Calcium‐dependent chloride secretion in rat colon epithelium.

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Abstract
Epithelia, dissected from the descending rat colon, were studied under short-circuit conditions in Ussing chambers. The latter were modified to accept flexible light guides, so that the tissue could be irradiated, with white light, normal to its surface. Irradiation alone had no effect on short-circuit current (SCC). In the presence of erythrosine B (which by itself had no effect on the SCC) on the basolateral side of the tissue, irradiation produced a substantial increase in SCC; this increase was sustained after irradiation had ceased and the dye had been washed away. The photodynamic effect of erythrosine B required the presence of O2 in the bathing solution. Ca was essential for the SCC response to occur. Thus, irradiation in the presence of the dye in the absence of Ca had no effect on SCC, but a SCC increase could be revealed by subsequent addition of Ca after irradiation had ceased. Co and Mg ions antagonized the effect of Ca in the conditions described above. Ion flux measurements with 36Cl and 22Na showed that the photodynamic effect of erythrosine B abolished net sodium absorption and reversed net Cl- absorption to secretion. The data are consistent with abolition of electroneutral NaCl absorption and the stimulation of electrogenic Cl- secretion to an extent equivalent to the SCC responses. Using Ca-containing buffers it was possible to compare the SCC responses at low, known ionized Ca concentrations with the maximal Cl- secretory effect following photodynamic activation. Cl- secretion was half-maximally activated when the basolateral bathing fluid contained 1 .mu.M-ionized Ca and after the basolateral face of the tissue was permeabilized by the photodynamic action of erythrosine B. The relation between ionized Ca concentration in the basolateral fluid and the Cl secretory response was steep.