Figure 2. A cartoon of active trans-membrane water cycling (AWC).

The rate constant for steady-state cellular water efflux is k_io; that for steady-state cellular water influx is k_oi. The Na⁺,K⁺-ATPase enzyme is indicated as NKA, while AQP, KCC4, and SGLT are defined in Figure 1. The actions of NKA substrates intracellular Na⁺ and ATP (Na_i⁺ and ATP_i) and extracellular K⁺ (K_o⁺) are indicated in red, as are the NKA inhibitor extracellular ouabain (ouabain_o) and an extracellular AQP inhibitor. Generic transporters I, II, III, and IV are exemplified in Figure 1: the actions of AQP, KCC4, and SGLT are shown here as specific examples. The quantity x is the AWC water stoichiometry integrated over the entire cell. Thus, for example, ^cMR_H2O(influx) = ^cMR_H2O(efflux) = x^cMR_NKA = x(^cMR_Na+(influx))/3 = x(^cMR_K+(efflux))/2, where ^cMR represents a cellular metabolic rate. First and second approximations of this cartoon have appeared in (15) and (2), respectively.