CARDIAC EFFECTS OF INJECTIONS OF EPINEPHRINE INTO THE SPINAL INTERMEDIOLATERAL COLUMN

Small doses of epinephrine (0.008, 0.05, and 0.1 pmol, i.e., 20-nl vol umes of 0.40,2.5, and 5 muM solutions) produced a dose-dependent incre ase in heart rate when microinjected into the right intermediolateral column (IML) at T2 spinal level. These effects were mediated via alpha 1-adrenergic receptors because prazosin blocked them. The presence of alpha1-adrenergic receptors at this site was confirmed by microinjecti ons of phenylephrine (a specific agonist for these receptors); phenyle phrine elicited tachycardia. Larger doses of epinephrine (320, 2,000, and 3,200 pmol, i.e., 20-nl volumes of 16, 100, and 160 mM solutions) caused bradycardia when microinjected into the IML. These effects were mediated via alpha2-adrenergic receptors because idazoxan blocked the m. The presence of alpha2-adrenergic receptors at this site was confir med by microinjections of clonidine (a specific agonist for these rece ptors); clonidine elicited bradycardia. Injections of the vehicle (20 nl of normal saline containing 0.3% ascorbic acid, pH 7.4) did not evo ke a response. Epinephrine, prazosin, or idazoxan did not alter the re sponses to L-glutamate. None of the doses of epinephrine elicited any response when injected intravenously. The aforementioned results provi de pharmacological evidence for the presence of alpha1- and alpha2-adr energic receptors in the IML at T2. Thus a basis is provided for inves tigating the role, if any, of alpha-adrenergic receptors in the IML in mediating cardiovascular responses elicited by the stimulation of dif ferent brain stem areas.