Recent concepts in bile formation and cholestasis.
Progress has recently been made in the understanding of normal bile secretion mechanisms. The membrane carriers for bile acids have been identified and new insights into intracellular transport mechanisms have been obtained. In particular, characterization of a vesicular pathway involving the Golgi apparatus is well under way. Hypercholeretic bile acids, such as ursodeoxycholic acid, have been discovered. Their choleretic effect is far greater than that of physiological bile acids and they stimulate bicarbonate secretion. Testable hypotheses to explain their hypercholeretic effect have been proposed, in particular the chole-hepatic shunt hypothesis. Several mechanisms capable of inducing cholestasis have been identified: a) inhibition of Na+, K(+)-ATPase; b) increased permeability of the paracellular pathway leading to leakage of bile constituents back into plasma; c) microtubule or microfilament dysfunction; d) increased cytosolic free calcium concentration due to permeabilization of the endoplasmic reticulum calcium stores. It is not yet possible, in a given case, to establish which of these mechanisms is predominant. Several may operate. A better knowledge of the mechanisms involved may lead to improved treatment.