Inhibition of vascular adenosine triphosphate-sensitive potassium channels by sympathetic tone during sepsis.

OBJECTIVE: Excessive opening of the adenosine triphosphate-sensitive potassium channel in vascular smooth muscle is implicated in the vasodilation and vascular hyporeactivity underlying septic shock. Therapeutic channel inhibition using sulfonylurea agents has proved disappointing, although agents acting on its pore appear more promising. We thus investigated the hemodynamic effects of adenosine triphosphate-sensitive potassium channel pore inhibition in awake, fluid-resuscitated septic rats, and the extent to which these responses are modulated by the high sympathetic tone present in sepsis. Temporal changes in ex-vivo channel activity and subunit gene expression were also investigated. DESIGN: In vivo and ex vivo animal study. SETTING: University research laboratory. SUBJECTS: Male adult Wistar rats. INTERVENTIONS AND MEASUREMENTS: Fecal peritonitis was induced in conscious, fluid-resuscitated rats. Pressor responses to norepinephrine and PNU-37883A (a vascular adenosine triphosphate-sensitive potassium channel inhibitor acting on the Kir6.1 pore-forming subunit) were measured at 6 or 24 hrs, in the absence or presence of the autonomic ganglion blocker, pentolinium. The aorta and mesenteric artery were examined ex vivo for rubidium efflux as a marker of adenosine triphosphate-sensitive potassium channel activity, and for adenosine triphosphate-sensitive potassium channel subunit gene expression using quantitative reverse transcription-polymerase chain reaction. MAIN RESULTS: A total of 120 rats (50 sham-operated controls, 70 septic) were included. Septic rats became hypotensive after 12 hrs, with a 24-hr mortality of 51.7% (0% in controls). At 6 hrs, there was an attenuated pressor response to norepinephrine (p < .01) despite blood pressure being elevated (p < .01). PNU-37883A had no pressor effect, except in the presence of pentolinium (p < .01). Kir6.1 subunit mRNA increased significantly in the mesenteric artery while rubidium efflux was increased in both the aorta and mesenteric artery at 24 hrs. CONCLUSIONS: Despite evidence of increased adenosine triphosphate-sensitive potassium channel activity in sepsis, it appears to be inhibited in vivo by high sympathetic tone. This may explain, at least in part, the reduced efficacy of adenosine triphosphate-sensitive potassium channel blockers in human septic shock.

Variable effects of inhibiting iNOS and closing the vascular ATP-sensitive potassium channel (via its pore-forming and sulfonylurea receptor subunits) in endotoxic shock.

Excess production of NO and activation of vascular ATP-sensitive potassium (K(ATP)) channels are implicated in the hypotension and vascular hyporeactivity associated with endotoxic shock. Using a fluid-resuscitated endotoxic rat model, we compared the cardiovascular effects of an iNOS inhibitor and two distinct inhibitors of the K(ATP) channel. Endotoxin (LPS) was administered to anesthetized, spontaneously breathing, fluid-resuscitated adult male Wistar rats, in which MAP, aortic and renal blood flow, and hepatic microvascular oxygenation were monitored continuously. At 120 min, the iNOS inhibitor, GW273629, and the K(ATP)-channel inhibitors, PNU-37883A and glyburide, were administered separately, and their effects on hemodynamics and oxygenation were examined. We found that GW273629 increased MAP over and above the pressor effect achieved in sham animals. Inhibiting K(ATP) channels via the pore-forming subunit (PNU-37883A and high-dose glyburide) produced significant pressor effects, whereas inhibiting the sulfonylurea receptor with low-dose glyburide was ineffective. No agent reversed the fall in aortic or renal blood flow, the fall in hepatic microvascular oxygenation, or the metabolic acidosis that occurred in LPS-treated animals. We conclude that inhibition of the K(ATP) channel via the pore-forming, but not the sulfonylurea receptor subunit, increases blood pressure in a short-term endotoxic model. However, this was not accompanied by any improvement in macrocirculatory or microcirculatory organ blood flow nor reversal of metabolic acidosis. It therefore remains uncertain whether the iNOS pathway or the K(ATP) channel represents a potential target for drug development in the treatment of endotoxic shock.

Differential effects of vasopressin and norepinephrine on vascular reactivity in a long-term rodent model of sepsis.

OBJECTIVE: There is escalating interest in the therapeutic use of vasopressin in septic shock. However, little attention has focused on mechanisms underlying its pressor hypersensitivity, which contrasts with the vascular hyporesponsiveness to catecholamines. We investigated whether a long-term rodent model of sepsis would produce changes in endogenous levels and pressor reactivity to exogenous norepinephrine and vasopressin comparable with those seen in septic patients. DESIGN: In vivo and ex vivo animal study. SETTING: University research laboratory. SUBJECTS: Male adult Wistar rats. INTERVENTIONS AND MEASUREMENTS: Fecal peritonitis was induced in conscious, fluid-resuscitated rats. Biochemical and hormonal profiles were measured at time points up to 48 hrs. Pressor responses to intravenous norepinephrine, vasopressin, and F-180, a selective V1 receptor agonist, were measured at 24 hrs. Contractile responses to these drugs were assessed in mesenteric arteries taken from animals at 24 hrs using wire myography. Comparisons were made against sham operation controls. MAIN RESULTS: Septic rats became unwell and hypotensive, with a mortality of 64% at 48 hrs (0% in controls). Plasma norepinephrine levels were elevated in septic animals at 24 hrs (1968 +/- 490 vs. 492 +/- 90 pg/mL in controls, p = .003), whereas vasopressin levels were similar in the two groups (4.5 +/- 0.8 vs. 3.0 +/- 0.5 pg/mL, p = not significant). In vivo, the pressor response to norepinephrine was markedly reduced in the septic animals, but responses to vasopressin and F-180 were relatively preserved. In arteries from septic animals, norepinephrine contractions were decreased (efficacy as measured by maximum contractile response, Emax: 3.0 +/- 0.3 vs. 4.7 +/- 0.2 mN, p < .001). In contrast, the potency of vasopressin (expressed as the negative log of the concentration required to produce 50% of the maximum tension, pD2: 9.1 +/- 0.04 vs. 8.7 +/- 0.05, p < .001) and F-180 (pD2 8.2 +/- 0.04 vs. 7.6 +/- 0.02, p < .001) was enhanced (n > or = 6 for all groups). CONCLUSIONS: This long-term animal model demonstrates changes in circulating vasoactive hormones similar to prolonged human sepsis, and decreased pressor sensitivity to norepinephrine. Ex vivo sensitivity to vasopressin agonists was heightened. This model is therefore appropriate for the further investigation of mechanisms underlying vasopressin hypersensitivity, which may include receptor or calcium-handling alterations within the vasculature.