ELECTROGENIC BICARBONATE SECRETION IN MOUSE GALLBLADDER

Mouse gallbladders (4 mm(2)) were investigated using the short-circuit current (I-sc) technique. Responses of 50 mu A/cm(2) were obtained in response to forskolin and agents that stimulated the adenylate cyclas e system (IBMX and dibutyryl-cAMP). The calcium ionophore ionomycin in creased I-sc to 30% of the forskolin-stimulated increase. The forskoli n-dependent current was inhibited 40% by acetazolamide but was insensi tive to furosemide. Forskolin responses were dependent on the presence of bicarbonate ions; removal from both sides of the membrane or the b asolateral side alone caused a significant reduction in responses. Rem oval of chloride ions from the basolateral side had no effect, while r emoval from the apical side caused a significant reduction in the fors kolin responses, but only by 30%. It is argued that the remaining curr ent (70%) cannot result from a parallel arrangement of a chloride chan nel and a chloride-bicarbonate exchanger and that bicarbonate is secre ted through the apical membrane by a predominantly conductive mechanis m. Apparently, forskolin converts a near electrically silent epitheliu m to an electrogenically secreting tissue.