Effects of melatonin on pulmonary and coronary vessels are exerted through perivascular nerves

The effects of melatonin on porcine pulmonary and coronary vessels have been studied. Vessels were isolated from normal pigs, placed in a tissue bath, and precontracted with 30 mM KCl. The biophysical responses to cumulative doses of melatonin were then assessed. In the pulmonary artery, melatonin caused a dose-dependent relaxation which was blocked by vasoactive intestinal peptide antagonists or prior 6-hydroxydopamine treatment of the vessels. In the coronary artery, melatonin caused the reverse, a dose-dependent contraction which was blocked by the alpha antagonist prazosin or prior 6-hydroxydopamine treatment. These experiments indicate that melatonin has different mechanisms of action in the coronary and pulmonary circulation, although both seem to depend on the integrity of perivascular 6-hydroxydopamine sensitive nerves. Such differing mechanisms may provide insight into pathophysiological events in the lungs (such as nocturnal asthma) and heart (early morning coronary infarction).