NO and KATP channels underlie endotoxin-induced smooth muscle hyperpolarization in rat mesenteric resistance arteries

Abstract

1 Smooth muscle membrane potential and tension measurements were made in isolated mesenteric resistance arteries from rats exposed to bacterial endotoxin (lipopolysaccharide, LPS; 10 mg kg(-1), i.p.) for 3 h to mimic septic shock syndrome. 2 Over this period, rats developed an endotoxaemic response, assessed in vivo as a 41+/-4 mmHg drop in mean blood pressure, vascular hyporeactivity to noradrenaline (1 microg kg(-1), i.v.) and a significant increase in core body temperature. 3 In mesenteric small resistance arteries from these rats (o.d. 180 - 240 microm), phenylephrine (0.01-3 microm)-evoked contraction was not altered when compared with arteries from sham-operated animals, but the concentration-relaxation curve to acetylcholine (ACh; 0.01 - 3 microm) displayed a small, but significant, shift to the right. 4 The smooth muscle resting membrane potential (-70.3+/-1.6 mV) in arteries from LPS-treated rats was significantly greater than in control arteries (-55.4+/-1.2 mV), but in both cases the smooth muscle was depolarized to a similar potential by the application of N(omega)-nitro-L-arginine methyl ester (L-NAME; 0.3 mm; -54.1+/-2.3 vs -52.4+/-2.5 mV) or glibenclamide (10 microm; -55.0+/-2.1 vs -50.4+/-2.0 mV). 5 ACh (1 microm) elicited a maximal hyperpolarization, which ranged from -14.7+/-3.2 mV (in arteries from LPS-treated rats) to -20.6+/-2.4 mV (in arteries from sham-operated rats), and was not altered by the presence of L-NAME. Levcromakalim (1 microm) increased the smooth muscle membrane potential by around -24 mV in arteries from both sets of experimental animals. 6 These results indicate that at the level of the resistance vasculature, endotoxaemia is associated with pronounced smooth muscle hyperpolarization reflecting the action of NO on KATP channels. These changes were not associated with vascular hyporeactivity or depressed endothelial cell function in vitro, suggesting that mesenteric resistance arteries may not contribute to equivalent changes in vivo.

Figure 1

Effects of endotoxin on (a) MAP, (b) pressor responses to NA and (c) rectal temperature in the anaesthetized rat. Depicted are the changes in MAP, pressor responses to NA (1 μg kg⁻¹ i.v.) and rectal temperature during the experimental period in animals, which received injection of vehicle (SOP; n=7) or E. coli LPS (10 mg kg⁻¹; n=9) for 3 h. Data are expressed as mean±s.e.m. ^*P<0.05 represents significant differences when compared with the SOP group.

Figure 2

Effects of endotoxin on (a) the contraction induced by PE and (b) the relaxation induced by ACh ex vivo. Depicted are the changes in the contraction induced by PE (0.01–3 μM) and in the relaxation induced by ACh (0.01–3 μM) in mesenteric small resistance arterial rings obtained from animals that received injection of vehicle (saline; sham-operated, SOP; n=7) or E. coli LPS (10 mg kg⁻¹; n=9) for 3 h. Data are expressed as mean±s.e.m. ^*P<0.05 represents significant differences when compared with the SOP group.

Figure 3

Effects of L-NAME on the basal membrane potential changes in mesenteric small resistance arteries from rats treated with saline or endotoxin. Depicted are the changes in the membrane potential of basal (n=7) and when challenged with L-NAME (0.3 mM, n=5–7) in mesenteric small resistance arterial rings obtained from animals that received injection of vehicle (saline; sham-operated, SOP) or E. coli LPS (10 mg kg⁻¹) for 3 h. Data are expressed as mean±s.e.m. ^*P<0.05 represents significant differences when compared with the SOP group.

Figure 4

Changes of membrane potential caused by L-NAME and glibenclamide (GB) in mesenteric small resistance arteries from rats treated with saline or endotoxin. Depicted are the changes in the membrane potential at the presence of L-NAME (n=8–11) and GB (n=3–4) in mesenteric small resistance arterial rings obtained from animals that received injection of vehicle (saline; sham-operated, SOP) or E. coli LPS (10 mg kg⁻¹) for 3 h. Data are expressed as mean±s.e.m. ^*P<0.05 represents significant differences when compared with the SOP group.

Figure 5

Changes of membrane potential caused by ACh, L-NAME plus ACh, and LV in mesenteric small resistance arteries from rats treated with saline or endotoxin. Depicted are the changes in the membrane potential in the presence of ACh (n=7–8), L-NAME plus ACh (1 μM, n=4–5), and LV (n=3–5) in mesenteric small resistance arterial rings obtained from animals that received injection of vehicle (saline; sham-operated, SOP) or E. coli LPS (10 mg kg⁻¹) for 3 h. Data are expressed as mean±s.e.m.