Ligand effects on membrane lipids associated with sodium, potassium- activated adenosine triphosphatase: comparative spin probe studies with rat brain and heart enzyme preparations

RT Coughlin, T Akera, EJ McGroarty and C Steinbach

Physical properties of membrane lipids associated with rat or dog brain and heart Na+, K+-ATPase preparations were compared using an electron spin resonance probe, 5-doxyl stearate. The degree of acyl chain order of the membrane lipids was greater for brain enzyme than for heart enzyme preparations; membrane lipids in the rat heart enzyme preparations were the most disordered. In the absence of added ligands, membrane lipids did not appear to undergo a detectable temperature- dependent rearrangement or structural transition. An apparent transition was observed in the simultaneous presence of Na+, Mg2+, and ATP. These ligands increased lipid order at temperatures above the structural transition, but not below it. In the presence of the above ligands, K+ caused a marked decrease in the transition temperature in the rat brain enzyme preparations, but only a modest decrease in rat heart enzyme preparations. Arrhenius plots of rat brain and heart Na+, K+-ATPase activity revealed a break point roughly corresponding to respective membrane lipid transition temperatures observed in the presence of Na+, K+, Mg2+, and ATP. A low concentration of ouabain (1 microM) failed to affect either the lipid transition temperature estimated by the spin probe or the value of lipid order of the rat brain enzyme preparations observed in the presence of Na+, Mg2+, and ATP, but markedly reduced the effect of K+ to lower the transition temperature observed in the presence of the above ligands. A high concentration (100 microM) of ouabain which was needed to completely inhibit rat heart enzyme eliminated the thermally induced structural rearrangement observed in the presence of Na+, Mg2+, and ATP, apparently through a nonspecific lipid perturbation. These results indicate that differences in the physical properties of the membrane lipids per se are unlikely to account for the low affinity of rat heart Na+, K+-ATPase for ouabain and also suggest that the use of high concentrations of ouabain required to completely inhibit Na+, K+-ATPase activity may cause nonspecific changes in addition to inhibition of Na+, K+-ATPase or the sodium pump.

Volume 32, Issue 1, pp. 147-153, 07/01/1987
Copyright © 1987 by American Society for Pharmacology and Experimental Therapeutics