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, measured as the dP/dt max (maximum rate of change of pressure), occurring in response to selective stimulation of the carotid and aortic body chemoreceptors by sodium cyanide, were studied in the cat anaesthetized with a mixture of chloralose and urethane. 2. The animals were artificially ventilated with an open pneumothorax. The heart rate and mean arterial blood pressure were maintained constant. 3. With on-going central respiratory activity, stimulation of the carotid bodies caused an increase in respiratory movements. Variable changes in left ventricular dP/dt max occurred, the predominant response being an increase. The mean change was 8.3 +/- 2.9 % from a control value of 6850 +/- 450 mmHg s-1. Stimulation of the aortic bodies resulted in a smaller increase 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 end-diastolic pressure occurred in response to stimulation of either group of chemoreceptors. 4. Tests of chemoreceptor stimulations were repeated during temporary suppression of the secondary respiratory mechanisms: the central respiratory drive was suppressed reflexly by electrical stimulation of the central cut ends of both superior laryngeal nerves and lung stretch afferent activity was minimized by stopping artificial respiration. Carotid body stimulation again evoked variable responses, 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 was significantly different to that occurring during on-going spontaneous respiration. Aortic body stimulation caused an increase in left ventricular dP/dt max similar to the response during on-going spontaneous respiration. 5. The positive inotropic responses were mediated via the sympathetic nervous system, as indicated by their abolition as a result of intravenous injections of the beta-adrenoceptor blocking agent, propranolol. 6. It is concluded that the carotid bodies exert a small variable 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 response to excitation of the aortic chemoreceptors is not respiratory modulated.