Catecholamine activation in the vasomotor center on emergence from anesthesia: the effects of alpha2 agonists.

Abstract

The rostral ventrolateral medulla (RVLM) controls the vascular system and may contribute to postoperative hypertension. It comprises adrenergic cardiovascular neurons, a site for action of alpha2-adrenergic agonists. Because alpha2 agonists minimize perioperative circulatory activation, we asked the following question: do alpha2 agonists, such as clonidine and mivazerol, blunt the catecholamine activation observed in the RVLM on emergence from anesthesia? Halothane-anesthetized, paralyzed rats had their ventilatory, circulatory, and acid-base stability controlled. All pressure points and incisions were infiltrated with local anesthetics. With in vivo electrochemistry, a catechol signal was recorded in the RVLM during 150 min of stable halothane anesthesia (saline-halothane group); for 120 min after halothane discontinuation (saline-emergence group); after emergence and administration of the reference alpha2 agonist, clonidine 7 microg/kg or 21 microg/kg I.V. (50% or 90% effective dose [ED50 or ED90], respectively); and after emergence and administration of a new alpha2 agonist, mivazerol 20 microg/kg or 150 microg/kg I.V. (ED50 or ED90). Under halothane, dose-response curves for the RVLM catecholamine signal were constructed for mivazerol and an alpha2 antagonist, idazoxan (ED50 2.3 mg/kg I.V.). Stable halothane anesthesia (n = 5) led to no change in mean arterial pressure (MAP), heart rate (HR), or catechol signal (CAOC). During emergence from anesthesia, the MAP, HR, and CAOC increased (n = 5). Clonidine led to a near total suppression of the RVLM catecholamine activation noticed on emergence from anesthesia (n = 5). Hypertension was partially blunted with clonidine 7 microg/kg (n = 5). Tachycardia was partially blunted with mivazerol 20 microg/kg (n = 5). Pretreatment with idazoxan suppressed all the effects of mivazerol (n = 5). On emergence from anesthesia, alpha2 agonists modify the activity of adrenergic cardiovascular neurons located within the vasomotor center, as assessed by in vivo electrochemistry. We provide a rationale for the use of alpha2 agonists on emergence from anesthesia in coronary/hypertensive patients.