RESPIRATORY MODULATION OF CAROTID AND AORTIC BODY REFLEX LEFT-VENTRICULAR INOTROPIC RESPONSES IN THE CAT

1. The reflex changes in the inotropic state of the left ventricle, me asured as the dP/dt(max) (maximum rate of change of pressure), occurri ng in response to selective stimulation of the carotid and aortic body chemoreceptors by sodium cyanide, were studied in the cat anaesthetiz ed with a mixture of chloralose and urethane. 2. The animals were arti ficially ventilated with an open pneumothorax. The beast rate and mean arterial blood pressure were maintained constant. 3. With on-going ce ntral respiratory activity, stimulation of the carotid bodies caused a n increase in respiratory movements. Variable changes in left ventricu lar dP/dt(max) occurred, the predominant response being an increase. T he mean change was 8.3+/-2.9% from a control value of 6850 +/- 450 mmH g s(-1). Stimulation of the aortic bodies resulted in a smaller increa se in respiration or no effect, but a significant increase occurred in left ventricular dP/dt(max) of 19.6 +/- 2.9% from a control value of 6136 +/- 228 mmHg s(-1). No significant changes in left ventricular en d-diastolic pressure occurred in response to stimulation of either gro up of chemoreceptors. 4. Tests of chemoreceptor stimulations were repe ated during temporary suppression of the secondary respiratory mechani sms: the central respiratory drive was suppressed reflexly by electric al stimulation of the central cut ends of both superior laryngeal nerv es and lung stretch afferent activity was minimized by stopping artifi cial respiration. Carotid body stimulation again evoked variable respo nses, the predominant now being a reduction in left ventricular dP/dt( max) of 3.1% from a control value of 5720 +/- 320 mmHg s(-1), which wa s significantly different to that occurring during on-going spontaneou s respiration. Aortic body stimulation caused an increase in left vent ricular dP/dt(max) similar to the response during on-going spontaneous respiration. 5. The positive inotropic responses were mediated via th e sympathetic nervous system, as indicated by their abolition as a res ult of intravenous injections of the P-adrenoceptor blocking agent, pr opranolol. 6. It is concluded that the carotid bodies exert a small va riable effect on left ventricular dP/dt(max), the predominant positive inotropic response being due to the concomitant neurogenic effects of the increase in respiration. In contrast, the positive inotropic resp onse to excitation of the aortic chemoreceptors is not respiratory mod ulated.