α-Adrenergically mediated changes in membrane lipid fluidity and Ca 2+ binding in isolated rat liver plasma membranes

Abstract

Noradrenaline (0.1–5 μM, in the presence of 5 μM propranolol to block β-receptors), ATP (100 μM) and angiotensin II (0.1 μM), which are thought to increase cytosolic Ca 2+ concentration by mobilizing Ca 2+ from internal stores, increased the lipid fluidity as measured by diphenylhexatriene fluorescence polarization in plasma membranes isolated from rat liver. The effect of noradrenaline was dose-dependent and blocked by the α-antagonists phenoxybenzamine (50 μM) and phentolamine (1 μM). The response to a maximal dose of noradrenaline (5 μM) and that to ATP (100 μM) were not cumulative, suggesting that both agents use a common mechanism to alter the membrane lipid fluidity. In contrast, the addition of noradrenaline (5 μM) along with the foreign amphiphile Na +-oleate (1–30 μM) resulted in an increase in membrane lipid fluidity which was equivalent to the sum of individual responses to the two agents. In the absence of Mg 2+, reducing free Ca 2+ concentration by adding EGTA increased membrane lipid fluidity and abolished the effect of noradrenaline, suggesting that Ca 2+ is involved in the mechanism by which the hormone exerts its effect on plasma membranes. Noradrenaline (5 μM) and angiotensin II (0.1 μM) also promoted a small release of 45Ca 2+ (16 pmol/mg membrane proteins) from prelabelled plasma membranes. The effect of noradrenaline was suppressed by the α-antagonist phentolamine (5 μM). It is proposed that noradrenaline, via α-adrenergic receptors and other Ca 2+-mobilizing hormones, increases membrane lipid fluidity by displacing a small pool of Ca 2+ bound to phospholipids, removing thus the mechanical constraints brought about by this ion.