Effects of Ca2+ on ileal transport and electrically induced secretion.

[Ca2+] affects nerves and target cells in stimulus-secretion coupling. In flux-chamber studies of full-thickness rabbit ileum, we determined the effects of 1) ethylene glycol bis-(beta-aminoethylether)-N,N'-tetraacetic acid (EGTA) 1.25 mM, 2) verapamil 0.1 mM and nifedipine 0.1 mM, and 3) trifluoperazine 0.1 mM on ion transport and its response to electrical field stimulation (EFS). EGTA increased JClm leads to s, JNam leads to s, Cl absorption and conductivity (G), and reduced Isc. In the absence of EGTA, EFS increased transmural PD and Isc and caused secretion of Na and Cl. EGTA prevented the responses to EFS, but the Isc responses to aminophylline and to glucose were normal. Verapamil reduced the response of Isc and Cl transport to EFS. Nifedipine reduced Isc but not the Isc response to EFS. Trifluoperazine reduced Isc and almost eliminated the Isc response to EFS. EFS did not increase the tissue concentration of cAMP. We conclude: 1) low extracellular [Ca2+] enhances net Cl absorption; 2) extracellular Ca2+ is required for the response of ion transport to EFS; 3) cAMP does not mediate Isc response to EFS; and 4) Isc response to EFS is blocked by trifluoperazine. The findings suggest that EFS stimulates secretion by increasing Ca entry into the epithelial cells, either directly or indirectly.