The role of endothelium in the calcium-induced reduction of the contractile response of the rabbit aorta

1. 1. Increase of Ca 2+ concentration in the bath solution diminishes the contractile response of isolated rabbit aorta rings to α 1 -adrenoceptor agonists and KCI. 2. 2. In intact preparations the contractions of methoxamine and phenylephrine were maximal when a 0.3- to 0.6-mM Ca 2+ bath solution was used, and those of KCI were maximal with a 2.5-mM Ca 2+ concentration. 3. 3. The contractions of methoxamine and phenylephrine also were decreased by increasing the Ca 2+ concentration above 1.25 mM in disrupted endothelium preparations and in those incubated in indomethacin (10 −5 M), N ω -nitro-L-arginine methyl ester (10 −4 M), or methylene blue (10 −6 M). 4. 4. High organ bath Ca 2+ concentrations also caused a decrease in KCI contractions using endotheliumdenuded and the treated preparations, the responses being similar with 1.25 mM and 2.5-mM Ca 2+ in the methylene blue-treated preparations, whereas they were greater with 1.25 mM Ca 2+ in the others.