposure of cells to elevated magnesium (Mg++) concentrations elicited a similar spike in Ca(i) but blocked the spike in Ca(i) in response to subsequent addition of neomycin and vice versa. Thus, Mg++ and neomycin mobilize Ca++ from the same intracellular store(s). These results indicate that a polycation, neomycin, closely mimics the effects of polyvalent cations on parathyroid function, suggesting that both agents regulate parathyroid function via similar biochemical pathways.