Voltage-dependent calcium current in human decidual cells and its relation to prolactin secretion

Human decidual cells synthesize and release decidual PRL (dPRL) immunologically and biochemically identical to human pituitary PRL. However, stimulators and inhibitors of PRL secretion such as TRH, bromocriptine or dopamine have no effect on dPRL release. The evidence for the involvement of Ca2+ in dPRL release is based on contradictory or unclear data. Since little is known about Ca2+ movement in human decidual cells we studied the membrane Ca2+ conductance of cultured decidual cells using the patch-clamp technique in the whole-cell configuration. We report the existence of Ca(2+)-dependent action potentials triggered by hyperpolarizing or depolarizing pulses and blocked by cobalt (Co2+; 5 mM). Spontaneous action potentials were observed in the cell-attached mode and found also to be Co(2+)-sensitive. A tetrodotoxin-insensitive and Ca(2+)-dependent rapidly inactivating inward current was investigated in voltage clamp. Its activation threshold was between -60 and -45 mV. Indo-1 measurements of free intracellular Ca2+ concentrations ([Ca2+]i, 169 +/- 14 nM and 141 +/- 8 nM in short-term culture vs. 149 +/- 5 nM in cells cultured for 3-6 days) showed that decidual cells have spontaneous transient fluctuations of [Ca2+]i and that [Ca2+]i was decreased by Ca2+ channel blockers. The existence of Ca2+ movements in decidual cells in culture is thus demonstrated. The occurrence of action potentials in decidual cells derived from fibroblasts, reputed to be inexitable cells, is an interesting biological observation. However, Ca2+ is not involved in the short-term release of PRL by decidual cells, and its effects on long-term secretion still requires further investigation.