Calmodulin stimulation of adenylate cyclase of intestinal epithelium.

Abstract

The present work analyzes the calmodulin stimulation of rat intestinal epithelial adenylate cyclase. It is shown that: (1) activation of intestinal adenylate cyclase by calmodulin occurs in the presence of 1 – 30 μM free Ca2+; (2) in the 0.5 – 10 mM MgCl2 concentration range, the pattern of calmodulin action is not modified by Mg2+; calmodulin acts by increasing the maximal velocity of the intestinal adenylate cyclase without altering its affinity for ATP and Mg2+; (3) the calmodulin effect does not require the presence of guanine nucleotide and is even additive to that induced by GTP or its synthetic non‐hydrolyzable analog guanosine 5′‐[β,γ‐imido]triphosphate; (4) calmodulin and sodium fluoride exert an additive effect on the enzyme activity; (5) calmodulin and prostaglandins E1 or E2 display additive stimulatory actions, whatever the dose of prostaglandins tested; calmodulin, therefore, does not modify the affinity of intestinal adenylate cyclase for its hormonal effectors; (6) trifluopeazine in the 25–200 μM range of concentrations (K0.5= 85 μM) suppresses the adenylate cyclase activity which has been stimulated by 5 μM calmodulin; (7) calmodulin is active in the 0.1 – 10 μM concentration range, half‐maximal stimulation is induced by 0.9 μM protein, i.e. a dose of calmodulin that is much below the calmodulin content of the intestinal epithelial cell; (8) at low (0.5 mM) and high (5 mM) Mg2‐ concentration, a sixfold and threefold maximal increase of basal activities are observed, respectively.Our data indicate that calmodulin stimulates intestinal adenylate cyclase by a mechanism that is different from the hormonal activation and suggest the presence of a specific regulatory site for calmodulin on the enzyme.