title: Alpha-adrenergically mediated changes in membrane lipid fluidity and Ca2/ binding in isolated rat liver plasma membranes.
abstract: Noradrenaline (0.1-5 microM, in the presence of 5 microM propranolol to block beta-receptors), ATP (100 microM) and angiotensin II (0.1 microM), which are thought to increase cytosolic Ca2+ concentration by mobilizing Ca2+ 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 alpha-antagonists phenoxybenzamine (50 microM) and phentolamine (1 microM). The response to a maximal dose of noradrenaline (5 microM) and that to ATP (100 microM) were not cumulative, suggesting that both agents use a common mechanism to alter the membrane lipid fluidity. In contrast, the addition of noradrenaline (5 microM) along with the foreign amphiphile Na+-oleate (1-30 microM) 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 Mg2+, reducing free Ca2+ concentration by adding EGTA increased membrane lipid fluidity and abolished the effect of noradrenaline, suggesting that Ca2+ is involved in the mechanism by which the hormone exerts its effect on plasma membranes. Noradrenaline (5 microM) and angiotensin II (0.1 microM) also promoted a small release of 45Ca2+ (16 pmol/mg membrane proteins) from prelabelled plasma membranes. The effect of noradrenaline was suppressed by the alpha-antagonist phentolamine (5 microM). It is proposed that noradrenaline, via alpha-adrenergic receptors and other Ca2+ -mobilizing hormones, increases membrane lipid fluidity by displacing a small pool of Ca2+ bound to phospholipids, removing thus the mechanical constraints brought about by this ion.
mesh_terms: D000255:Adenosine Triphosphate / Q000494:pharmacology; D000804:Angiotensin II / Q000494:pharmacology; D000818:Animals; D002118:Calcium / Q000378:metabolism*; D002462:Cell Membrane / Q000378:metabolism; D005260:Female; D005454:Fluorescence Polarization; D008099:Liver / Q000378:metabolism*; D008560:Membrane Fluidity / Q000187:drug effects; D008563:Membrane Lipids / Q000378:metabolism*; D009638:Norepinephrine / Q000494:pharmacology; D051381:Rats; D011919:Rats, Inbred Strains; D011941:Receptors, Adrenergic / Q000378:metabolism*; D011942:Receptors, Adrenergic, alpha / Q000378:metabolism*
chemical_list: D008563:Membrane Lipids; D011941:Receptors, Adrenergic; D011942:Receptors, Adrenergic, alpha; D000804:Angiotensin II; D000255:Adenosine Triphosphate; D002118:Calcium; D009638:Norepinephrine