HETEROGENEOUS CHOLINERGIC REACTIONS OF RINGED SEAL CORONARY-ARTERIES

Coronary blood flow of some seal species is unusual in that it is high ly variable in both non-diving and diving conditions and shows intermi ttent fluctuations, especially during dives when it frequently ceases for brief periods. We sought regulatory mechanisms governing these rea ctions by studying isometric tension recordings of isolated left circu mflex (LC) and left anterior descending (LAD) coronary arteries of rin ged seals, Phoca hispida, during reactions to a variety of agents for stimulating or blocking autonomic responses of the vascular smooth mus cle. Micromolar acetylcholine (ACh) produced constriction of the small diameter segments of the LAD, but relaxation of the LC and larger seg ments of LAD. Both constrictions and dilations were prevented by atrop ine. Small vessel constriction by ACh was prevented by micromolar indo methacin and by a thromboxane receptor antagonist. Large vessel ACh di lations were prevented or reduced by rubbing off the endothelium and b y the L-arginine analog, L-NG-nitro-arginine, an inhibitor of nitric o xide synthesis. We conclude that cholinergic, muscarinic, dilation of ringed seal large coronary arteries is mediated by endothelial-derived relaxing factor (EDRF), whereas ACh constriction of small arteries is mediated by a prostaglandin. (C) 1998 Elsevier Science Inc. All right s reserved.