Effects of phenothiazines on the membrane-bound guanylate and adenylate cyclase in tetrahymena pyriformis

The particulate-bound guanylate cyclase activity of Tetrahymena pyriformis was shown previously to be Ca²⁺-dependent and to be activated by an endogenous calmodulin-like protein (Tetrahymena Ca²⁺-binding protein, TCBP) S. Nagao, Y. Suzuki, Y. Watanabe and Y. Nozawa, Biochem. biophys. Res. Commum.90, 261 (1979)]. Phenothiazine derivatives, such as chlorpromazine and trifluoperazine, that interact with calmodulin were found to inhibit the Ca²⁺-dependent guanylate cyclase activity and the TCBP-induced activation of the guanylate cyclase activity. Ethylene glycol-bis (β-aminoethyl ether)-N, N'-tetraacetic acid (EGTA), a Ca²⁺ chelator, also inhibited the activation of guanylate cyclase. However, the mechanisms by which EGTA and trifluoperazine act were different. The EGTA-induced inhibition could not be overcome by increasing the concentration of TCBP, whereas the trifluoperazine-induced inhibition could be overcome by increasing the concentration of TCBP, but not by increasing the concentration of Ca²⁺. These findings suggest that the mechanism by which trifluoperazine inhibits the activation of guanylate cyclase involves competition with TCBP.