THE RELEASE OF HUMAN CHORIONIC-GONADOTROPIN AND PLACENTAL-LACTOGEN BY PLACENTAL EXPLANTS CAN BE STIMULATED BY CA2+ ENTRY THROUGH A NA+-CA2+ EXCHANGE

Isosmotical replacement of extracellular Na+ ([Na+]0) by K+, choline and to a lesser extent by saccharose stimulated the release of chorionic gonadotrophin and placental lactogen from human term placental explants. The effect of [Na+]0 removal on the release of both hormones was concentration-dependent and was inhibited in the absence of extracellular Ca2+ or in the presence of 0.5 mM Co2+, a Ca2+ entry blocker. Blockers of the voltage-sensitive Ca2+ channels (20μM nifedipine and 50μM methoxyverapamil) or Na+ channels (5μM tetrodotoxin) did not affect the stimulatory effects of [Na+]0 omission. By contrast, Mg2+ and Sr2+ (10 mM) as well as amiloride (2 mM) and its analogue 2′, 4′-dimethylbenzamil (50μm), all known to affect the Na+−Ca2+ exchange, markedly reduced the increase in hormone release elicited by [Na+]0 removal. Lastly, the secretory responses to [Na+]0 deprivation were increased in the presence of 2 mM ouabain, an inhibitor of the Na+−K+ ATPase. These results indicate for the first time that [Na+]0 omission provokes a Ca2+-dependent stimulation of human chorionic gonadotrophin and placental lactogen releases. The pharmacological dissection of the secretory effects of [Na+]0 removal supports the existence of a process of Na+−Ca2+ exchange in placental cells. © 1994, W. B. Saunders Company Ltd.. All rights reserved.