Characterization of rat myocardial sarcolemma

Abstract

Sarcolemma (SL) was isolated from the hearts of rats by the procedure of Philipson et al. and was found to be of high purity by the use of electron microscopy and marker enzyme analysis. In the SL fraction the sarcolemmal marker enzyme Na +, K +-ATPase was enriched about 33-fold (without SDS stimulation) compared to the crude homogenate. The SL phospholipid (PL) specific content was 0.76 μmol mg −1 SL protein and although this is about three-fold higher than that previously seen in the rat, it is substantially lower than the [PL] in SL reported by others isolated from other mammals. The SL cholesterol was almost entirely non-esterified and the cholesterol: PL molar ratio was 0.547. The individual PL were separated by thin-layer chromatography (TLC). Sarcolemmal PL from the rat have a distribution that is distinct from that observed by others in both rabbit and dog hearts. Although phosphatidyl choline (PC) and phosphatidyl ethanolamine (PE) together accounted for slightly more than 70% of the total PL as in other mammals, the PC PE ratio in the rat was 0.94 and is substantially lower than the 1.21 to 1.40 range observed by others in rabbit and dog hearts. The anionic PL phosphatidyl serine (PS), known to be an important component of SL Ca 2+ binding, had a concentration in the rat of about 40% of the [PS] of other mammalian SL. Conversely, sphingomyelin was found in significantly higher concentrations in the rat. The SL fatty acid composition was also determined and was significantly more saturated than the SL from rabbit heart. The significance of the differences in rat SL composition compared to other mammals is not known but may be related to its profoundly different E-C coupling characteristics.