C. B. Hensley

Physiologic Relevance of the α2 Isoform of Na,K-ATPase in Muscle and Heart

The existence of isoforms of Na,K-ATPase suggests the potential for isoform specific function, distribution or regulation. Under the distribution category is the potential for tissue specific expression as well as subcellular compartment specific isoform expression. Under the regulation category is the potential for isoform specific acute regulation of existing pumps as well as chronic regulation of the pool size of isofoms. The picture is further complicated by the fact that there are isoforms of not only the α catalytic subunit but also the s glycoprotein subunit that make up the heterodimeric sodium pump. Skeletal muscles express the fairly ubiquitous α1 and s1 subunits as well as the more restricted α2 and s2 subunits. Cardiac muscles express these subunits (s2 only reported at mRNA to our knowledge) as well as α3 (24). Thus, there is the potential for four and six different heterodimers to be expressed in skeletal and cardiac muscle, respectively, and the potential for each having a unique function, pattern of subcellular distribution and/or regulation, as summarized in the compartmental model in Figure 1.

Use of Phase Partitioning in Multidimensional Subcellular Fractionation

Conventional subcellular fractionation procedures incorporate two separation dimensions, one based on membrane sedimentation coefficient, the other on equilibrium density. Questions about the homogeneity of samples obtained with these procedures have been addressed with additional separations based on manipulation of membrane density or on partitioning in dextran-poly(ethylene glycol) two-phase systems. In some cases it has proven useful to employ a total of 4 separation dimensions, i.e. sedimentation, equilibrium density in 2 differently designed density gradients, and phase partitioning. In other cases, parallel third dimensions analyses in two-phase systems with different pH values have been used to delineate the multiplicity of populations present in conventional subcellular samples.