Cyclic AMP and Ca‐binding in Microsomal Fractions Isolated from Rabbit Colon Smooth Muscle

Abstract

From a homogenate of rabbit colon muscle 2 ATP dependent Ca-accumulating microsomal fractions were isolated by differential centrifugation on a sucrose density gradient at 35% and 35-45% sucrose. Adenylate cyclase and phosphodiesterase activities were found in the fractions. The Ca-accumulation and the ATPase activity of these fractions were stimulated by cyclic[c]AMP (10-5 M) at an ATP concentration of 0.35 mM. In the presence of higher concentrations of ATP (5 mM) cAMP had no effect on the Ca-binding. The higher concentration of ATP markedly increased the cAMP formation in relation to the activity found at the lower concentration of ATP. Isoprenaline (2 .times. 10-6 M) stimulated the Ca-accumulation in the 35-45% fraction and increased the hydrolysis of ATP. These effects were absent in the fraction isolated at 35% sucrose. In the former fraction isoprenaline stimulated the adenylate cyclase activity at 0.35 mM but not at 5 mM ATP. Both the effect of isoprenaline on the Ca-binding and the adenylate cyclase activity were inhibited by the adrenergic .beta.-receptor blocking agent sotalol. In the 35-45% fraction papaverine (1 .times. 10-3 M) stimulated the Ca-accumulation and inhibited the phosphodiesterase activity. cAMP and agents which influence the cAMP metabolism in the microsomes may have a regulatory role on the Ca-binding of the microsomes.