TEMPERATURE DEPENDENCY OF THE VAGAL CHRONOTROPIC RESPONSE IN THE YOUNG PUPPY - AN ENVIRONMENTAL-AUTONOMIC INTERACTION

We investigated the effects of mild hypothermia (34.3 +/- 0.2 degrees C [mean +/- SD]), hyperthermia (40.8 +/- 0.2 degrees C) and hypoxia (P a-O2 = 43 +/- 4 mmHg) on the response of heart rate to continuous righ t vagus nerve stimulation (the 'vagal chronotropic response') in young puppies, aged 5-22 days. Puppies were anesthetized with a-chloralose, vagotomized and pre-treated with propranolol (1 mg/kg i.v.) and phent olamine(1 mg/kg, 1-2 mg/kg/h i.v.). Hypoxia (n = 9) did not significan tly alter the resting sinus cycle length and did not alter the respons e of sinus cycle length to a 30 s train of 8 Hz right vagal stimulatio n. Mild hypothermia tn = 8) increased the resting sinus cycle length b y 16 +/- 4% and greatly augmented the vagal chronotropic response (fro m 76 +/- 27% change in the sinus cycle length (normothermia) to 155 +/ - 38% (hypothermia)). Both the sinus cycle length and the vagal chrono tropic response returned towards pre-hypothermia values with rewarming . Mild hypothermia also increased the negative chronotropic response t o 20 mu g/kg/min i.v. of methacholine (12 +/- 2% change in the sinus c ycle length (normothermia) versus 24 +/- 14% (hypothermia)), suggestin g 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 sin us cycle length (normothermia) to 50 +/- 26% (hyperthermia)). These ch anges were also reversible with the re-establishment of normothermia. This demonstrates that clinically relevant, environmentally-induced ch anges in body temperature can directly and reversibly modify parasympa thetic efferent responses.