Is intracellular pH and/or intracellular bicarbonate a determinant of bile salt independent canalicular bile formation? The subject revisited.

Canalicular bile formation is a complex process that involves basolateral and apical cell membrane transport, paracellular transport and vesicular transport, all of which may be subject to regulation by pH. We review the concept that apical cell membrane bicarbonate secretion promotes bile salt independent canalicular bile formation. We show that the presence of paracellular electrolyte transport imposes a severe restriction in interpreting data from ion substitution experiments aimed at demonstrating pH or bicarbonate dependent bile formation. Furthermore, we report on experiments that all show stimulation of bile flow under three disparate experimental conditions: i) intracellular alkalinization in the absence of [HCO3-]i or associated with a decrease of [HCO3-]i, ii) intracellular alkalinization with an increase of [HCO3-]i, and iii) intracellular acidification with increase of [HCO3-]i. It is suggested that both, intracellular pH and intracellular bicarbonate may modulate canalicular bile salt independent bile formation, but it remains conjectural which mechanism is the prevailing one under a given experimental setting.