Differential molecular dynamics and transmembrane fluidity gradients in canine myocardial sarcolemma and sarcoplasmic reticulum

Abstract

The molecular dynamics of highly purified preparations of canine myocardial sarcolemma (SL) and sarcoplasmic reticulum (SR) were quantified by electron spin resonance spectroscopy (ESR). Canine myocardial SL and SR have substantially different motional regimes in their membrane interiors as demonstrated by alterations in the relative peak height ratios, peak widths and peak splittings in ESR spectra of 16-doxylstearate incorporated into SL and SR. Quantification of the apparent order parameters (S) of 16-doxylstearate in SL and SR by analyses of ESR spectra demonstrated that the interior of the SL membrane was substantially more immobilized than the interior of the SR membrane (e.g. S = 0.168 ± 0.002 for SL and S = 0.128 ± 0.003 for SR). In contrast, only modest differences in membrane dynamics near the hydrophobic-hydrophilic interface were present in SL and SR as ascertained by ESR spectra of the probe 5-doxylstearate incorporated into these membranes. Myocardial sarcolemma contained heretofore unsuspected amounts of cholesterol (1.4 ± 0.1 μmol cholesterol/mg protein) while sarcoplasmic reticulum contained only small amounts of cholesterol (0.17 ± 0.06 μmol cholesterol/mg protein). Model systems employing binary mixtures of plasmenylcholine/cholesterol and phosphatidylcholine/cholesterol demonstrated that the observed alterations in molecular dynamics were due, in large part, to the differential cholesterol content in these two subcellular membrane compartments. Taken together, these results demonstrate that these two functionally distinct myocardial subcellular membranes have markedly disparate molecular dynamics and transmembrane fluidity gradients which may facilitate their performance of specific functional roles during excitation-contraction coupling in myocardium.