IMPLICATIONS FROM PHYSIOLOGICAL STUDIES OF MARINE MAMMALS1

Some aspects of captive marine mammal research have become matters of controversy, largely due to a misunderstanding of the value of such studies and of their importance to effective management and conservation of these species. Understanding of the physiological mechanisms underlying the special adaptations for aquatic life that marine mammals demonstrate provides essential information relating to their health and survival. The need for such information is increasing as disturbance of the world's marine environments steadily escalates. While studies of these animals in their natural habitats contribute much to our comprehension of their biology, a more complete understanding of the physiological basis of their behavior depends upon experimental laboratory investigations. Some of the new knowledge thus gained has potentially important implications for human medicine.     

Heterogeneous cholinergic reactions of ringed seal coronary arteries

Coronary blood flow of some seal species is unusual in that it is highly variable in both non-diving and diving conditions and shows intermittent fluctuations, especially during dives when it frequently ceases for brief periods. We sought regulatory mechanisms governing these reactions by studying isometric tension recordings of isolated left circumflex (LC) and left anterior descending (LAD) coronary arteries of ringed seals, Phoca hispida, during reactions to a variety of agents for stimulating or blocking autonomic responses of the vascular smooth muscle. Micromolar acetylcholine (ACh) produced constriction of the small diameter segments of the LAD, but relaxation of the LC and larger segments of LAD. Both constrictions and dilations were prevented by atropine. Small vessel constriction by ACh was prevented by micromolar indomethacin and by a thromboxane receptor antagonist. Large vessel ACh dilations were prevented or reduced by rubbing off the endothelium and by the -arginine analog, -NG-nitro-arginine, an inhibitor of nitric oxide 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.