VENTILATORY ACCLIMATIZATION TO CHRONIC HYPOXIA - RELATIONSHIP TO NORADRENALINE METABOLISM IN THE RAT SOLITARY COMPLEX

1. The relationship between ventilatory acclimatization to chronic hyp oxia (10 % O-2-90 % N-2) and noradrenaline metabolism was examined in two regions located immediately caudal and rostral to the obex within the rat solitary complex. 2. Three experimental protocols were establi shed. In protocol 1, the percentage changes in respiratory tidal volum e, frequency and minute ventilation elicited by 4, 7, 10 and 14 days o f hypoxia were assessed by flow plethysmography in awake rats, and the n the content of tyrosine hydroxylase was measured in the solitary com plex. In protocol 2, the time course response of tyrosine hydroxylase protein level was determined after 3, 7, 14 and 22 days of hypoxia by using a quantitative immunoblotting method for the protein assay. In p rotocol 3, the turnover of noradrenaline was estimated in the solitary complex after 14 days of hypoxia. 3. A progressive increase in ventil ation was observed to reach a maximum (+105 +/- 15 %, mean +/- S.E.M.) above normoxic control after 10 days of hypoxia, at which time it sta bilized. Furthermore, tyrosine hydroxylase protein increased progressi vely and reached a maximal level at 14 days of hypoxia (+36 +/- 4 %, m ean +/- S.E.M.). Return to the basal level of tyrosine hydroxylase was observed after 22 days of hypoxia. 4. Tyrosine hydroxylase content (36 +/- 4 %) and noradrenaline turnover (+394 +/- 3 %) increased exclus ively in the caudal part of the solitary complex. 5. The ventilatory a cclimatization to chronic hypoxia preceded the increase in tyrosine hy droxylase and these two parameters were significantly correlated. 6. T hese data suggest that ventilatory acclimatization to chronic hypoxia is associated with topical modifications of the brainstem catecholamin e metabolism.