Isolated control of pressure in the carotid sinus was exercised bilaterally by means of pump-driven autoperfusion in anaesthetized heparinized cats breathing 100% O2. In cats with vagus nerves intact, a rise of intrasinus pressure resulted in a non-adapting fall in arterial blood pressure and a short-lasting depression in tidal volume and frequency of breathing that adapted fully in the steady state. Vagotomy increased the steady-state gain of the intrasinus pressure-vasodepressor relationship and resulted sometimes in a measurable sustained inhibition of breathing, following its initial adaptation, at the upper levels of intrasinus pressure tested. Pharmacological stabilization of the blood pressure with the use of guanethidine plus phenylephrine did not detectably affect the adapting character of the sinus reflex respiratory response. Denervation of the carotid sinuses abolished all responses previously evoked by shifts of intrasinus pressure. It is concluded that the reflex effects of carotid sinus autoperfusion are produced by selective activation of the baroreceptors and that the respiratory adaptation results from signal processing in the central nervous system different from the processing involved in the non-adapting vasodepressor effect.
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