Influence of respiratory acidosis or alkalosis on pressor responses mediated byα1- andα2-adrenoceptors in pithed normotensive rats

The effect of respiratory acidosis and alkalosis on the vasoconstriction toα1- andα2-adrenoceptor stimulation was studied in pithed normotensive rats. The selectiveα1-adrenoceptor agonists (-)amidephrine, cirazoline, (±)erythro methoxamine (-)phenylephrine, Sgd 101/75 and St 587 were used, as well as the selectiveα2-adrenoceptor agonists B-HT 920, B-HT 933, DP-6,7-ADTN, M-7 and UK 14,304. The non-selectiveα-adrenoceptor agonists xylazine, noradrenaline and adrenaline were included as well. The latter two were also studied under selective doses of the antagonists rauwolscine and prazosin, thus yielding the respectiveα1- andα2-adrenoceptor components of the vasoconstriction to these agonists. The effect of acid-base balance disturbances on presynaptically released noradrenaline elicited by electrical stimulation of preganglionic nerves was studied as well. Dose response curves for the agonists were generated under various conditions of ventilation, yielding either alkalotic, normal or acidotic values of arterial blood pH. Pressor responses to all agonists were maximally affected by changes in acid-base status at the low doses of the agonists. Acidosis was found to inhibit increases in diastolic pressure mediated by theα1- as well as theα2-adrenoceptor agonists studied, although not to the same extent. Alkalosis exerted either an obvious potentiation or did not significantly influenceα1-adrenoceptor mediated pressor responses. On the basis of acid-base sensitivity the following groups of agonists were distinguised: (1) Cirazoline, phenylephrine, methoxyamine, electrically released noradrenaline from presynaptic sites, of which pressor responses are obviously potentiated and attenuated by alkalosis and acidosis, respectively. (2) Sgd 101/75, St 587, noradrenaline-α1, amidephrine and adrenaline-α1, eliciting pressor responses which are strongly acid-sensitive and base-insensitive. (3) B-HT 920, B-HT 933, DP-6, 7-ADTN (lower doses), of which vasoconstriction is markedly inhibited by both acidosis and alkalosis. (4) Noradrenaline-α2, UK 14,304, M-7 (lower doses), adrenaline-α2 and high doses of the agonists of the former group. Pressor responses of these agonists were found to be not or slightly base-sensitive, but profoundly acid-sensitive. Xylazine does not fit into this classification. The present data are not in accordance with purported subdivisions ofα-adrenoceptor agonists by others. It is therefore concluded that the differential effect of acid-base status onα-adrenoceptor mediated vasoconstriction in pithed rats is an exponent of the differential way of interaction of the agonists with the adrenoceptors involved.