Effects of myocardial hypertrophy on neural reflexes controlling cardiovascular function

There are clinical and experimental evidences that the cardiopulmonary reflex function is impaired in chronic hypertension, but it could be due to myocardial hypertrophy rather than to hypertension itself. To test this hypothesis we evaluated the Bezold-Jarisch reflex in experimental conditions of myocardial hypertrophy and arterial normotension. Adult male Wistar rats were subjected to myocardial hypertrophy (MHR) treating them with the beta-adrenoceptor agonist isoproterenol (0.3 mg/kg/day, s.c.) for 15 days and compared with vehicle injected control rats (CR). No significant changes in body weight (283+/-14 vs. 299+/-9 g), resting mean arterial pressure (104+/-4 vs. 110+3 mm Hg) or heart rate (330+/-11 vs. 358+/-18 bpm) were observed in MHR compared to CR. As expected, MHR showed left and right ventricular and left atrial hypertrophy when compared to CR. The bradycardia and hypotension that characterizes the Bezold-Jarisch reflex, induced by the 5-HT3, agonist phenyldiguanide (1.5-24.0 microg/kg, i.v.), were significantly decreased in MHR compared to CR. Cardiac muscarinic responsiveness, which was assessed by electrical stimulation of the efferent vagus in anesthetized animals or by stimulation of muscarinic receptors in isolated hearts, was unchanged or increased, respectively, in MHR compared to CR. Additional studies showed that the baroreflex and chemoreflex were also attenuated in MHR compared to CR. These data indicate that cardiac hypertrophy impairs the Bezold-Jarisch reflex probably due to changes at central integrative areas of the reflex.