Effect of verapamil on ventilation and chemical control of breathing in anesthetized rats.

Abstract

The calcium channel blocker, verapamil (0.1-1.0 mg/kg, i.v.) was administered to anesthetized rats to determine its effects on ventilation and on ventilatory responses to hypoxia and CO2. Verapamil produced a dose-dependent increase in tidal volume (VT) and a decrease in respiration rate (f). The bradypnea due to verapamil was characterized by an increase in expiratory duration (TE) and no change of inspiratory duration (TI). Verapamil produced similar changes in VT and f in vagotomized rats. The increase in respiration rate and minute volume due to hypoxia were inhibited by verapamil (0.5 and 1.0 mg/kg) but the increase in tidal volume due to hypoxia was depressed only with the 1.0 mg/kg dose. On the other hand, the increase in VT due to breathing CO2 was not changed by verapamil (0.1-1.0 mg/kg), but depression of the respiratory frequency response to CO2 occurred with 1.0 mg/kg of verapamil. These results indicate that verapamil produced slow, deep breathing and these responses were not mediated by vagal mechanisms. Ventilatory responses to hypoxia were depressed by verapamil. However, since the calcium blocker demonstrated no effect on the VT-CO2 relationship, verapamil did not change ventilatory chemosensitivity to CO2. The data also suggest that mechanisms governing the control of respiratory frequency are more sensitive to verapamil than tidal volume responses.