Effects of selective β1-adrenoceptor blockade on cardiovascular and renal function and circulating cytokines in ovine hyperdynamic sepsis

Widespread activation of the sympathetic nervous system is a well-known phenomenon in sepsis and septic shock [1]-[3]. An increase in sympathetic nerve activity would be expected to be beneficial in sepsis to maintain arterial pressure and organ perfusion, and indeed treatment with noradrenaline is the first-choice vasopressor to maintain arterial pressure in sepsis [4]. In addition, inotropic treatment with the β-1 adrenoceptor agonist dobutamine has been used to increase cardiac output in sepsis, although a recent study found dobutamine treatment was associated with increased mortality [5] and recent guidelines recommend that dobutamine is used only in selected circumstances [4].

Despite the overall beneficial effects of catecholamines in sepsis, there are indications that a protracted and excessive increase in sympathetic nerve activity during critical illness may become maladaptive and exert adverse effects. For example, an increasing body of evidence suggests that septic myocardial depression and failure can in part be mediated by excessive and prolonged activation of the sympathetic nervous system [6],[7]. Indeed, there is evidence in septic shock that inhibiting sympathetic outflow, or blocking the action of catecholamines, may improve survival. Previous studies in sepsis have found contrasting effects depending on whether non-selective or selective β-blockade was used. Non-selective β-blockade with propranolol, started in the first hour of sepsis, improved survival in dogs with endotoxin-induced septic shock [8], and in patients in the late stages of septic shock propranolol improved blood pressure and urine output [9]. In contrast, in mice with sepsis induced by cecal ligation puncture, propranolol caused clinical deterioration and reduced survival [10]. Selective β1-adrenergic blockade with esmolol has been tested in patients with septic shock without a major increase in adverse events [11]. In mice, selective β1-blockade starting two days before treatment with lipopolysaccharide (LPS) or cecal ligation puncture significantly improved survival, although this effect was substantially reduced if β-blockers were not given until 6 h after induction of endotoxemia [12]. In this study, metoprolol reduced plasma levels of interleukin 6 (IL-6) hepatic expression of pro-inflammatory cytokines and cardiac expression of IL-18.

We hypothesised that selective β₁-adrenoceptor blockade with atenolol would be beneficial in developed hyperdynamic sepsis in conscious sheep by reducing the effects of increased cardiac sympathetic activation and also by decreasing the release of inflammatory cytokines, which in excess may contribute to multiple organ failure. We therefore examined the effects of treatment with the selective β₁-adrenoceptor antagonist atenolol on arterial pressure and pro- and anti-inflammatory plasma cytokine levels in an ovine model of hyperdynamic, hypotensive sepsis. Since atenolol may also induce cardiovascular decompensation, reduced organ perfusion and organ failure, we measured cardiac output (CO), renal blood flow (RBF) and renal function.

The main findings of this study are that in an ovine model of hyperdynamic sepsis selective β1-adrenoceptor blockade with atenolol reduced HR and CO, but increased SV. Despite the greater fall in CO during atenolol treatment, the level of hypotension that developed during sepsis was similar to that in the control group. Atenolol did not alter the sepsis-induced renal vasodilatation and increase in RBF and by the end of the atenolol infusion there was a similar level of acute kidney injury. Atenolol did not decrease oxygen extraction or increase arterial lactate levels during sepsis. During sepsis plasma levels of TNF-α, IL-6 and IL-10 increased; atenolol further increased the level of the anti-inflammatory cytokine IL-10, but had no effect on the pro-inflammatory cytokines TNF-α and IL-6.

There is increasing interest in the effects of sympatholytic drugs in sepsis. Recent human trials have shown that esmolol treatment in patients with pneumonia and in septic shock was not associated with major adverse events despite a reduction in CO and DO₂ [11],[14]. In addition, patients on chronic β-blocker therapy admitted to intensive care with sepsis had reduced mortality [15]; such findings may promote the more widespread use of these agents. Similarly, in some experimental studies, β-blockers improved survival in sepsis [8],[12], but it remains unclear why these drugs, which would be expected to be detrimental in a hypotensive disease, had a beneficial action and reduced mortality. It may depend on the reduction in tachycardia leading to improved cardiac function [16] or a reduction in the level of inflammatory cytokines [12],[17], since these molecules can themselves cause organ damage and even death, independently of the underlying infection [18].

In the present study the dose of atenolol used completely blocked cardiac β1-adrenoceptors, as judged by the inhibition of the tachycardic effect of isoproterenol. Interestingly, this dose of atenolol did not reduce HR to control levels, indicating that the sepsis-induced tachycardia was not completely dependent on increased cardiac sympathetic activation. This is in agreement with a previous study in which we examined the arterial baroreflex control of HR in sepsis and concluded that, in addition to sympathetic activation, a reduction in cardiac vagal activity and/or a direct effect of cytokines on the heart accounted for the tachycardia in sepsis [2]. Although atenolol treatment increased SV, this was insufficient to offset the effect of the reduction in HR and throughout the infusion CO remained lower in the atenolol group. This reduction in CO did not result in a greater degree of hypotension during sepsis because there tended to be less peripheral vasodilatation in the atenolol-treated group. The mechanism for this is unclear but is most likely to be a peripheral vascular effect because atenolol is hydrophilic and does not cross the blood-brain barrier. Indeed there is evidence that atenolol causes peripheral vasoconstriction, which has been proposed to be either by a reflex increase in sympathetic activity or blockade of prejunctional β-adrenoceptors [19].

A similar maintenance of blood pressure, with a fall in CO and increase in peripheral resistance, has been reported with propranolol treatment in patients with hyperdynamic septic shock [9]. The finding that requirement for norepinephrine was reduced in patients with septic shock treated with the β-₁ adrenoceptor blocker, esmolol [20], suggests that treatment with β-blockers may improve vascular reactivity in sepsis. The present findings indicate that in sheep with hyperdynamic sepsis arterial pressure is maintained in the presence of selective cardiac sympathetic denervation, whereas we have recently shown that selective renal denervation resulted in a greater degree of hypotension [21].

As we have previously shown, hyperdynamic sepsis was accompanied by renal vasodilatation, an increase in RBF and acute kidney failure [22],[23]. In septic sheep, atenolol had minor effects on renal hemodynamics and renal function despite causing a decrease in CO, probably because atenolol did not induce a greater degree of hypotension. Critically, renal function in the atenolol group, estimated by changes in creatinine clearance, plasma creatinine, urine output, FENa, FEK and FEUN, was similar to that in the control group. In addition, indices of tissue perfusion, oxygen consumption and oxygen extraction, were similar in the atenolol and control groups, as was the increase in blood lactate, indicating similar levels of disease severity in both groups.

Our findings of simultaneous increases in both pro-inflammatory and opposing anti-inflammatory cytokines in this ovine model of sepsis are in accord with findings in septic patients [24]-[26]. Although the levels of TNF-α and IL-10 declined towards normal from 8 to 24 hours of sepsis, the increase in IL-6 was maintained throughout the septic period indicating a maintained hyper-inflammatory phase. Treatment with atenolol did not alter the levels of TNF-α or IL-6, but increased IL-10, indicating no effect of β1-adrenoceptor blockade on the pro-inflammatory response, but an enhancement of the anti-inflammatory response. Currently the mechanism by which atenolol increased IL-10 in ovine sepsis is unclear since in isolated macrophages β-agonists stimulate IL-10 release [27],[28], suggesting that in the whole animal other mechanisms override this effect.

These findings vary from those in murine sepsis where β1-blockade with metoprolol, started two days before sepsis, reduced plasma IL-6 and hepatic expression of key inflammatory genes and reduced mortality, although if treatment was given 6 hours after endotoxaemia there was no significant improvement in survival [12]. The β1-adrenoceptor blocker landiolol reduced both TNF-α and IL-6 and protected against acute lung injury and cardiac dysfunction in LPS-treated rats [29]. In rats, following cecal ligation puncture, esmolol reduced plasma TNF-α, but not IL-1β, and increased myocardial β1-receptor expression [16]. These differences from the present study are probably in part species related considering the vastly different sensitivity to LPS across species, with humans and sheep having a 500 to 1,000 times greater sensitivity than rats and mice [30]. In addition, the different genomic responses to inflammatory stimuli in mice and humans have also questioned the use of murine models of sepsis [31].

Our study has both strengths and limitations. The major strength was that it reproduced severe septic acute kidney injury in a large conscious mammal and described the effects of selective β₁-adrenoceptor blockade, started at a clinically relevant time when signs of sepsis were present, on cardiovascular function, renal function and cytokine levels. The indirect estimation of glomerular filtration rate (GFR) using creatinine clearance is of limited accuracy, but this method is widely used and clinically relevant. Furthermore, its changes were concordant with changes in plasma creatinine. We only investigated the effects of selective β1-adrenocetor blockade so we are not able to distinguish the extent to which the findings are due to β₁-antagonism or selective stimulation of β₂-adrenoceptors. As in all experimental animal studies, the relevance of the findings to the clinical situation is unclear, but as most human sepsis is hyperdynamic in nature it is likely that this model has clinical relevance.

In experimental hyperdynamic sepsis treatment with atenolol, started at a clinically relevant time when sepsis had developed, did not lead to greater hypotension even though CO was reduced. Renal function and indices of tissue perfusion were not adversely affected by atenolol and disease severity appeared similar in the control and treated groups, indicating the safety of the treatment. Atenolol did not influence the plasma levels of the pro-inflammatory cytokines TNF-α and IL-6 but it induced a greater increase in the anti-inflammatory cytokine IL-10. These findings indicate in hyperdynamic sepsis, with a relatively small decrease in arterial pressure and a large increase in CO, that treatment with atenolol had no detrimental cardiovascular or renal effects and had an anti-inflammatory effect. Further studies are required to determine if atenolol is safe in septic shock with circulatory failure.