Selective Inhibitors of Ca2+-Binding Modulator of Phosphodiesterase Produce Vascular Relaxation and Inhibit Actin-Myosin Interaction

To elucidate the role of Ca 2+ -regulated activator protein (a protein modulator) of cyclic nucleotide phosphodiesterase in smooth muscle contraction, we determined the effects of various drugs that selectively inhibit a protein modulator-induced stimulation of cyclic nucleotide phosphodiesterase on contractile response of isolated rabbit aortic strip, and superprecipitation of bovine aorta smooth muscle actomyosin, considered to be an in vitro analogue of muscle contraction. Actomyosin preparation from bovine aorta smooth muscle contained approximately 200 units (2 µg) of protein modulator per mg actomyosin and superprecipitated by addition of ATP. Selective inhibitors of the stimulation of the phosphodiesterase by a protein modulator such as N-(6-aminohexyl)-5-chloro-1-naphthalenesulfonamide (W-7) and its derivatives, or psychotropic agents such as chlorpromazine, chlorprothixene, amitriptyline and desipramine produced relaxation of isolated aortic strips contracted by various agonists such as KCl, CaCl 2 , norepinephrine, histamine, and prostaglandin F 2α . The relaxation induced by these agents was not affected by treatment with adrenergic or cholinergic blocking agents such as propranolol and atropine, suggesting that such do not work through these receptors. These drugs not only reduced the extent of actomyosin superprecipitation but also prolonged the time required to attain the maximum superprecipitation, suggesting that they inhibit the superprecipitation of aorta smooth muscle actomyosin. The addition of various amounts of protein modulator prevented in dose-dependent fashion the inhibition by these compounds of actomyosin superprecipitation. The order of the potency of the drugs was much the same in assay systems such as inhibiting the stimulation of the phosphodiesterase by a protein modulator, superprecipitation of actomyosin and producing a relaxation of aortic strip. Other vasodilating drugs such as isoproterenol, sodium nitrite and fusaric acid had no effect on smooth muscle contraction process (superprecipitation of actomyosin). Papaverine, which is a potent inhibitor of cyclic nucleotide phosphodiesterase and not a selective inhibitor, served also as the control. However, even papaverine failed to inhibit the superprecipitation. Our results provide pharmacological evidence that a protein modulator participates in the regulation in smooth muscle contraction.