Effects of vagal stimulation and carotid body chemoreceptor stimulating agents on phrenic nerve activity in vagotomized rabbits.

Effects of vagal stimulation and carotid body chemoreceptor stimulating agents on phrenic nerve activity were studied in vagotomized rabbits. The decreases of TI and IPA were proportional to the increase of stimulus frequency and these inhibitory effects were rapidly reversed after vagal stimulation was discontinued. The continuous vagal stimulation at low frequency resulted in a marked potentiation of TI accompanied by a decrease of IPA. The prolongation of TI caused by subthreshold vagal stimulation which failed to terminate the inspiration may probably be due to the inhibition of the PC phasic activity. An intracarotid injection of NaCN led to a decrease of TI and it showed a marked potentiation of IPA. DA and 5-HT showed apneustic respiratory patterns in phrenic responses associated with a slight increase of IPA. Therefore, it may be suggested that the afferent inputs from carotid body chemoreceptor by DA and 5-HT are projected to the I beta neurons in the NTS whereas the input from NaCN-stimulated carotid body chemoreceptor is projected into the I alpha neurons.