Control of the Myocardial Contractile State by Carotid Chemo- and Baroreceptor and Pulmonary Inflation Reflexes in Conscious Dogs

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The effects of carotid chemoreceptor stimulation with intracarotid injections of either nicotine, 0.2 μg/kg, or cyanide, 2 μg/kg, were compared with the effects of bilateral carotid occlusion on left ventricular (LV) pressure, dP/dt, and diameter in conscious dogs instrumented with ultrasonic diameter gauges and miniature pressure gauges. With heart rate maintained constant, carotid chemoreceptor stimulation increased mean arterial pressure by 27±3%, LV and diastolic diameter by 4±0.9% and LV dP/dt by 21±2%. With ventilation controlled during succinylcholine infusion, carotid chemoreceptor stimulation increased mean arterial pressure by 43±2% and dP/dt by 37±5%, values significantly greater, P < 0.01, than were observed in dogs with spontaneous ventilation. Similarly, in dogs with spontaneous ventilation after vagotomy, carotid chemoreceptor stimulation also increased dP/dt by a greater amount, i.e., by 48±9%. The increases in LV end diastolic diameter were not affected significantly by either cholinergic blockade with atropine or beta adrenergic blockade with propranolol. Although cholinergic blockade did not affect the inotropic or pressor responses significantly, beta adrenergic blockade attenuated the pressor response and essentially abolished the inotropic response. Bilateral carotid occlusion increased mean arterial pressure and LV end diastolic diameter by similar amounts to those observed with chemoreceptor stimulation, but increased dP/dt significantly less, P < 0.02, i.e., by 13±2%. As was observed with chemoreceptor stimulation, inotropic responses were not affected significantly by cholinergic blockade, but were essentially abolished by beta adrenergic blockade. Thus, in the conscious dog with heart rate constant, carotid chemoreceptor stimulation induces a clear positive inotropic effect, which is greater in the absence of the attenuating influences of pulmonary inflation reflexes, and for an equal elevation in arterial pressure appears to exert a greater increase in myocardial contractility than does carotid baro-receptor unloading.

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