CAROTID-BODY NORADRENERGIC SENSITIVITY IN VENTILATORY ACCLIMATIZATIONTO HYPOXIA

Norepinephrine inhibits ventilation in awake goats under normoxic, res ting conditions. This inhibition is carotid body (CB) mediated and may be due to stimulation of noradrenergic receptors on the CB. Cao et al . (FASEB J. A118, 1991) recently suggested that CB noradrenergic recep tors may be down regulated following 24-36 hours of hypoxic exposure i n cats. Our study was aimed at determining whether a change in noradre nergic receptor sensitivity during ventilatory acclimatization to hypo xia (VAH) was responsible for the increased sensitivity of the CB to h ypoxia during prolonged exposure to hypoxia in goats. We tested this h ypothesis using intracarotid infusions of norepinephrine (NE) (0.5, 1. 0, 5.0 mug.kg-1.min-1) and dopamine (DA) (1.0 mug.kg-1.min-1) in awake goats under control normoxic conditions, during 4 h of isocapnic hypo xia, and upon return to normoxia. NE and DA (1.0 mug.kg-1.min-1) both inhibited control ventilation significantly during the intracarotid in fusions (56% and 62% decreases, respectively). No significant differen ces were found between the pre- and post-hypoxic infusions of NE and D A in normoxia, During hypoxia, inhibition of VE during NE and DA infus ions was attenuated relative to control. Time-dependent changes of the NE response were not apparent during the acclimatization period sugge sting that a decreased carotid body chemoreceptor sensitivity to NE an d DA is not responsible for the increased drive to breathe characteris tic of ventilatory acclimatization to hypoxia.