Evidence for compartmentalization of adenosine 3',5'-monophosphate (cAMP)-dependent protein kinases in rat Leydig...

It is generally believed that LH-stimulated steroidogenesis in Leydig cells is via activation of the cAMP second messenger system. The present study used cAMP analogs selective in their binding to the two types of cAMP-dependent protein kinase (PK-A) and/or selective in their binding to the two cAMP-binding sites on the regulatory subunits of PK-A. As the two cAMP-binding sites show positive cooperativity, synergistic increases in androgen production by rat Leydig cells occurred when they were incubated in vitro with analog pairs that selectively activated PK-A type I or II. This confirmed that both types of PK-A are present in Leydig cells and demonstrated that both PK-A types are capable of activating steroidogenesis. Synergistic increases in androgen production were also observed when either a type I selective analog (8-aminohexylamino-cAMP) or a type II selective analog (8-thiomethyl-cAMP) was paired with added cAMP. However, when these analogs were paired with LH or the adenylate cyclase activator forskolin, a synergistic increase in steroidogenesis occurred only with the type I selective analog. These results suggest that PK-A type I is compartmentalized in Leydig cells so that it has preferential access to endogenously produced cAMP.