SQ22536 and W-7 inhibit forskolin-induced cAMP elevation but not relaxation in newborn ovine pulmonary veins

The role of cAMP in forskolin-induced relaxation was studied in isolated pulmonary veins of newborn lambs (7–12 days). In vessels preconstricted with endothelin-1, forskolin at concentrations ≤10 −7 M had no effect on cAMP content and adenylyl cyclase activity but caused up to 50% relaxation. At higher concentrations, forskolin markedly elevated cAMP content and adenylyl cyclase activity and caused a further relaxation. SQ22536 [9-(tetrahydro-2-furanyl)-9 H-purin-6-amine; an adenylyl cyclase inhibitor] and W-7 [ N-(6-aminohexyl)-5-chloro-1-naphthalensulfonamide; a calmodulin-dependent adenylyl cyclase inhibitor] had no significant effect on forskolin-induced relaxation but markedly inhibited the elevation of cAMP content and adenylyl cyclase activity caused by forskolin. Rp-8-CPT-cAMPS [8-(4-chlorophenylthio)-adenosine-3′,5′-cyclic monophosphorothioate; an inhibitor of cAMP-dependent protein kinases] and Rp-8-Br-PET-cGMPS (β-phenyl-1, N 2-etheno-8-bromoguanosine-3′,5′-cyclic monophosphorothioate; an inhibitor of cGMP-dependent protein kinases) attenuated the relaxation caused by a cAMP analog but not that caused by forskolin. These results suggest that cAMP may not play a major role in forskolin-induced relaxation of pulmonary veins of newborn lambs.