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

Abstract

The alpha (catalytic) subunit of the Na+ pump (Na+, K(+)-ATPase) has three isoforms; alpha1 is ubiquitous, skeletal muscle expresses predominantly alpha2, and alpha3 has been localized to specific types of neurons and, possibly, to axonal processes. The alpha3 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 alpha subunit and labeled alpha-bungarotoxin as a probe for the neuromuscular junction (NMJ). We found that alpha3 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 alpha3 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 alpha3-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.

Subunit Composition and Role of Na+,K+-ATPases in Ventricular Myocytes

“…Although the α3 subunit has been reported to be present in the hearts of humans [22] and dogs [23], its distribution in the ventricular myocytes and corresponding β subunits for heterodimer formation have not yet been elucidated. As a result, the present experiment extended Zahler et al's findings [21] and also demonstrated for the first time, the presence of the α3β2 isozyme in the intercalated disc of ventricular myocytes.…”

“…This possibility, however, may not be feasible. First, the α3-like IR was shown to be present in the junctional site of the rat cardiac conduction system [21]. Secondly, RT-PCR revealed the relative level of PCR products for the α3 in the rat heart to be about 30% of that in the rat brain, a value which was in agreement with that for the α3 signal in the guinea-pig heart.…”

Deletion of the Na/K-ATPase $alpha;1-subunit gene (Atp1a1) does not prevent cavitation of the preimplantation mouse embryo

“…cavitation (Betts et al, 1998), similar to its role in other cell systems that deal with high intracellular Na þ loads (Zahler et al, 1996). In this model, Na þ -pump a1-subunit is considered to be the ubiquitous 'house keeping' enzyme which functions at near saturation under conditions of basal and elevated [Na þ ] i , whereas the lower affinity a3-isoform becomes relevant as [Na þ ] i increases beyond the turnover capacity of a1-isozymes preventing disruption to cellular processes inhibited by high [Na].…”

Phorbol Diacetate Potentiates Na + -K + ATPase Inhibition by a Putative Endogenous Ligand, Marinobufagenin

“…NKA isoforms are distributed throughout cardiovascular system in an inhomogeneous and tissue-specific manner. 14,15,20 We have shown that the ␣ 1 -NKA isoform in rat aortae is mainly localized to the sarcolemma, whereas the ␣ 3 -NKA predominates in the nerve ending membranes. 11 The present results, similar to our previous observations, 17 demonstrate that HMA sarcolemma contains a greater amount of the ␣ 1 -NKA than of the ␣ 3 -NKA isoform protein.…”