Heart rate responses to intravenous serotonin in rats with acute chagasic myocarditis.

We administered serotonin to rats with experimentally induced chagasic myocarditis in order to study the Bezold-Jarisch reflex. Sixteen 4-month old Wistar rats were inoculated with 200,000 T. cruzi parasites ("Y" strain). Between days 18 and 21 (acute stage), 8 infected rats and 8 age-matched controls received intravenous serotonin as a bolus injection at the following doses: 0.5, 1.0, 2.0, 4.0, 6.0, 8.0, 10.0, 12.0, and 14.0 micrograms/kg. Heart rate was recorded before, during and after each dose of serotonin. The remaining 8 infected animals and 8 controls were subjected to the same experimental procedure during the subacute stage, i.e., days 60 to 70 after inoculation. The baseline heart rate of the infected animals studied during the acute stage (327 +/- 62 beats/min, mean +/- SD) was higher than that of the controls (248 +/- 52, P < 0.01). The heart rate changes were expressed as percent changes to correct for the higher baseline heart rate of the infected animals. A dose-response curve was constructed for each group of animals. The slope for the acutely infected animals (r = -0.95, b = -3.98) was not different from that for the control animals (r = -0.92, b = -3.50). The infected animals studied during the subacute stage (r = -0.92, b = -4.33) were not different from the age-matched controls (r = -0.87, b = -4.03). These results suggest that the afferent and efferent pathways which mediate the Bezold-Jarisch reflex are functionally preserved in rats with histologically proved chagasic myocarditis.