Biochemical characterization of plasma membrane isolated from human skeletal muscle

Abstract

Specific components of ion translocation systems were studied in excitable plasma membranes isolated from normal human muscle. Na⁺ ‐ K⁺ ATPase and ouabain‐sensitive K⁺ phosphatase activities were 8.9 ± 1 per mg protein and 96 ± 9 per mg protein, respectively. Scatchard analysis of equilibrium binding assays with [³H]ouabain showed non‐linear curves consistent with high‐ and low‐affinity sites (estimated K _d 3 nM and 0.22 μM). Two families of receptors with different affinities for a tritiated TTX derivative (estimated K _d 0.4 and 4 nM) were also identified suggesting the existence in human muscle of at least two classes of voltage‐dependent Na⁺ channels. In addition (+)‐[methyl‐³H]PN200‐110, a potent Ca²⁺ antagonist used for labeling voltage‐dependent Ca²⁺ channels, was observed to bind to a homogeneous population of receptors in the plasma membrane (K _d = 0.2 nM).