Bicarbonate secretion in rat distal colon in vitro: a measurement technique.

To study HCO3- secretion in rat distal colon, we utilized a technique that permits control of electrical and chemical transepithelial gradients. With symmetrical solutions (pH 7.4, [HCO3-] 25 mM, and CO2 tension 40 mmHg) bathing both tissue surfaces and under short-circuit conditions, HCO3- secretion remained stable for greater than 4 h at 1 mueq. h-1.cm-2. As the mucosal solution was alkalinized, the serosal solution was acidified at 3.1 mueq.h-1.cm-2. Ninety-four percent of serosal acidification was accounted for by the rate of metabolic lactic acid generation and transepithelial HCO3- secretion. Clamping transepithelial voltage reversibly affected net HCO3- secretion, and a linear relationship existed between clamped mucosal voltage and net HCO3- flux (r = 0.99); mucosal voltage of -68 mV completely inhibited net secretion. The apparent permeability coefficient of the colon to HCO3- is 2.8 X 10(-6) cm/s. One millimolar ouabain completely inhibited net HCO3- secretion. Acetazolamide (10(-4) M) inhibited secretion by approximately 50%, whereas a 10(-3) M concentration inhibited secretion by 90%. These data demonstrate that net colonic HCO3- secretion can be measured without imposed electrical and chemical gradients and that this flux is voltage sensitive and depends on carbonic anhydrase and Na+-K+-ATPase activities.