Turnover rates of the canine cardiac Na,K-ATPases

Two functional isoforms (₁) and ⁺ (₃) of the Na,K-ATPase catalytic subunit coexist in canine cardiac myocytes J. Biol. Chem. (1987) 262, 8941-8943]. The in vitro turnover rates of ATP hydrolysis have been determined in sarcolemma preparations by comparing ³H]ouabain-binding and Na,K-ATPase activity at various doses of ouabain (0.3–300 nM). The correlation between the occupancy of the ouabain-binding sites and the degree of Na,K-ATPase inhibition was not linear. The results showed that the form of low-affinity for ouabain (K_d = 300–700 nM) exhibited a lower turnover rate (88 ± 10 vs. 147 ± 15 molecules of ATP hydrolyzed per second per ouabain-binding site) than the high affinity form (K_d = 1–8 nM). Thus our results indicate this specific isoform kinetic difference could contribute to differences in the cardiac cellular function.