Ileal HCO3 secretion: relationship to Na and Cl transport and effect of theophylline

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.