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Interrelationships among Na, Cl, and HCO3 transport processes were examined in short-circuited rabbit ileal mucosa. As serosal (HCO3) was increased from 10 to 50 mM (pH from 7.1 to 7.8), net Na absorption decreased from 4.6 to 0.3 mueq/h-cm2, net Cl flux changed from absorption of 0.9 to secretion of 0.9 and a net HCO3 secretion of 3.0 developed. A similar change in net Cl flux was also observed when serosal Pco2 was altered at constant (HCO3). In Cl-free SO4-Ringer, serosal alkalinization produced net HCO3 secretion which was not significantly less than that observed in Cl-containing Ringer. Theophylline caused secretory changes in net Na and Cl fluxes at both 10 and 50 mM serosal (HCO3). Theophylline did not alter net HCO3 flux in Cl-Ringer but increased net HCO3 flux in SO4-Ringer. Total dc conductance was decreased by both serosal alkalinization and theophylline. Shortcircuit current was consistently increased by theophylline but not by serosal alkalinization. The results indicate that ileal ion transport is regulated in part by serosal pH and/or (HCO3) and that resulting changes in Cl and HCO3 transport are coupled one-for-one with changes in Na transport. Furthermore, HCO3 secretion does not require the presence of Cl in the bathing medium.

Addition of theophylline to isolated rabbit ileal mucosa resulted in a persistent increase in short-circuit current (SCC) and a persistent decrease in DC conductance. Under short-circuit condition, net Na flux (JNanet) was reversed by theophylline from absorption to secretion (-0.6 mueq/h cm2) and was due to a decrease in unidirectional mucosal- (M) toserosal (S) flux (JNams). Addition of L-alanine to theophylline-treated m-ucosa increased SCC, JNanet, and JNams. Theophylline reduced JAlanet by 25%. Na flux ratios, determined in the presence of theophylline at +26 and -11 mV (mucosal reference), differed from flux ratios expected for a diffusional process by less than 8%. In contrast, Na flux ratios, determined in the absence of theophylline and presence of glucose at +26 and O mV, differed from diffusional ratios by 35%. The results are consistent with the hypothesis that theophylline inhibits active Na absorption, but the data do not conclusively demonstrate the diffusional nature of unidirectional Na fluxes in the presence of theophylline.