Contractile responses and calcium movements induced by alpha 1-adrenoceptor stimulant, norepinephrine, in rabbit iris...

1. The mechanisms involved in contraction of rabbit iris dilator muscle induced by norepinephrine (NE) were studied. 2. The concentration-response curve of NE was not influenced by Ca2+ blockers in the normal physiological saline solution (PSS) and removal of Ca2+ from PSS. 3. In 0.01 mM EGTA containing Ca(2+)-free PSS, the NE-induced contraction was phasic, which was suppressed by TMB-8, cyclopiazonic acid, ionomycin and A23187 but still partly remained. 4. In 2 mM EGTA containing Ca(2+)-free PSS, NE increased the intracellular Ca2+ ([Ca2+]i) and muscle tension. Ryanodine abolished the increase in [Ca2+]i induced by NE but slightly inhibited the tension. 5. These results suggest that the NE-induced contraction of rabbit iris dilator in normal PSS is mainly due to the increase in the release of intracellularly sequestered Ca2+ and partly due to the Ca(2+)-independent processes.