Interaction between the responses to stimulation of peripheral chemoreceptors and baroreceptors: the importance of chemoreceptor activation of the defence areas.

It has been shown recently that in the cat anesthetized with althesin, stimulation of carotid chemoreceptors evokes the autonomic components of the alerting stage of the defence reaction including cholinergic vasodilatation in skeletal muscle. The discrepancy between this result and those of previous experiments on animals anethetized with chloralose or barbiturates may be reconciled when it is realized that these more conventional anesthetics prevent normal transmission through the defence areas. The obvious conclusion, that activation of the defence area is an integral part of the response to peripheral chemoreceptor stimulation indicates that the question of interaction between baro- and chemoreceptor responses should be reconsidered, particularly as it is known that the baroreceptor reflex is suppressed when the defence areas are activated by central stimulation. The present paper describes experiments performed on Althesin-anesthetized cats in which carotid baroreceptors were stimulated by inflating a blind sac and carotid chemoreceptors were stimulated by injections of inorganic phosphate solution or saline equilibrated with CO2. The results showed that the baroreceptor reflex may be fully suppressed when the autonomic components of the alerting response were evoked by chemoreceptor stimulation. In some cases however, when the activation of the defence areas was mild, at least as judged by the magnitude of the cholinergic vasodilatation evoked by the chemoreceptor stimulus alone, there appeared to be algebraic summation of the baro- and chemoreceptor response. It is concluded that the extent to which a given chemoreceptor stimulus suppresses the baroreceptor reflex is dependent on its potency as a stimulus to the defence areas. It is suggested that the chemoreceptor input may be more important than hitherto suspected in setting the level of arterial blood pressure.