The influence of blood gases on α1‐ and α2‐adrenoceptor‐mediated pressor responses in the pithed rat

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Abstract
1 The influence of blood gases on α1- and α2-adrenoceptor-mediated pressor responses was studied in the pithed rat by varying the inspired gas mixture or the ventilation stroke volume. 2 Acidosis favoured the peak responses to the α2-adrenoceptor agonist, xylazine, while alkalosis favoured the peak responses to the α1-adrenoceptor agonist, phenylephrine. A combination of hypoxia and hypercapnia greatly depressed the α1 response to phenylephrine whereas the α2 response to xylazine remained relatively unaffected. 3 When PaO2 was varied in either acidotic or alkalotic conditions the response to the phenylephrine increased as PaO2 increased. 4 To prevent hypoxia in air ventilated rats, large stroke volumes were required. This caused alkalosis and hence decreased responsiveness to xylazine. Consequently, air ventilated pithed rats gave poorer responses to xylazine than did those ventilated on 100% O2. 5 The results show that α1- and α2-adrenoceptor-mediated pressor responses can be differentially affected by blood gases. The relative contribution of α1- and α2-adrenoceptors to vascular tone may be either under- or over-estimated depending on the arterial blood gases.