Model simulation of the steady-state values of intracellular Na (Na

_{i}) for altered ratios of Na pump subunit isoforms α1 to α3. The steady state is assumed to be the result of the Na pump operating to offset the inward net leakage of Na

_{o} (external concentration of Na). The following equation, based on previous formulations (32, 33) was used to estimate the values given in the figure:

where

*L* is the leak rate constant in sec

^{−1}. Na

_{o} is held constant and

*X* is the fraction of pumps that are α3, with 1 −

*X* being the fraction of pumps that are α1.

*K* and

*K* represent the relative affinities the two isoforms have for Na

_{i} as indicated in the inset together with their respective

*V*_{max} values. These kinetic parameters, determined at room temperature, are taken from ref. 28. It should also be noted that the turnover numbers given in table 1 (ref. 28) are taken to represent the

*V*_{max} values given in the

*Inset*. The leak flux was determined by using Na

_{i} ≈ 7.0, Na

_{o} = 145, assuming only α1 present and the

*K*_{1/2} and

*V*_{max} values given in the

*Inset*. The leak value so obtained (0.025 sec

^{−1}) was assumed to remain constant independent of the ratio of α1 to α3. The values in the graph were obtained by choosing values of Na

_{i} and calculating

*X*, keeping the total number of pumps, α1 + α3, constant. The denominators in the equation are raised to the power 3 to take into account that three sodium ions are effluxed per pump cycle. See text for discussion.

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