Erythrocyte water, Na+-K+ cotransport, and forearm vascular function in humans.

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Abstract
We examined the relationships between erythrocyte (RBC) composition (Na+,K+, and water content) and ouabain-insensitive transports (Na+-K+ cotransport, Li+-Na+ countertransport) and forearm vascular hemodynamics under standardized basal conditions and during vasoconstriction (intra-arterial infusion of graded doses of norepinephrine and angiotensin II) and vasodilation (intraarterial phentolamine and postischemic exercise). RBC water content correlated positively and significantly (r = 0.53, p = 0.001) with minimum forearm vascular resistance, a measure of vascular structural change, and negatively with maximal forearm blood flow (r = -0.55, p < 0.001). Similar correlations with forearm vascular resistance and blood flow were observed under all experimental conditions. RBC Na+-K+ cotransport correlated positively and significantly (r = 0.43, p = 0.01) with the change in forearm blood flow produced by phentolamine, a functional measure of .alpha.-adrenergic tone, and was as strong an independent predictor of phentolamine-induced blood flow change as was arterial norepinephrine concentration. RBC Na+-K+ cotransport was also significantly positively correlated with residual forearm blood flow and resistance after phentolamine administration, where nonadrenergic influences predominate. RBC water correlated negatively with Li+-Na+ countertransport (r = -0.33, p < 0.05) and Na+-K+ cotransport (r = -0.44, p < 0.01). We propose that RBC water is a marker for a vascular structural property that contributes to vascular reactivity. RBC Na+-K+ cotransport seems to relate most strongly to the sympathetically mediated control of forearm blood flow and may also be linked to the intrinsic myogenic tone of the forearm vasculature. Li+-Na+ countertransport is not significantly correlated with vascular hemodynamic measures but may, along with Na+-K+ cotransport, relate to the control of cell composition.