Effects of chloride substitution on electromechanical responses in the pulmonary artery of Dahl normotensive and hypertensive rats

Abstract

1. We have investigated the in vitro interaction between chloride ions and endothelium as revealed by alterations in vascular contractility and smooth muscle cell membrane potential in isolated pulmonary arteries from Dahl salt-resistant normotensive and salt-sensitive hypertensive rats. 2. Exposure to nitro-l-arginine methyl ester (l-NAME) of tissues from normotensive but not hypertensive rats augmented contractions to cirazoline. While chloride removal did not alter cirazoline-induced contractions, it completely abolished the augmentation by l-NAME in normotensive rats. However, in hypertensive rats, removal of chloride ions significantly attenuated contractions elicited by cirazoline, and l-NAME effectively reversed this inhibition. 3. Methacholine-induced endothelium-dependent relaxations of the same magnitude were evident in both normotensive and hypertensive rats. However, basal cyclic GMP levels were found to be significantly higher (7.8-fold) in blood vessels of normotensive rats compared to hypertensive rats. 4. The resting membrane potential in pulmonary arteries of hypertensive rats (-52.1+/-1.04 mV) revealed a significant hyperpolarisation when compared with that of normotensive rats (-46.4+/-1.58 mV). Cirazoline did not produce a significant depolarisation in blood vessels of either normotensive or hypertensive rats. Perfusion with chloride-free solution resulted in a modest but significant hyperpolarisation (-8.0 mV) in the blood vessels of hypertensive but not in normotensive rats. 5. We conclude that salt-dependent hypertension in Dahl rats is accompanied by functional and biochemical changes in low-pressure blood vessels. These changes can, in part, be attributed to impairment in the basal, but not methacholine-stimulated, release of nitric oxide, and to altered chloride ion handling.

Figure 1

Concentration–contraction curves for cirazoline in endothelium-intact pulmonary artery rings (a) from DSR normotensive rats in normal Krebs solution (NK) plus twice-distilled water (TDW; closed circles), in normal Krebs solution containing L-NAME (10 μM; open circles), or from DSS hypertensive rats in normal Krebs solution plus twice-distilled water (closed triangles), in normal Krebs solution containing L-NAME (10 μM; open triangles), and (b) from salt-resistant normotensive rats in chloride-free buffer (Cl.Free) plus twice-distilled water (closed circles), in chloride-free buffer containing L-NAME (10 μM; open circle), or from salt-sensitive hypertensive rats in chloride-free buffer plus twice-distilled water (closed triangles), and in chloride-free buffer containing L-NAME (10 μM; open triangle). Each point represents a mean of six experiments±s.e.m.

Figure 2

Concentration–relaxation curves for methacholine in endothelium-intact pulmonary artery rings (a) from DSR normotensive rats in normal Krebs solution (NK) plus twice-distilled water (TDW; closed circles; pD₂=6.33±0.06^a; n_H=1.22±0.15^a; %E_max=75±6^a), in normal Krebs solution containing L-NAME (10 μM; open circles; pD₂=6.79±0.16; n_H=0.6±0.04; %E_max=11±1; P<0.05^a), or from DSS hypertensive rats in normal Krebs solution plus twice-distilled water (closed triangles; pD₂=6.49±0.17^b; n_H=1.49±0.20^b; %E_max=77±8^b), in normal Krebs containing L-NAME (10 μM; open triangles; pD₂=5.81±0.12; n_H=0.53±0.02; %E_max=9±1; P<0.05^b), and (b) from salt-resistant normotensive rats in chloride-free buffer (Cl.Free) plus twice-distilled water (closed circles), in chloride-free buffer containing L-NAME (10 μM; open circles; n_H=0.56±0.09; %E_max=14±3; P<0.05^b), or from salt-sensitive hypertensive rats in chloride-free buffer plus twice-distilled water (closed triangles), and in chloride-free buffer containing L-NAME (10 μM; open triangles; n_H=0.43±0.09; %E_max=12±1; P<0.05^b). Each point represents a mean of six experiments±s.e.m.

Figure 3

(a) Concentration–contraction curves for cirazoline in endothelium-denuded pulmonary artery rings from DSR normotensive rats in normal Krebs (NK) plus twice-distilled water (TDW; closed circles), in chloride-free buffer (Cl.Free) plus twice-distilled water (open circles), or from DSS hypertensive rats in normal Krebs plus twice-distilled water (closed triangles), chloride-free buffer plus twice-distilled water (open triangles). (b) Concentration–response curves for methacholine in endothelium-denuded pulmonary artery rings from salt-resistant normotensive (closed circle) or salt-sensitive hypertensive (closed triangles) rats, and concentration–relaxation curves for sodium nitroprusside in endothelium-denuded pulmonary artery rings from salt-resistant normotensive rats (open circles; pD₂=8.4±0.05; n_H=0.76±0.06; n=6; mean±s.e.m.), and salt-sensitive hypertensive (open triangles; pD₂=8.7±0.09; n_H=0.81±0.09; n=6; mean±s.e.m.) in normal Krebs solution. Each point represents a mean of six experiments±s.e.m.