Temperature Dependency of the Vagal Chronotropic Response in the Young Puppy: An 'Environmental-Autonomic Interaction'

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We investigated the effects of mild hypothermia (34.3 ± 0.2°C [mean ± SD]), hyperthermia (40.8 ± 0.2°C) and hypoxia (Pa(O2) = 43 ± 4 mmHg) on the response of heart rate to continuous right vagus nerve stimulation (the 'vagal chronotropic response') in young puppies, aged 5-22 days. Puppies were anesthetized with α-chloralose, vagotomized and pre-treated with propranolol (1 mg/kg i.v.) and phentolamine(1 mg/kg, 1-2 mg/kg/h i.v.). Hypoxia (n = 9) did not significantly alter the resting sinus cycle length and did not alter the response of sinus cycle length to a 30 s train of 8 Hz right vagal stimulation. Mild hypothermia (n = 8) increased the resting sinus cycle length by 16 ± 4% and greatly augmented the vagal chronotropic response (from 76 ± 27% change in the sinus cycle length (normothermia) to 155 ± 38% (hypothermia)). Both the sinus cycle length and the vagal chronotropic response returned towards pre-hypothermia values with rewarming. Mild hypothermia also increased the negative chronotropic response to 20 μg/kg/min i.v. of methacholine (12 ± 2% change in the sinus cycle length (normothermia) versus 24 ± 14% (hypothermia)), suggesting that a postsynaptic mechanism is involved in the hypothermia-induced augmentation of the cardiac vagal chronotropic response. In contrast to hypothermia, mild hyperthermia (n = 8) decreased the resting sinus cycle length slightly (-5 ± 5% change) and significantly attenuated the cardiac vagal chronotropic response (from 88 ± 28% change in sinus cycle length (normothermia) to 50 ± 26% (hyperthermia. These changes were also reversible with the re-establishment of normothermia. This demonstrates that clinically relevant, environmentally-induced changes in body temperature can directly and reversibly modify parasympathetic efferent responses.



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