Immunologic identification of Na⁺,K⁺-ATPase isoforms in myocardium: Isoform change in deoxycorticosterone acetate-salt hypertension

There are three isoforms of the catalytic (α) subunit of the Na⁺,K⁺- ATPase, each derived from a different gene, that differ in their sensitivity to inhibition by cardiac glycosides. Antibodies specific for the three isoforms were used to study Na⁺,K⁺-ATPase isoform expression in ventricular myocardium, where an understanding of digitalis receptor diversity is most important. In the rat heart, there is simultaneous expression of two isoforms in adult ventricle, and immunofluorescence studies demonstrated that both isoforms are expressed uniformly in cardiomyocytes. Hypertension and hypertrophy have been reported to selectively depress α2 isoform mRNA levels, and we show in the present study that α2 protein levels were correspondingly depressed in rats made hypertensive by uninephrectomy and treatment with deoxycorticosterone acetate and a high-salt diet. In the human heart, where mRNA for all three α isoforms has been reported, we detected all three isoform proteins (α1, α2, and α3). Two isoforms (α1 and α3) predominated in the macaque heart; dissection of the heart showed uniformity of isoform expression in different ventricular regions but markedly less α3 in the atrium. Finally, isoform-specific antibodies were used to detect which α isoforms were expressed in the ventricles of several commonly used experimental animals to test the correlation of isoform expression with cardiac glycoside-response heterogeneity. Two isoforms (α1 and α3) were found in canine myocardium, whereas only one (α1) was found in sheep and guinea pig. Expression of Na⁺,K⁺-ATPase isoforms can thus be readily followed and related to the physiology of the digitalis receptor.