Considerable evidence suggests that liver plasma membrane (LPM) NaK-ATPase [(Na+ + K+)-dependent adenosinetriphosphatase] and Mg-ATPase (Mg2+-dependent adenosinetriphosphatase) activity and lipid fluidity are important in liver cell functions such as bile formation. However, little is known regarding factors which might alter these membrane properties in vivo. Bile salts are actively concentrated by liver cells under normal conditions and reach even higher tissue concentrations in cholestasis. Since current methodology does not permit investigation of the effect of bile salts on LPM in vivo, we have examined the effects of bile salts in vitro on isolated rat LPM essentially free of organelle contamination and enriched 52.9-, 23.8-, and 27.8-fold in NaK-ATPase, 5′-nucleotidase, and alkaline phosphatase, respectively, compared with homogenate. We found that taurocholate (0.5 to 4.0 mM), taurochenodeoxycholate (0.25 to 4.0 mM), deoxycholate (0.25 to 4.0 mM), but not dehydrocholate (0.25 to 4.0 mM) caused immediate, concentration-dependent inhibition of LPM Mg- and NaK-ATPase which at low bile salt concentration (1.0 mM) was reversible. By contrast, taurocholate (1 to 4 mM) had no effect on LPM 5′-nucleotidase activity. Both 1 mM taurocholate and taurochenodeoxycholate caused a decrease in the apparent Km for ATP of NaK-ATPase, but not of Mg-ATPase, and 1 mM taurochenodeoxycholate caused no significant change in the activation of either Mg- or NaK-ATPase by cations. Finally, LPM lipid fluidity measured by fluorescence polarization using 1,6-diphenyl-1,3,5-hexatriene as a probe was reversibly increased by taurocholate, taurochenodeoxycholate, and deoxycholate, but not dehydrocholate; the effect of taurochenodeoxycholate on lipid fluidity was confirmed by electron spin resonance using 5-(2,2-dimethyl-N-oxyl-oxayalidine)-stearic acid as a probe. However, by both techniques, the effect of low concentrations of these bile salts on ATPase activity was independent of detectable changes in fluidity.
These findings indicate that at concentrations comparable to those reported for liver tissue endogenous, micelle-forming bile salts reversibly inhibit LPM ATPases and reversibly increase LPM lipid fluidity in vitro; these two effects being dissociable at low bile salt concentrations. It is possible that bile salts may have similar effects in vivo, particularly under cholestatic conditions.