Expression of β subunit isoforms of the Na+,K+‐ATPase is muscle type‐specific

Abstract

Hindlimb skeletal muscles of the rat express two isoforms of the α (αl and α2) and two isoforms of the β (β1 andβ2) subunits of the Na⁺,K⁺-ATPase. Because several muscles constitute the hindlimb, we investigated if specific isoforms are expressed in particular muscles. Northern blot analysis using isoform-specific cDNA probes demonstrated that soleus muscle expressed only the β1 transcript, whereas EDL or white gastrocnemius muscles expressed only the β2 transcript, and red gastrocnemius muscle expressed both mRNAs. All muscles tested expressed both α1 and α2 transcripts, albeit to various degrees: α1 transcripts were present to about the same extent in all muscles but α2 mRNA was 4-fold more abundant in soleus than in EDL for the same amount of total RNA. β subunit protein levels were investigated in purified plasma membrane fractions of pooled red (soleus + red gastrocnemius + red quadriceps) or white (white gastrocnemius + white quadriceps) muscles using isoform-specific antibodies. Red muscles expressed mostly the β1 protein while white muscles expressed mostly the β2 subunit. Both muscle groups had similar levels of α1 or α2 subunits, and crude membranes isolated from red muscles had 30% higher Na⁺,K⁺-ATPase activity than white muscle membranes. We conclude that oxidative muscles (slow and fast twitch) express β1 subunits, whereas glycolytic, fast twitch muscles express β2 subunits, and that both β isoforms support the Na⁺,K⁺-ATPase activity of the a subunits.