[Alpha 2 adrenergic control of ventilation in the rat]

In anaesthetized rats, ventilatory stimulation induced by phentolamine, an alpha sympatholytic agent, emphasizes the role of some adrenergic mechanisms in the control of the respiratory centres activity. Phentolamine (5 and 10 mg.kg-1, iv) stimulates ventilation after a 4 s latency, tidal volume and respiratory rate being both increased. A same response can also be provoked 10 min later, by a second identical iv administration, systemic blood pressure remaining then stable at its previous low level. Hyperventilation is also observed when phentolamine is injected in totally denervated rats, without any remaining baro- or chemosensitivity. Stimulation is thus due to a central activity in relation with the release of inhibitory influences. Phentolamine also causes hyperventilation after prazosin pretreatment indicating that the alpha 1 adrenergic blockade is not involved in the post-phentolamine stimulation. This is an alpha 2 adrenergic transmission dependent mechanism. Variation of the systemic blood pressure is not the main mechanism involved in the hyperventilation induced by phentolamine. Meanwhile, baroreceptor activity modulates the central response to the drug, as shown by the negative influence of the post-vasopressin arterial hypertension. Hyperoxia is also a modulating factor acting by two ways: an inhibition of the peripheral chemoreceptors activity is added to an arterial hypertension. On the other side, activation of these chemoreceptors by almitrine bismesilate increases the respiratory responses to phentolamine. As already shown by one of us (Lagneuax, 1986), phentolamine pretreated rats are more responsive to hypoxia and to almitrine. Moreover, these phentolamine pretreated rats are protected against cardiovascular collapses and against apnea, frequently observed during hypoxia without CO2 compensation.