STATIC AND DYNAMIC CAROTID CHEMORECEPTOR RESPONSES

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.