Pial Arterial Response to Systemic Hypoxia in Anaesthetised Cats

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Abstract
Pial arterial responses to reduction in arterial oxygen tension were studied in anaesthetised cats. In 12 cats under chloralose, dilatation occurred in vessels of all sizes between 20 and 200 μm, at variable levels of Pao2, and to a very variable extent. At Pao2 25–35 mm Hg, dilatation ranged from negligible to 175% above initial diameter. The variations in response were largely dependent on associated blood pressure (BP) changes. Increase in BP counteracted dilatation; dilatation was greater during hypoxia when the BP change was prevented. At the induction of hypoxia, the first response of the vessels was a constriction, which occurred about 5 s after the chemoreflex increase in BP. Dilatation was delayed a further 30–90 s, and this delay was similar when BP was prevented from rising. Vessels of all sizes responded in the appropriate manner when only BP was transiently changed within the autoregulatory range. In 3 cats in which similar procedures were compared under pentobarbitone anaesthesia, there were smaller and less consistent changes in responses to BP changes alone and to hypoxia and its associated BP changes. The findings were compatible with a local effect of lowered Po2. From the time course of the changes there was no indication of a chemoreflex component in the responses of these vessels at the induction of hypoxia.