Objective: Osmotic gradient-induced volume change and sarcolemmal water permeability of cardiac myocytes were evaluated to characterize the mechanism of water flux across the plasma membranes.
Methods: Cell surface dimensions were measured from isolated guinea-pig and rat ventricular myocytes by digital videomicroscopy, and membrane hydraulic conductivity (L(p)) was obtained by analyzing the time course of cell swelling and shrinkage in response to osmotic gradients.
Results: Superfusion with anisosmotic solution (0.5-4 times normal osmolality) caused a rapid (<3 min to steady states) and reversible myocyte swelling or shrinkage. L(p) was approximately 1.9 x 10(-10) l N(-1) s(-1) for guinea-pig myocytes and approximately 1.7 x 10(-10) l N(-1) s(-1) for rat myocytes at 35 degrees C. Arrhenius activation energy (E(a)), a measure of the energy barrier to water flux, was approximately 3.7 (guinea-pig) and approximately 3.6 kcal mol(-1) (rat) between 11 and 35 degrees C; these values are equivalent to E(a) of self-diffusion of water in bulk solution ( approximately 4 kcal mol(-1)). Treatment with 0.1 mM Hg(2+), a sulfhydryl-oxidizing reagent that blocks membrane water channels, reduced L(p) by approximately 80%, and the sulfhydryl-reducing reagent dithiothreitol (10 mM) antagonized the inhibitory action of Hg(2+). Inhibition of the volume-sensitive cation (30 microM Gd3+) and anion (1 mM 4,4'-diisothiocyanostilbene-2,2'-disulfonate) channels and Na+-K+ pump (10 microM ouabain) modified the size of osmotic swelling but had little effect on L(p).
Conclusions: Although the observed L(p) is relatively small in magnitude, the low E(a) and the sulfhydryl reagent-induced modification of L(p) are characteristic of channel-mediated water transport. These data suggest that water flux across the sarcolemma of guinea-pig and rat heart cells occurs through parallel pathways, i.e., the majority passing through water channels and the remainder penetrating the lipid bilayers.