Modulation of Na+/alanine cotransport in liver sinusoidal membrane vesicles by internal divalent cations

Abstract

Rat liver basolateral plasma membrane (bILPM) vesicles resuspended in 5 mM Mg2+-, Ca2+-, Mn2+- or Co2+-containing media exhibited a markedly lower rate of Na+-stimulated -alanine transport. Divalent cation inhibition of -alanine uptake was dose dependent, and was observed only when the vesicles were pre-loaded with the divalent cations. The presence or absence of the metal ions in the extravesicular incubation media had no effect on -alanine transport. Conversely, pretreatment of the vesicles with 0.2 mM of either EGTA or EDTA resulted in higher initial rates of -alanine transport. This stimulation was overcome by addition of excess divalent cation to the vesicle suspension solution. Since these bILPM vesicles are primarily oriented right-side-out, the divalent cation inhibition of -alanine transport appears to be a result of their interaction with cytosolic components of the cell membrane. Total Na+ flux as measured with 22Na+ was not affected by intravesicular 5 mM Mg2+ or Ca2+, indicating that the inhibition was not due to dissipation of the Na+ gradient. These observations suggest that intracellular divalent cations may serve to modulate -alanine transport across the liver cell plasma membrane.