EFFECTS OF DOPAMINE RECEPTOR AND ALPHA-2-ADRENOCEPTOR AGONISTS AND ANTAGONISTS ON MUSCARINIC RECEPTOR STIMULATION IN CARDIAC SYMPATHETIC-GANGLIA OF THE DOG

The effects of dopamine (DA)-receptor and alpha(2)-adrenoceptor agonists and antagonists on ganglionic muscarinic stimulation were examined in anesthetized dogs. All drugs were injected or infused intra-arterially into the blood supply of the cardiac sympathetic ganglia. The muscarinic agonists McN-A-343 (10, 20, and 30-mu-g) and muscarine (1, 2, and 3-mu-g) increased heart rate. The muscarinic receptor stimulation elicited by McN-A-343 or muscarine was significantly inhibited by infusion of the DA2-receptor agonist quinpirole (3 and 10-mu-g/min) but not by the DA1-receptor agonist SK&F 38393 (10 and 30-mu-g/min). The alpha(2)-adrenoceptor agonist BHT 933 (3 and 10-mu-g/min) also inhibited muscarinic receptor stimulation. The DA2-receptor antagonist domperidone (10-mu-g/min) and the alpha(2)-adrenoceptor antagonist yohimbine (1-mu-g/min) had no effects on muscarinic receptor stimulation, but they antagonized the inhibitory effects of quinpirole and BHT 933, respectively. The nicotinic transmission elicited by preganglionic cardiac sympathetic nerve stimulation (1 and 2 Hz) was also inhibited by DA-receptor and alpha(2)-adrenoceptor agonists and antagonists. These results suggest that DA2-receptors and alpha(2)-adrenoceptors suppress muscarinic transmission as well as nicotinic transmission in the cardiac sympathetic ganglia of the dog.