Distribution of (Na++K+)ATPase and sodium channels in skeletal muscle and electroplax

Summary The distributions of (Na⁺ + K⁺)ATPase and sodium channels in skeletal muscle fibres and electrocytes were determined by immunofluorescent and immunoelectron microscopic techniques using antibodies against rat and eel (Na⁺ + K⁺)ATPase and the eel electric organ sodium channel. The extrajunctional sarcolemma of skeletal muscle was uniformly stained by polyclonal antibodies against (Na⁺ + K⁺)ATPase and the sodium channel. The T-tubule system of skeletal muscle was also labelled heavily for both (Na⁺ + K⁺)ATPase and the sodium channel. The terminal cisternae of the sarcoplasmic reticulum was stained for (Na⁺ + K⁺)ATPase but not sodium channels. At the motor endplate, (Na⁺ + K⁺)ATPase-like immunoreactivity was present along the plasmalemma of motor nerve terminals but not along the postsynaptic junctional sarcolemma. Paradoxically, a monoclonal antibody that binds to the a form of the catalytic subunit of (Na⁺ + K⁺)ATPase from rat hepatocytes and renal tubule cells did not label the enzyme in rat skeletal muscle. In electrocytes, (Na⁺ + K⁺)ATPase-like irnmunoreactivity was concentrated primarily along the plasmalemma and calveolae of the non-innervated face. In contrast, sodium channel-like immunoreactivity was concentrated along the plasmalemma of the innervated face except in the clefts of the postsynaptic membrane. Thus, we conclude that at endplates both the (Na⁺ + K⁺)ATPase of rat skeletal muscle and sodium channels of eel electrocytes are not concentrated in the juxtaneuronal postsynaptic membrane. We also interpret the failure of the monoclonal anti-a (Na⁺ + K⁺)ATPase antibodies to bind to the enzyme in muscle to indicate that the catalytic subunit of skeletal muscle (Na⁺ + K⁺)ATPase displays different epitopes than does the a subunit of kidney and liver.