Cardiovascular responses to carotid chemoreceptor stimulation in the dog: their modulation by urinary bladder distension

Respiratory, heart rate and hindlimb vascular responses were studied in response to increasing levels of stimulation of the carotid body chemoreceptors, together with an examination of the modulation of their effects by distension of the urinary bladder in the dog anaesthetized with a mixture of chloralose and urethane. The vascularly isolated carotid bifurcation regions were perfused with blood, stimulation of the carotid bodies being carried out by three different levels of hypoxic isocapnic blood (PO2 approximately 58, 40 and 22 mmHg) obtained from a donor animal. A vascularly isolated hindlimb was autoperfused at constant blood flow through its femoral artery. In spontaneously breathing animals, increasingly intense hypoxic stimulation of the carotid bodies caused a progressive augmentation of respiratory minute volume. Superimposition of distension of the bladder increased ventilation further, by the same amount during hypoxic as during normoxic blood perfusion of the chemoreceptors. Prevention of the effects of lung stretch afferent stimulation by artificial ventilation modified the heart rate and hindlimb vascular responses to excitation of the carotid bodies by revealing or accentuating the primary cardiovascular responses, bradycardia and vasoconstriction. In contrast, no such respiratory modulation was apparent in the cardiovascular responses to bladder distension. When, under conditions of artificial ventilation and in the absence of changes in the arterial baroreceptor input, the primary cardio-inhibitory and vasoconstrictor responses to carotid chemoreceptor stimulation predominated, the heart slowed progressively as the stimulus was increased. At the same time the cardio-accelerator effects of bladder distension progressively diminished, indicating an interaction between the cardiac reflex responses evoked by the two inputs. In contrast, the reflex vascular responses resulting from stimulation of the two inputs were additive, at least for PO2 levels of carotid body perfusate down to approximately 40 mmHg. In conclusion these experiments demonstrate the differential nature of the integration of respiratory and cardiovascular responses evoked by stimulation of the carotid chemoreceptors and bladder distension.