The regulation of steroidogenesis is different in the two types of ovine luteal cells

Abstract

Ovine luteal tissue contains two distinct steroidogenic cell types, small (8–20 μm) and large (>20 μm), which differ based on morphological and biochemical criteria. Unstimulated small cells secrete low levels of progesterone, respond to LH or dibutyryl cAMP (dbcAMP) with enhanced secretion of progesterone, and contain most of the receptors for LH. The unstimulated large cells, conversely, secrete high levels of progesterone, have few, if any, receptors for LH, and do not respond to LH –or dbcAMP with increased progesterone secretion. The lack of response to dbcAMP by large cells was investigated. Large cells incubated in the presence of cholesterol, ram serum, or 25-hydroxycholesterol did not demonstrate substrate limitation. Hormone-independent stimulation of adenylate cyclase by cholera toxin or forskolin resulted in increased adenylate cyclase activities (P < 0.01), cAMP accumulation (P < 0.05), and the binding of endogenous cAMP (P < 0.05) by type 1 cAMP-dependent protein kinase in both small and large cells. These treatments were accompanied by enhanced secretion of progesterone (P < 0.05) in small cells. In contrast, large cells did not respond with an increase in progesterone secretion under these conditions. These observations suggest that the high rate of secretion of progesterone in unstimulated large cells is not regulated by cAMP.