Temporal responses of oxidative vs. glycolytic skeletal muscles to K+ deprivation: Na+ pumps and cell cations.
When K+ output exceeds input, skeletal muscle releases intracellular fluid K+ to buffer the fall in extracellular fluid (ECF) K+. To investigate the mechanisms and muscle specificity of the K+ shift, rats were fed K+-deficient chow for 2-10 days, and two muscles at phenotypic extremes were studied: slow-twitch oxidative soleus and fast-twitch glycolytic white gastrocnemius (WG). After 2 days of low-K+ chow, plasma K+ concentration ([K+]) fell from 4.6 to 3.7 mM, and Na+-K+-ATPase alpha2 (not alpha1) protein levels in both muscles, measured by immunoblotting, decreased 36%. Cell [K+] decreased from 116 to 106 mM in soleus and insignificantly in WG, indicating that alpha2 can decrease before cell [K+]. After 5 days, there were further decreases in alpha2 (70%) and beta2 (22%) in WG, not in soleus, whereas cell [K+] decreased and cell [Na+] increased by 10 mM in both muscles. By 10 days, plasma [K+] fell to 2.9 mM, with further decreases in WG alpha2 (94%) and beta2 (70%); cell [K+] fell 19 mM in soleus and 24 mM in WG compared with the control, and cell [Na+] increased 9 mM in soleus and 15 mM in WG; total homogenate Na+-K+-ATPase activity decreased 19% in WG and insignificantly in soleus. Levels of alpha2, beta1, and beta2 mRNA were unchanged over 10 days. The ratios of alpha2 to alpha1 protein levels in both control muscles were found to be nearly 1 by using the relative changes in alpha-isoforms vs. beta1- (soleus) or beta2-isoforms (WG). We conclude that the patterns of regulation of Na+ pump isoforms in oxidative and glycolytic muscles during K+ deprivation mediated by posttranscriptional regulation of alpha2beta1 and alpha2beta2 are distinct and that decreases in alpha2-isoform pools can occur early enough in both muscles to account for the shift of K+ to the ECF.
Thompson
C B
CB
Department of Physiology and Biophysics, University of Southern California School of Medicine, Los Angeles, California 90033, USA.
Choi
C
C
Youn
J H
JH
McDonough
A A
AA
D000818:Animals; D002412:Cations / Q000378:metabolism*; D005110:Extracellular Space / Q000378:metabolism; D006019:Glycolysis; D007008:Hypokalemia / Q000201:enzymology / Q000378:metabolism*; D008297:Male; D018482:Muscle, Skeletal / Q000201:enzymology / Q000378:metabolism*; D009994:Osmolar Concentration; D010084:Oxidation-Reduction; D011188:Potassium / Q000378:metabolism; D020033:Protein Isoforms / Q000378:metabolism; D012333:RNA, Messenger / Q000378:metabolism; D051381:Rats; D017207:Rats, Sprague-Dawley; D000254:Sodium-Potassium-Exchanging ATPase / Q000235:genetics / Q000378:metabolism*; D013997:Time Factors
D002412:Cations; D020033:Protein Isoforms; D012333:RNA, Messenger; D000254:Sodium-Potassium-Exchanging ATPase; D011188:Potassium