[Evaluation of the vagal efferent pathway in rats in the acute and chronic phases of myocardial infarction]. / Avaliação da via eferente vagal em ratos nas fases aguda e crônica do infarto do miocárdio.

ABSTRACT

PURPOSE: To investigate the sensitivity of the muscarinic receptors to acetylcholine (Ach) and to vagal stimulation in rats during the acute and the chronic phases of myocardial infarction (MI). METHODS: Male albino rats were submitted to ligature of the descending anterior branches of the left coronary artery to produce MI. Control rats (Con) were submitted to a sham surgery. The animals were studied 1-3 days (acute phase) or 30 days (chronic phase) after surgery. Under anesthesia (ketamine+xylazine) the right vagus nerve was isolated at the neck and stimulated with suprathreshold pulses (2ms, 1-64Hz). Atrial and ventricular rates were measured in the ECG recording. Dose-response curve to Ach (5-80 micrograms) was studied in the isolated hearts perfused according to the Langendorftechnique. Atrial and ventricular rates were evaluated through the surface electrogram recording. The left ventricular pressure was measured with an intraventricular balloon. RESULTS: Basal heart rate in the anesthetized animals was similar in Con and MI rats. The vagal stimulation produced a frequency dependent reduction of the heart rate. This reduction was less intense in the MI groups to stimulation rates of 32 and 64Hz. It was not observed any difference in the sensitivity of sinus and AV nodes to exogenous Ach in infarcted hearts. The reduction of the systolic pressure obtained after Ach administration to the hearts paced artificially (3.3Hz) was similar in MI and Con hearts. CONCLUSION: MI hearts were less sensitive to vagal stimulation than Con hearts. Since the in vitro effects of Ach remained unchanged after infarction, these results suggest an impairment of the cardiac neuroeffector vagal synapse. This may contribute to a less efficient control of the heart rate by the parasympathetic pathway in infarcted individuals.