Effect of time on endothelium-dependent relaxations in mice and rat thoracic aortas

Temporal variations in the endothelium-dependent relaxant effects of acetyl choline (ACh) in mice and histamine (IIA) in rat thoracic aorta have been s tudied. The relaxations induced by higher concentrations of ACh and HA were signifi cantly dependent on the time the tissues were obtained. However, neither EC S, (the concentration inducing half of the maximum response) values for ACh and WA, nor Ks (antagonist dissociation constant) values for atropine and diphenhydramine were found to be statistically significant depending upon t he time of obtaining aorta preparations. These results show that the in vitro responsiveness of mice and rat thoraci c aortas to endothelium-dependent relaxant effects of ACh and HA, respectiv ely, changes over a 24-h period. These Variations might he dependent on a t emporal rhythm in post-receptor events, i.e., guanylate cyclase-cGMP-phosph odiesterase system which mediates responses to Endothelium-derived relaxing factor(s).