The α3 Isoform Protein of the Na + ,K + -ATPase Is Associated With the Sites of Cardiac and Neuromuscular Impulse Transmission

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Abstract
The α (catalytic) subunit of the Na + pump (Na + ,K + -ATPase) has three isoforms: α1 is ubiquitous, skeletal muscle expresses predominantly α2, and α3 has been localized to specific types of neurons and, possibly, to axonal processes. The α3 isoform mRNA is also expressed in the rat cardiac conduction system. Thus, we studied rat heart and quadriceps muscles by immunohistochemistry using isoform-specific antibodies to the Na + pump α subunit and labeled α-bungarotoxin as a probe for the neuromuscular junction (NMJ). We found that α3 pump protein is localized to three sites important for impulse transmission: the junctional complex between cardiac myocytes, the heart conduction system, and the NMJ. Specifically, all levels of the conduction system expressed α3 immunoreactive protein, as assessed by two isoform-specific antibodies and histological conduction system markers. Specific expression at the junctional complex was confirmed by immuno-EM. Double-labeling and denervation analysis indicated that α3-positive areas in skeletal muscle were presynaptic and adjacent to postsynaptic bungarotoxin-positive regions, which had the classic morphology of NMJs. Thus, specific Na + ,K + -ATPase pump isoforms may be adapted to maintenance of membrane potential and/or intracellular ion concentrations required for impulse transmission in both heart and presynaptic motor terminals contacting skeletal muscle.