Cholinergic modulation of non-adrenergic, non-cholinergic relaxation in isolated, small coronary arteries from lambs

The presence of cholinergic innervation of small coronary arteries in the l amb was investigated by measuring choline acetyltransferase (ChAT) and acet ylcholinesterase (AChE) activities and by performing in vitro experiments i n a microvascular myograph to establish whether or not there is a cholinerg ic component in the response to electrical field stimulation (EFS). ChAT-sp ecific activity was present in proximal coronary segments, but was signific antly higher in small coronary arteries. AChE-positive ganglia and fibres w ere distributed within the adventitia and outer third of the media in proxi mal coronary segments, and dense perivascular nerve plexuses were observed in small coronary arteries. Acetylcholine induced contractions in all prepa rations examined and relaxations in 20% of the segments contracted with the thromboxane analogue U46619. EFS did not induce neurogenic contractions in lamb small coronary arteries. In the presence of the alpha-adrenoceptor an tagonist, phentolamine, EFS caused frequency-dependent reproducible relaxat ions that were enhanced by the blocker of cholinergic transmission, botulin um neurotoxin. An inhibitor of AChE, physostigmine, had no significant effe ct on the relaxations caused by EFS, while both the muscarinic receptor ant agonist, atropine, and the muscarinic M-2-receptor antagonist, AFDX 116, en hanced these responses. Blockade of sympathetic neurotransmission with guan ethidine or incubation with the Pt-receptor antagonist, suramin, abolished the relaxations induced by EFS, whereas propranolol was without effect. Low -frequency EFS caused less relaxation in preparations activated by acetylch oline than in those contracted with U46619, while sensitivity and maximal r elaxation induced by adenosine 5'-triphosphate (ATP) were not different in U46619- and acetylcholine-contracted arteries. The presence of the enzymes necessary for both biosynthesis and degradation of acetylcholine and the fi nding that blockers of cholinergic neurotransmission enhance EFS-induced re laxations suggest that small coronary arteries are cholinergically innervat ed.