THE ROLE OF ENDOTHELIUM IN THE CALCIUM-INDUCED REDUCTION OF THE CONTRACTILE RESPONSE OF THE RABBIT AORTA

1. Increase of Ca2+ concentration in the bath solution diminishes the contractile response of isolated rabbit aorta rings to alpha(1)-adreno ceptor agonists and KCl. 2. In intact preparations the contractions of methoxamine and phenylephrine were maximal when a 0.3- to 0.6-mM Ca2 bath solution was used, and those of KCl were maximal with a 2.5-mM C a2+ concentration. 3. The contractions of methoxamine and phenylephrin e also were decreased by increasing the Ca2+ concentration above 1.25 mM in disrupted endothelium preparations and in those incubated in ind omethacin (10(-5) M), N-omega-nitro-L-arginine methyl ester (10(-4) M) , or methylene blue (10(-6) M). 4. High organ bath Ca2+ concentrations also caused a decrease in KCl contractions using endothelium-denuded and the treated preparations, the responses being similar with 1.25 mM and 2.5-mM Ca2+ in the methylene blue-treated preparations, whereas t hey were greater with 1.25 mM Ca2+ in the others. (C) 1997 Elsevier Sc ience Inc.