Mechanisms of vasodilation induced by NKH477, a water-soluble forskolin derivative, in 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⁺]_i, isometric force, and cellular concentrations of cAMP and inositol 1,4,5-trisphosphate were measured. NKH477 (0.1-1.0 μM), isoproterenol (0.01-0.1 μM), or forskolin (0.1-1.0 μM) increased cAMP and attenuated the contraction induced by 128 mM K⁺ or 10 μM ACh in a concentration-dependent manner. These agents, at concentrations up to 0.3 μM, did not change the amount of cGMP. NKH477 (0.1 μM) attenuated the contraction induced by 128 mM K⁺ without corresponding changes in the evoked [Ca²⁺]_i responses. ACh (10 μM) produced a large phasic increase followed by a small tonic increase in [Ca²⁺]_i and produced a sustained contraction. The ACh-induced phasic increase in [Ca²⁺]_i, but not the tonic increase, disappeared after application of 0.1 μM ionomycin. NKH477 (0.1 μM) attenuated both the increase in [Ca²⁺]_i and the force induced by 10 μM 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 μM NKH477 shifted the [Ca²⁺]_i-force relation to the right in the presence or absence of 10 μM ACh. In β-escin-skinned smooth muscle strips, 0.1 μM 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.