A Futile Cycle? Tissue Homeostatic Trans-Membrane Water Co-Transport: Kinetics, Thermodynamics, Metabolic Consequences
The phenomenon of active trans-membrane water cycling (AWC) has emerged in little over a decade. Here, we consider H2O transport across cell membranes from the origins of its study. Historically, trans-membrane water transport processes were classified into: A) compensating bidirectional fluxes (exchange), and B) unidirectional flux (net flow) categories. Recent literature molecular structure determinations and molecular dynamic (MD) simulations indicate probably all the many different hydrophilic substrate membrane co-transporters have membrane-spanning hydrophilic pathways and co-transport water along with their substrates, and that they individually catalyze category A and/or B water flux processes, although usually not simultaneously. The AWC name signifies that, integrated over the all the cell co-transporters, the rate of homeostatic, bidirectional trans cytolemmal water exchange (category A) is synchronized with the metabolic rate of the crucial Na+,K+ ATPase (NKA) enzyme. A lit