The Calcium-Mobilizing Agent, Thapsigargin, Inhibits Progesterone Production in Rat Luteal Cells by a Calcium-Independent Mechanism*

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Abstract
In rat luteal cells, an increase in intracellular [Ca]i impairs luteal function similar to that of prostaglandin F2a (PGF2a). However, calcium per se is not the mediator of the antigonadotropic action of PGF2a. Thapsigargin, a plant sesquiterpene lactone, increases intracellular calcium concentration concentration ([Ca]i) in several cell types by a mechanism that involves specific inhibition of the endoplasmic reticulum Ca2+- ATPase. To further investigate the antigonadotropic role of [Ca]i and the mechanism of action of PGF2a in rat luteal cells, the action of thapsigargin on cellular functional responses was examined in the absence and presence of PGF2a. Thapsigargin dose dependently increased [Ca]i and inhibited cAMP accumulation and progesterone production in response to LH. The inhibitory effect of thapsigargin on cAMP accumulation was calcium dependent but in contrast, inhibition of LH-stimulated progesterone production was independent of calcium mobilization by thapsigargin. Steroidogenesis stimulated by (Bu)2cAMP was inhibited by thapsigargin. Thus, thapsigargin mimicked some effects of PGF2a with inhibitory sites of action on both cAMP accumulation and progesterone production. Thapsigargin also blocked the mobilization of [Ca]i by PGF2a, but when coincubated with PGF2a an additive effect on inhibition of LH-stimulated progesterone production occurred. However, no additive effects of thapsigargin and PGF2a on gonadotropin-sensitive cAMP accumulation were evident. In conclusion, although thapsigargin and PGF2a may share some similar actions, their antigonadotropic effects are mediated differently.