Characterization of cyclic AMP accumulation in cultured human corpus cavernosum smooth muscle cells

Intracavernous pharmacotherapy relies heavily on the use of vasoactive agents which act by increasing intracellular cAMP levels in human corpus cavernosum smooth muscle. Yet little is known about the cAMP generating system in this tissue, and how it may affect observed patient variability. Thus, the goal of these studies was to better characterize the biochemistry of cAMP formation in human corpus cavernosum smooth muscle, and thus provide more insight into the mechanisms of corporal smooth muscle relaxation in vivo. We studied both receptor and nonreceptor mediated increases in cAMP formation in short-term cultures of human corpus cavernosum smooth muscle cells. Both isoproterenol (ISO) and prostaglandin E1 (PGE1) produced concentration-dependent increases in cAMP, but histamine, serotonin and vasoactive intestinal polypeptide did not. Forskolin, a relatively specific activator of adenylate cyclase, was also a potent stimulant of cAMP formation in these cells. Moreover, there was a direct correlation between the degree of forskolin-induced cAMP accumulation in cultured corporal smooth muscle cells and the magnitude of the forskolin-induced relaxation response of precontracted isolated corporal smooth muscle strips. Prostaglandin E1 and ISO concentration response curves (CRCs) were then assayed in the absence and presence of subthreshold forskolin (0.1 microM.). In the presence of forskolin, the calculated maximal PGE1-induced cAMP accumulation (Emax) was significantly greater than that elicited by PGE1 alone, ISO alone, or ISO + forskolin (p < or = 0.02). In addition, a fixed molar ratio (FMR) (PGE1:ISO) protocol was used to demonstrate that both 80:20 and 70:30 FMRs (but not 95:5 or 90:10), were associated with significantly greater cAMP Emax values than that observed for PGE1 alone (p < or = 0.01). These data provide direct evidence that the degree of cAMP formation in cultured corporal smooth muscle cells is strongly correlated with the magnitude of relaxation of isolated corporal smooth muscle strips. In addition, since simultaneous activation of distinct components of the cAMP generating system produces significant increases in maximal intracellular cAMP accumulation, this suggests that such drug combinations may also augment corporal smooth muscle relaxation in vitro and in vivo.