SHORT-TERM EFFECT OF PROLACTIN ON INTRACELLULAR CALCIUM IN CHINESE-HAMSTER OVARY CELLS STABLY TRANSFECTED WITH PROLACTIN RECEPTOR COMPLEMENTARY DEOXYRIBONUCLEIC-ACID

The mechanism of transduction of the PRL signal in target cells is poorly understood. We examined the effects of PRL on the intracellular free Ca-2+ concentration in Chinese hamster ovary cells overexpressing functional PRL receptors. [Ca2+](i) was determined by dual emission microspectrofluorimetry using indo-1 as the Ca2+ fluorescent probe. We demonstrate that at physiological concentrations (0.5-5 nM), PRL stimulates Ca2+ entry (type I) and/or induces a mobilization of calcium ions stored in intracellular compartments (type II). Two types of Ca2+ mobilization, distinguishable by their onset kinetics, were observed, a slow mobilization (type IIa; transition time to peak, similar to 10 sec) and a fast mobilization (type IIb; transition time to peak, <2 sec). PRL responses were delayed (15-120 sec) compared to the well known activation by phosphatidylinositol 4,5 bisphosphate hydrolysis-coupled receptors. This suggests that inositol trisphosphate is not involved in PRL response or that phosphatidylinositol 4,5 bisphosphate hydrolysis is not directly coupled to the PRL receptor. The amplitude of the PRL-induced Ca2+ increases (300-1400 nM) would be sufficient to provoke several physiological responses, such as stimulation of secretion, cell proliferation, or gene activation. However, the relation between the increase in Ca2+ and activation of milk protein genes remains to be established.