EFFECTS OF PTH ON INTRACELLULAR CALCIUM OF MDCK CELLS IN CULTURE

Intracellular calcium [Ca2+](i) exerts multiple functions in renal epi thelia and is regulated by several hormonal factors, among them parath yroid hormone (PTH). The effect of PTH on [Ca2+](i) was investigated i n Madin-Darby canine (MDCK) kidney cells in culture. Changes in [Ca2+] (i) were monitored fluorometrically with the Ca2+-sensitive probe fura 2/AM, at 37 degrees C. The addition of PTH, in three cumulative doses , led to a significant and sustained raise in [Ca2+](i), in a medium w ith 1.36 mM [Ca2+](e). When Ca2+ was subtracted from the extracellular medium, [Ca(2+)0](e), the addition of PTH caused discrete changes on [Ca2+](i), suggesting that the hormone-induced alteration in [Ca2+](i) depend not only on [Ca2+](e), but also on intracellular stores of the ion. Nifedipine (NF) and verapamil (VP) partially blocked the PTH-ind uced increase in [Ca2+](i), when calcium was present in the medium, su ggesting that dihydropyridine sensible-type channels are present in th e plasma membrane (PM). PTH showed no effect on [Ca2+](i) in MDCK cell s previously treated with 2 mu M thapsigargine (TP) in [Ca(2+)0](e) co nditions but the hormone led to further fluctuations in [Ca2+](i) when [Ca2+](e) was restored to normal values. VP did not interfere with th e sharp increase in [Ca2+](i) induced in cells depleted from Ca2+, whi ch were reincubated in a normal Ca2+ medium, showing that calcium-rele ase-activated channels (CRAC) may be also present in PM. Our results s upport the view that MDCK cells are responsive to PTH, and that both v oltage-induced calcium channels (Ca2+-VOC) and CRAC are present in the PM of MDCK cells.