Abstract

Cyclopropane (25 to 50%) was added to the inspired air of dogs and cats, while nerve impulses were recorded from the nerves of carotid sinus baroreceptors and carotid body chemo-receptors. During pulsatile distension of the carotid sinus, the gas usually caused a greater peak discharge frequency and greater number of impulses per heart beat even when the blood pressure fell. With constant pressure distensions of the carotid sinus cyclopropane invariably lowered the pressure threshold of the receptors, and increased their discharge frequencies over the whole range of distensions. Cooling the vagosympathetic trunk to block conduction in the sympathetic nerve fibers to the carotid sinus did not change the responses of baroreceptors to constant pressure excitation either with or without administration of cyclopropane. Acute hemorrhage did not alter the responses of baro-receptors to constant pressure stimulations. It is concluded that cyclopropane acts by a direct effect on the baroreceptors or on the carotid sinus wall, and not via the sympathetic nervous system. The activity in nerve fibers from carotid body chemoreceptors was not appreciably changed by cyclopropane.