Effects of Carbachol and Calcium on the Cyclic Guanosine-3‘,5’-Monophosphate (Cyclic GMP) Metabolism in Intestinal Smooth Muscle

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Abstract
In a submaximal concentration carbachol contracted the rabbit colon muscle and increased the cyclic[c]GMP level. The cAMP level was reduced. In a Ca2+-depleted muscle carbachol reduced the cGMP level while the effect on the cAMP content of the muscle was unchanged. Carbachol had no effect on the guanylate cyclase activity of the plasma membrane fraction (the 35-45% fraction). In the homogenate and the microsomal fractions Ca2+ had no effect on the guanylate activity while it stimulated the enzyme in a soluble fraction. In the plasma membrane fraction cGMP released Ca2+ from the preloaded fraction and inhibited the Ca2+ accumulation. These effects were not found in the vesicular microsomal fraction (the 35% fraction). In both fractions, cGMP counteracted the stimulating effect of cAMP. cAMP and cGMP may have antagonistic roles on the Ca2+ metabolism in the colon muscle. cGMP may act as some kind of positive feedback mechanism which may have a modulating effect on the release of Ca2+ from one pool to another in rabbit colon.