Relaxation of pig coronary arteries by new and potent cGMP analogs that selectively activate type Iα, compared with type Iβ, cGMP-dependent protein kinase

Smooth muscle preparations of human aorta or pig coronary arteries contain nearly equal amounts of cGMP-dependent protein kinase isozymes (cGMP kinase Iα and Iβ). In order to understand the roles of these isozymes in relaxing vascular smooth muscle, several new cGMP analogs were synthesized and tested for potencies in activating each enzyme and in relaxing pig coronary arteries. Analogs modified with a derivatized phenylthio group at the 8-position were as much as 72-fold more potent in activating purified cGMP kinase Iα than cGMP kinase Iβ. Electron-donating substituents, such as hydroxy, amino, and methoxy, on the phenyl ring enhanced the potencies of these analogs in activating cGMP kinase Iα. The most potent of these cGMP analogs [8-(4-hydroxyphenylthio)-cGMP] was 17 times more potent (EC₅₀ = 1.1 μM) as a muscle relaxant than the most efficacious analog tested previously. Among derivatives with an 8-halo group, 8-iodo-cGMP was the most potent compound (K_a = 9 nM for Iα and 122 nM for Iβ) for both Iα and Iβ. Analogs modified at the 1,N²-position or at both the 1,N²- and 8-positions of cGMP were highly potent for activating both isozymes. Within this group, 8-I-β-phenyl-1,N²-etheno-cGMP had K_a values of 22 nM and 17 nM for cGMP kinase Iα and Iβ, respectively, whereas the K_a values of cGMP were 110 nM and 250 nM for the two isozymes. 8-I-β-phenyl-1,N²-etheno-cGMP was the most potent muscle relaxant tested, with EC₅₀ of 0.4 μM. For all cGMP analogs tested, there was a positive correlation between potency for activation of cGMP kinase Iα and that for relaxation of pig coronary arteries. Assuming that the kinase assay conditions yielded a cyclic nucleotide specificity similar to that which would exist in intact cells, it was concluded that the cGMP kinase Iα isozyme mediates the relaxation of pig coronary artery smooth muscle caused by cGMP elevation. However, an additional role for cGMP kinase Iβ in the relaxation process could not be ruled out.