Effect of oxytocin on free intracellular Ca2+ levels and progesterone release by human granulosa-lutein cells.

Oxytocin and its receptor are found in the corpus luteum in a variety of species, including the human. In the present study we used fura-2 microfluorimetry to investigate whether activation of the oxytocin receptor of cultured human granulosa-lutein cells causes intracellular calcium (Ca2+) signals and affects progesterone release. Although after 1 day in culture, cells were not responsive to oxytocin, the number of responsive cells increased steadily during the first 3 days in culture, reaching a maximum on days 4 and 5 (59-66%) and then declined again until day 8. Effective oxytocin concentrations were apparently independent of the culture day, and concentrations as low as 10 nmol/L increased intracellular free Ca2+ levels from 70-140 nmol/L (basal levels) to maximal peak levels of 800 nmol/L. The oxytocin-induced Ca2+ signal was not affected by removal of extracellular Ca2+ with EGTA. Moreover, depletion of intracellular Ca2+ stores by ionomycin treatment rendered the cells unresponsive to oxytocin, pointing also at the intracellular source of the oxytocin-inducible Ca2+ signal. Interestingly, after one single stimulation with oxytocin, cells became refractory to additional stimuli, and only extremely high concentrations of oxytocin induced a second increase in intracellular free Ca2+. To examine the possible effects of oxytocin on progesterone release by cultured cells, we incubated cells on culture day 2 (20% responsive cells in the fura measurements) and culture day 5 (66% responsive cells in the fura measurements) for 24 h with oxytocin (10 nmol/L) and hCG (10,000 IU/L). Although hCG significantly stimulated progesterone release, oxytocin alone was without a stimulatory effect on either day. However, a significant augmentation of the effect of hCG on progesterone release was found in incubations of cells on day 5. Interestingly, the effects of hCG also included stimulation of oxytocin release by cultured granulosa-lutein cells into the culture medium, as determined by RIA. In summary, our data indicate the presence of a functional oxytocin receptor on human granulosa-lutein cells that is linked to Ca2+ as a second messenger released from intracellular Ca2+ stores. The number of oxytocin-responsive cells increases during differentiation in culture. Moreover, oxytocin release induced by hCG and a stimulatory effect of oxytocin on the hCG-induced progesterone production during the period of maximal responsiveness of cultured cells were found. We, therefore, propose that oxytocin may have autocrine and/or paracrine functions in human granulosa-lutein cells, including fine-tuning of progesterone release.