EXPRESSION OF BETA-SUBUNIT ISOFORMS OF THE NA-ATPASE IS MUSCLE TYPE-SPECIFIC(, K+)

Hindlimb skeletal muscles of the rat express two isoforms of the alpha (alpha1 and alpha2)and two isoforms of the beta(beta1 and beta2) subun its of the Na+,K+-ATPase. Because several muscles constitute the hindl imb, we investigated if specific isoforms are expressed in particular muscles. Northern blot analysis using isoform-specific cDNA probes dem onstrated that soleus muscle expressed only the beta1 transcript, wher eas EDL or white gastrocnemius muscles expressed only the beta2 transc ript, and red gastrocnemius muscle expressed both mRNAs. All muscles t ested expressed both alpha1 and alpha2 transcripts, albeit to various degrees: alpha1 transcripts were present to about the same extent in a ll muscles but alpha2 mRNA was 4-fold more abundant in soleus than in EDL for the same amount of total RNA. Beta subunit protein levels were investigated in purified plasma membrane fractions of pooled red (sol eus + red gastrocnemius + red quadriceps) or white (white gastrocnemiu s + white quadriceps) muscles using isoform-specific antibodies. Red m uscles expressed mostly the beta1 protein while white muscles expresse d mostly the beta2 subunit. Both muscle groups had similar levels of a lpha1 or alpha2 subunits, and crude membranes isolated from red muscle s had 30% higher Na+,K+-ATPase activity than white muscle membranes. W e conclude that oxidative muscles (slow and fast twitch) express beta1 subunits, whereas glycolytic, fast twitch muscles express beta2 subun its, and that both beta isoforms support the Na+,K+-ATPase activity of the alpha subunits.