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Molecular water pumps.

¹Panum Institute, Copenhagen, Denmark.

Reviews of Physiology, Biochemistry and Pharmacology

|August 1, 2000

Summary

Cotransporters act as molecular water pumps, using substrate gradients to drive secondary active water transport. This mechanism, observed across various symports, is crucial for cellular water homeostasis and vectorial transport in epithelia.

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Area of Science:

Membrane protein function
Secondary active transport
Cellular physiology

Background:

Cotransporters, particularly symports, facilitate the movement of solutes across cell membranes.
Water transport across cell membranes is essential for maintaining cellular homeostasis.
The role of cotransporters in water movement has been an area of ongoing investigation.

Purpose of the Study:

To investigate the role of cotransporters as molecular water pumps.
To elucidate the mechanism by which water flux is coupled to substrate transport.
To explore the physiological implications of cotransporter-mediated water transport.

Main Methods:

Analysis of experimental data from various symport systems, including K+/Cl-, H+/lactate, Na+/glucose (SGLT1), and Na+/dicarboxylate cotransporters.
Flux measurements initiated by rapid changes in external solution composition.
Theoretical evaluations and experimental introduction of unstirred layers to assess their impact on water flux.

Main Results:

Evidence supports cotransporters functioning as molecular water pumps, coupling water flux to substrate fluxes via secondary active transport.
This phenomenon is observed across diverse symport proteins, suggesting a common mechanism based on conformational changes.
Experimental data indicate a molecular model where water is cotransported as a substrate, with a constant flux ratio independent of external parameters.

Conclusions:

Cotransporters possess an intrinsic water-pumping capability, utilizing stored energy in substrate gradients for uphill water transport.
This molecular water pump mechanism contributes to cellular water homeostasis and is relevant for vectorial water transport across epithelia.
The findings suggest a paradigm shift in understanding cellular water balance, viewing it as a interplay between water pumps and leaks.

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