Reduced ambient temperature exacerbates SIRS-induced cardiac autonomic dysregulation and myocardial dysfunction in mice

Sepsis-induced myocardial depression (SIMD) is an early and frequent consequence of the infection-induced systemic inflammatory response syndrome. In homiotherms, variations in ambient temperature (T-a) outside the thermoneutral zone induce thermoregulatory responses mainly driven by a gradually increased sympathetic activity, which may affect disease severity. We hypothesized that thermoregulatory responses upon reduced T-a exposition aggravate SIMD in mice. Mice were kept at neutral T-a (30 +/- 0.5 degrees C), moderately lowered T-a (26 +/- 0.5 degrees C) or markedly lowered T-a (22 +/- 0.5 degrees C), exposed to lipopolysaccharide- (LPS, 10 mu g/g, from Escherichia coli serotype 055:B5, single intraperitoneal injection) evoked shock and monitored for survival, cardiac autonomic nervous system function and left ventricular performance. Primary adult cardiomyocytes and heart tissue derived from treated mice were analyzed for inflammatory responses and signaling pathways of myocardial contractility. We show that a moderate reduction of T-a to 26 degrees C led to a 40% increased mortality of LPS-treated mice when compared to control mice and that a marked reduction of T-a to 22 degrees C resulted in an early mortality of all mice. Mice kept at 26 degrees C exhibited increased heart rate and altered indices of heart rate variability (HRV), indicating sympathovagal imbalance along with aggravated LPS-induced SIMD. This SIMD was associated with reduced myocardial -adrenergic receptor expression and suppressed adrenergic signaling, as well as with increased myocardial iNOS expression, nitrotyrosine formation and leukocyte invasion as well as enhanced apoptosis and appearance of contraction band necrosis in heart tissue. While ineffective separately, combined treatment with the (2)-adrenergic receptor (AR) antagonist ICI 118551 (10ng/gbw) and the inducible nitric oxide synthase (iNOS) inhibitor 1400W (5 mu g/gbw) reversed the increase in LPS-induced mortality and aggravation of SIMD at reduced T-a. Thus, consequences of thermoregulatory adaptation in response to ambient temperatures below the thermoneutral range increase the mortality from LPS-evoked shock and markedly prolong impaired myocardial function. These changes are mitigated by combined (2)-AR and iNOS inhibition.