Interference of lung distension with the cardiovascular response to chemoreceptor stimulation in anaesthetized rats

Experiments were performed to examine the role of the stimulation of pulmonary stretch receptors in cardiovascular response to peripheral chemoreceptor stimulation in spontaneously breathing anaesthetized rats. The effects of continuous positive tracheal pressure (0.1 to 0.4 kPa) were examined in normal rats and in rats pretreated with almitrine bismesylate, a potent stimulator of the arterial chemoreceptors. In the untreated group, an increase of a maximum 4 +/- 0.7 beats.min-1 in heart rate was found during testing at pressure of 0.2 and 0.3 kPa when interference with augmented breath was avoided. These pressures at the tracheal level had no effect on systemic blood pressure, so that baroreflex influences can be discounted. During strong almitrine-induced chemostimulation, a prolonged bradycardia developed with the long-lasting hyperventilation. The same continuous positive pressures were unable to overcome the chemoreflex bradycardia. No changes in heart rate were observed under these conditions, whatever the potentiation of the mecanoreceptors for distension in the presence of alveolar hypocapnia. It was concluded that the stretching of lung mecanoreceptors on the cardiac control is of little importance in anaesthetized rats and that bradycardia generally dominates during strong stimulation of the chemoreceptors even when lung distension is artificially increased.