MATERIALS AND METHODS

Segments of distal ileum from New Zealand white, male rabbits which weighed between 2 and 3 kg, were stripped of muscularis and mounted as flat sheets in chambers, as previously described (12). The tissues were bathed in an HCOa-Ringer solution (in mM: Na, 141; Ii, 10; Mg, 1.1; Ca, 1.25; Cl, 127; HC03, 25; HzP04, 1.6; HPO,, 0.3) gassed with 95% 02-5% COz. The transmural electric potential difference, short-circuit current (SCC), and electric resistance were determined as previously described (12). Tissues from adjacent segments of ileum that diff’ered in electrical resistance by less than 25% I were paired for flux measurements. Unidirectional and net fluxes of Na and Cl were determined by adding 22Na and Wl to the lurninal side of one tissue and the serosal side of the other. In most experiments, 24Na was added to the remaining sides. Net Cl fluxes were the differ-ence between opposite unidirectional fluxes nleasured on paired tissues. In 12 experiments, since only Wa was used, net Na fluxes were determined in the same fashion. In the other 16 experiments, both radioisotopes of Na were used and, therefore, net Na fluxes were determined separately for _____-1 The term HC03 secretion is used here and subsequently for COII-venience and is not intended to imply a precise mechanism. The active step in producing alkalinization of the lumen could involve active extrusion of HC03 across the luminal border, active extrusiorl of hydrogen ion across the serosal border, or yet allother process. 992