Effects of carbachol and calcium on the cyclic guanosine-3',5'-monophosphate (cyclic GMP) metabolism in intestinal smooth muscle.

In a submaximal concentration carbachol contracted the rabbit colon muscle and increased the cyclic GMP level. The cyclic AMP level was reduced. In a Ca++-depleted muscle carbachol reduced the cyclic GMP level while the effect on the cyclic AMP 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 Ca++ had no effect on the guanylate activity while it stimulated the enzyme in a soluble fraction. In the "plasma membrane fraction" cyclic GMP released Ca++ from the preloaded fraction and inhibited the Ca++ accumulation. These effects were not found in the vesicular microsomal fraction (the 35% fraction). In both fractions, however, cyclic GMP counteracted the stimulating effect of cyclic AMP. These results indicate that cyclic AMP and GMP may have antagonistic roles on the Ca++ metabolism in the colon muscle. It is suggested that cyclic GMP may act as some kind of positive feedback mechanism which may have a modulating effect on the release of Ca++ from one pool to another in rabbit colon.