Absence of Nicotinic Receptors Mediating the Release of Norepinephrine from Adrenergic Neurons in the Rat Heart 1

Abstract

Introduction. It appears well established that cholinergic receptors of the nicotinic type exist on some adrenergic nerve terminals (1-6), and that activation of these receptors results in a release of NE (4-8). Numerous drugs appear to have the ability to antagonize these responses (4, 5, 8). The physiological significance of these receptors remains obscure, although they have been postulated to mediate the cholinergic link in adrenergic transmission as proposed by Burn and Rand (9). The receptors are of great pharmacological interest since nicotine is the major ingredient in tobacco (10). In the course of experiments carried out to gain a better understanding of the action of nicotine on the release of NE from adrenergic nerve terminals experiments were carried out on the isolated perfused rat heart. The present studies demonstrate that unlike the rabbit, cat, dog, and guinea pig heart, nicotine does not induce a release of NE from the rat heart (5-8, 11, 12).

Methods. Male Sprague-Dawley rats weighing between 200 and 300 g were used for all experiments. Hearts were removed from rats under pentobarbital anesthesia and placed in an Anderson-Carver coronary perfusion apparatus (Metro Scientific Co.). The normal perfusion medium contained the following composition in millimoles per liter: NaCl, 119.8; KC1, 5.63; CaCl₂, 2.16; MgCl₂, 2.1; dextrose, 10.0; and NaHCO₃, 25.0. Ascorbic acid (10 mg/100 ml) and EDTA (10 mg/liter) were added to the perfusion medium to prevent oxidation of NE. The solution was bubbled with 95% O₂-5% CO₂; the temperature was 38 ± 1°, and pH 7.32 to 7.4. Hearts were perfused at a constant rate of 5.0 ± 0.25 ml/min. Following an initial equilibration period of 10 min, hearts were perfused with 1 ng/ml of l-[³H]norepinephrine (specific activity 4.1 Ci/mmole) for 20 min to label the endogenous amine stores.