Absence of a Direct Effect of Phosphate or 5'-AMP on the Contractile Proteins of Hog Carotid Arteries

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Abstract
Two to 4 m M 5'-adenosine monophosphate (AMP) caused a 28% inhibition of the ATPase activity of a crude "fibrillar" contractile protein preparation isolated from porcine vascular smooth muscle. This effect was absent in highly purified Ca 2+ -insensitive actomyosin extracted from carotid arteries as well as a Ca 2+ -dependent actomyosin preparation. Inorganic phosphate (P 1 ) caused no inhibition of ATPase activity in either actomyosin preparation, but 4 m M P 1 produced a slight depression in ATP hydrolysis of the crude fibrillar proteins. Superprecipitation of actomyosin remains unaffected by additions of either 4 m M P 1 or 4 m M AMP to the purified protein solutions. The effects of P 1 and AMP in combination were additive in the fibrillar system. However, the combination of the two agents had no effect on the ATPase activity of the two actomyosin preparations. These results indicate that concentrations of P 1 and AMP which might be produced under physiological conditions have no direct inhibitory action on the vascular smooth muscle actin-myosin interaction.