ENHANCED GTP-DEPENDENT ACTIVITIES OF THE ADENYLATE-CYCLASE SYSTEM - BASIS FOR INCREASED HORMONAL RESPONSIVENESS

Normal rat kidney (NRK) cells growth arrested by picolinic acid and isoleucine deprivation exhibit an increased response to certain agents (i.e., prostaglandin E1, (-)-isoproterenol, and cholera toxin) which elevate intracellular cyclic AMP levels. The enhanced hormonal response is apparently due, at least in part, to increased adenylate cyclase activity. Adenylate cyclase activities measured in the presence of GTP, GTP plus prostaglandin E1, and GTP plus (-)-isoproterenol are increased two- to threefold in membranes prepared from treated cells. In contrast, basal activity is potentiated only 20 to 50% and activity determined in the presence of fluoride is only marginally altered. Also of interest is the increase in cholera toxin activation of cyclase activity in the treated cells. Lower concentrations of cholera toxin (5 ng/ml) are required to achieve maximal stimulation of cyclase activity from picolinic acid-treated and isoleucine-deprived cells; maximal stimulation of control cell adenylate cyclase is attained with 25 to 50 ng/ml cholera toxin. Picolinic acid treatment and isoleucine deficiency both have been shown to arrest NRK cell growth in the G1 phase of the cell cycle. However, results with cells arrested in G1 by serum starvation and by growth to high cell population density indicate that G1 specific growth arrest does not appear to account for the increase in hormonal responsiveness. Chelation of inhibitory metals and proteolytic activation also do not appear to be involved in the mechanism by which picolinic acid enhances cyclic AMP formation. Rather, the results suggest that the treated cells have an increased amount of an active GTP-dependent function required for hormone and cholera toxin stimulation of adenylate cyclase. Thus, picolinic acid treatment and isoleucine deprivation may provide a useful means of modulating the GTP-dependent step required to potentiate hormonal responsiveness. © 1979.