SECRETIN STIMULATES BILE DUCTULES TO SECRETE BOTH H+ AND HCO-3-IONS

Secretin-dependent ductular HCO3- secretion into bile may involve secretion of H+ to interstitial fluid and HCO3- to bile by the ductular epithelium. To determine whether secretin causes bile ductules to secrete H+, we have examined the effect of secretin on the elimination of an intracellular acid load from bile ductular epithelium during pharmacological blockade of Na+-H+ exchange and in the absence of HCO3-. Microdissected bile ductules from pigs were suspended in HCO3- free HEPES buffer and loaded with acid using an NH4Cl prepulse technique. Intracellular pH was measured using dual-wavelength excitation of BCECF fluorescence. Na+-H+ exchange was defined as a Na+-dependent and amiloride- and 5-(N,N-hexamethylene)-amiloride-sensitive efflux of H+-ions following acid loading, We found that secretin stimulated ductular H+ secretion independent of Na+-H+ exchange. Blockade of Na+-H+ exchange by hexamethylene-amiloride did not affect secretin-dependent ductular HCO3- choleresis in vivo. We conclude that secretin stimulates bile ductules to secrete H+-ions to interstitial fluid as well as HCO3- ions to bile by a mechanism independent of Na+-H+ exchange.