STATIC AND DYNAMIC CAROTID CHEMORECEPTOR RESPONSES

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Although avian and mammalian carotid body chemoreceptors are known to be structurally similar, and to serve similar functions in the control of ventilation, the stimulus-response characteristics of single unit avian carotid chemoreceptors are poorly documented. The proposed study will establish functional similarities and differences using single unit neural recording techniques. Static and dynamic responses of avian carotid chemoreceptors to their adequate stimuli will be defined, and their responses to substances known to affect mammalian carotid bodies--eg. oligomycin, KCN, and norepinephrine--characterized. The pattern of avian carotid body discharge will be quantitated with interspike interval histograms. The independent and interactive effects of steady levels of pH, PCO2, and PO2 on avian carotid chemoreceptors will be determined from single unit stimulus-response curves. Receptor responses to hypocapnic-hypoxic stimuli will be of special interest because the superior hypoxic tolerance of birds compared to mammals may be partly due to a stronger hypoxic drive in birds. Arterial PCO2 oscillations linked to tidal breathing affect carotid chemoreceptor discharge patterns in mammals. Since arterial PCO2 oscillations are predicted to be even larger in birds, correlations will be made between avian carotid chemoreceptor discharge pattern, and the arterial PCO2 oscillations measured wiith an intravascular pH electrode. The hypothesis that dynamic responses of carotid chemoreceptors may help couple ventilation to metabolism by detecting exercise-induced changes in arterial PCO2 oscillations will be tested. Dynamic receptor responses will be quantitated while forcing arterial PCO2 oscillations of varying shape and frequency, using artificial, unidirectional pulmonary ventilation. Unidirectional ventilation of the avian lung offers a powerful tool for controlling arterial gas tensions, allowing a systems analysis approach to the study of receptor frequency response. Sinusoidal PCO2 oscillations will be used to determine the frequency response of avian carotid chemoreceptors, and ramp oscillations with varying up and down slopes, but constant period and amplitude, will be used to determine the effect of rate of PCO2 change on receptor discharge. Similar experiments would be much more difficult in mammals. The results of this study will broaden our knowledge of avian carotid body chemoreceptor responses, and serve to guide further studies of carotid bodies in general.