Effect of the Na-K-2Cl cotransporter NKCC1 on systemic blood pressure and smooth muscle tone

Abstract

Studies in rat aorta have shown that the Na-K-2Cl cotransporter NKCC1 is activated by vasoconstrictors and inhibited by nitrovasodilators, contributes to smooth muscle tone in vitro, and is upregulated in hypertension. To determine the role of NKCC1 in systemic vascular resistance and hypertension, blood pressure was measured in rats before and after inhibition of NKCC1 with bumetanide. Intravenous infusion of bumetanide sufficient to yield a free plasma concentration above the IC(50) for NKCC1 produced an immediate drop in blood pressure of 5.2% (P < 0.001). The reduction was not prevented when the renal arteries were clamped, indicating that it was not due to a renal effect of bumetanide. Bumetanide did not alter blood pressure in NKCC1-null mice, demonstrating that it was acting specifically through NKCC1. In third-order mesenteric arteries, bumetanide-inhibitable efflux of (86)Rb was acutely stimulated 133% by phenylephrine, and bumetanide reduced the contractile response to phenylephrine, indicating that NKCC1 influences tone in resistance vessels. The hypotensive effect of bumetanide was proportionately greater in rats made hypertensive by a 7-day infusion of norepinephrine (12.7%, P < 0.001 vs. normotensive rats) but much less so when hypertension was produced by a fixed aortic coarctation (8.0%), again consistent with an effect of bumetanide on resistance vessels rather than other determinants of blood pressure. We conclude that NKCC1 influences blood pressure through effects on smooth muscle tone in resistance vessels and that this effect is augmented in hypertension.

Figure 1

Effect of bumetanide on blood pressure in anesthetized rats. Solid symbols, vehicle control; open symbols, bumetanide. Bumetanide dose was 1.2 mg/kg given intravenously. Results are the means of 3 to 5 rats. *, p < 0.01 vs. vehicle; **, p < 0.001 vs. vehicle. Error bars, standard errors.

Figure 2

Effect of bumetanide on blood pressure in mice lacking NKCC1. Bumetanide dose was 1.2 mg/kg given intravenously. Results are means of 10-12 individual mice. *, p < 0.02 vs. vs. vehicle. Error bars, standard errors.

Figure 3

Effect of phenylephrine on Rb efflux in rat mesenteric arteries. A. Efflux before and after addition of 10 :M phenylephrine (arrow) in the absence (solid symbols) and presence (open symbols) of 50 :M bumetanide. B. Bumetanide-sensitive efflux. Results are the means of measurements in 10 separate vessels. Error bars, standard errors.

Figure 4

Force generation in mesenteric arteries in the absence (solid circles) and presence (open circles) of 10 :M bumetanide. Results are the means of measurements in 9 (control) or 12 (+ bumetanide) separate vessels. *, p < 0.001 vs. vehicle; **, p < 0.02 vs. vehicle. The EC₅₀ for phenylephrine was 0.59 ± 0.08 :M in control aortas and 1.07 ± 0.01 :M (p < 0.01) in bumetanide-treated aortas. Error bars, standard errors.

Figure 5

Effect of bumetanide on blood pressure in anesthetized, hypertensive rats. The data for normotensive rats are the same as presented in Fig. 1. Solid symbols, vehicle control; open symbols, bumetanide. Bumetanide dose was 1.2 mg/kg given intravenously. Results are the means of 3 to 5 rats. *, p < 0.01 vs. vehicle; **, p < 0.001 vs. vehicle. Error bars, standard errors.