Biochemical characterization of plasma membrane isolated from human skeletal muscle

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 mumol Pi/h per mg protein and 96 +/- 9 nmol/min 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 Kd 3 nM and 0.22 microM). Two families of receptors with different affinities for a tritiated TTX derivative (estimated Kd 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 (Kd = 0.2 nM).