12-O-tetradecanoyl-phorbol-13-acetate decreases influx of extracellular Ca2+ induced by depolarization in GH4C1 cells: effects of pretreatment with 1,25-dihydroxycholecalciferol.

In GH4C1 rat pituitary cells, 1,25-dihydroxycholecalciferol (1,25(OH)2D3) causes amplification of both the TRH-induced spike phase in cytosolic free calcium [( Ca2+]i) and the increase in [Ca2+]i induced by depolarization with K+. In the present study we investigated the actions of 12-O-tetradecanoyl-phorbol-13-acetate (TPA) on Ca2(+)-homeostasis in GH4C1 cells pretreated with 1,25(OH)2D3 for 24 h. In control and 1,25(OH)2D3-pretreated cells, incubation with TPA (0.1-300 nM) for 15 min in the presence of 45Ca2+ did not affect the basal uptake of 45Ca2+. However, if the cells were treated with 50 mM K+, TPA induced a time- and concentration-dependent decrease in depolarization-induced net 45Ca2+ uptake. A maximal decrease of 30-50% was observed with 100-300 nM TPA, 1,25(OH)2D3 pretreated cells being more responsive to the action of TPA than control cells. sn-1-Oleoyl-2-acetyl-glycerol, which mimics the action of TPA on protein kinase C (PKC), did not alter depolarization-induced uptake of 45Ca2+. Two agents which inhibit PKC activity, polymyxin B and K252A, did not prevent the effect of TPA on depolarization-induced uptake of 45Ca2+, whereas staurosporin totally inhibited the action of TPA. In Fura-2 loaded cells pretreated with 1,25(OH)2D3, incubation with 200 nM TPA for 9 min decreased the depolarization-induced spike and plateau phases of change in [Ca2+]i; only the spike phase was decreased in control cells. TPA did not affect basal [Ca2+]i in either group. Treatment with TPA for only 3 min decreased the TRH-induced spike in [Ca2+]i only in 1,25(OH)2D3 pretreated cells; however, after a 5-min treatment with TPA, the TRH-induced spike in [Ca2+]i was decreased in both control and 1,25(OH)2D3 pretreated cells. TPA did not affect the spike in [Ca2+] induced by 50 nM ionomycin. Na+/Ca2+ exchange was not altered by TPA, nor did TPA enhance efflux of 45Ca2+ from cells preloaded with 45Ca2+ for 2.5 h. We conclude that, in GH4C1 cells, TPA modulates plasma membrane calcium flux, probably via an inhibitory action on voltage-operated Ca2+ channels. This inhibitory action may be independent of activation of PKC, and 1,25(OH)2D3 pretreated cells are more responsive to the actions of TPA than are control cells. These results are consistent with our previous findings that 1,25(OH)2D3 enhances voltage-dependent Ca2+ channel activity in GH4C1 cells.