Simultaneous measurement of force and calcium uptake during acetylcholine-induced endothelium-dependent relaxation of rabbit thoracic aorta.

This study was designed to determine whether the endothelium-derived relaxing factor induced by acetylcholine (1 JJ.M) in rabbit thoracic aorta inhibits agonist-induced calcium mobilization, specifically calcium influx. Force generated in rings of rabbit thoracic aorta by norepinephrine (1 /xM) was measured under isometric conditions. At the appropriate time during 1 fiM acetylcholine-induced relaxation of 1 /AM norepinephrine-contracted rabbit thoracic aorta, the rings were pulse-labelled with calcium-45 to measure calcium influx. When measured in this fashion, 1 /xM acetylcholine decreased the 1 /u.M norepinephrine-induced increase in calcium influx. This effect was eliminated by removal of the endothelium and by atropine (1 /xM), but not by indomethacin (14 ju.M). Acetylcholine (1 fiM) also blocked the 60 mM potassium-chloride-induced increase in calcium influx without dramatically affecting force. The phasic contraction produced by norepinephrine (1 /AM) with 2 mM lanthanum pretreatment, which is caused by release of intracellular calcium, was inhibited by acetylcholine (1 /xM) in a fashion similar to 1 /xM nitroglycerin. The tonic contraction produced by norepinephrine (1 /xM) after depletion of the agonist-releasable pool of intracellular calcium, which is thought to be due to calcium influx, was depressed by acetylcholine (1 /xM). These data suggest that endothelium-derived relaxing factor relaxes 1 /xM norepinephrine-contracted rings of rabbit thoracic aorta by decreasing calcium entry and by producing an extracellular calcium-independent relaxant effect. (Circulation Research 1987;60:31-38)