Smooth Muscle of the Porcine Coronary Artery

To study the mechanism of vasodilation induced by 6-(3-dimethylaminopropionyl) forskolin (NKH477), a water-soluble forskolin derivative, its effects on the acetylcholine (ACh)-induced contraction of muscle strips of porcine coronary artery were examined. [Ca2"]1, isometric force, and cellular concentrations of cAMP and inositol 1,4,5-trisphosphate were measured. NKH477 (0.1-1.0 ,uM), isoproterenol (0.01-0.1 uM), or forskolin (0.1-1.0 ,uM) increased cAMP and attenuated the contraction induced by 128 mM K' or 10 ,M ACh in a concentration-dependent manner. These agents, at concentrations up to 0.3 LM, did not change the amount of cGMP. NKH477 (0.1 gM) attenuated the contraction induced by 128 mM K' without corresponding changes in the evoked [Ca2+11 responses. ACh (10 ,uM) produced a large phasic increase followed by a small tonic increase in [Ca2+1i and produced a sustained contraction. The ACh-induced phasic increase in [Ca2+1], but not the tonic increase, disappeared after application of 0.1 ,M ionomycin. NKH477 (0.1 ,uM) attenuated both the increase in [Ca2+]i and the force induced by 10 ,uM ACh in muscle strips that were not treated with ionomycin and inhibited the ACh-induced contraction without corresponding changes in [Ca2+]i in ionomycin-treated muscle strips. These results suggest that NKH477 inhibits ACh-induced Ca2+ mobilization through its action on ionomycin-sensitive storage sites. In ionomycin-treated and 128 mM K'-treated muscle strips, 0.1 ,uM NKH477 shifted the [Ca2+1f-force relation to the right in the presence or absence of 10 piM ACh. In 3-escin-skinned smooth muscle strips, 0.1 ,uM NKH477 shifted the pCa-force relation to the right but had no effects on Ca2+-independent contraction. We conclude that in smooth muscle of porcine coronary artery, NKH477 inhibits ACh-induced contraction by both attenuating ACh-induced Ca2+ mobilization and reducing the sensitivity of the contractile machinery to Ca2 , possibly by activating cAMP-dependent mechanisms. (Circulation Research 1992;71:70-81)