Reversible effects of phenothiazines on frog gastric stimulation

The calmodulin inhibitors trifluoperazine (TFP), chlorpromazine (CPZ), and promethazine (PZ) were tested for effects on stimulus-secretion coupling in in vitro bullfrog gastric mucosa. When added to histamine-stimulated tissues, the drugs caused H+ secretion to decrease and transepithelial resistance to increase over a 2-h time course. The potency sequence was TFP (IC50 = 40 microM) greater than CPZ (IC50 = 72 microM) congruent to PZ (IC50 = 72 microM). Anesthetics and other phenothiazines with weak anticalmodulin activity had no effect on secretory parameters. In the presence of histamine, further addition of isobutylmethylxanthine (IBMX, a phosphodiesterase inhibitor) plus dibutyryl cAMP (DBcAMP), IBMX alone, or forskolin (a specific activator of adenylate cyclase) to phenothiazine-inhibited tissues caused full resumption of secretory activity. If TFP (50 microM) was added before stimulation with histamine, the normal increases in tissue cAMP content (which occurs primarily in oxyntic cells), oxyntic cell apical membrane elaboration (morphometric analysis of electron micrographs), and H+ secretion were all blocked. Subsequent addition of IBMX or IBMX plus DBcAMP completely reversed the TFP effect. These results indicate that the histamine-sensitive adenylate cyclase may be the site of TFP inhibition and Ca2+-calmodulin regulation; since these drugs inhibited stimulation by DBcAMP plus IBMX, they may also be exerting additional effects distal to cAMP generation.