Abstract
1. 1. The mechanisms involved in contraction of rabbit iris dilator muscle induced by norepinephrine (NE) were studied. 2. 2. The concentration-response curve of NE was not influenced by Ca 2+ blockers in the normal physiological saline solution (PSS) and removal of Ca 2+ from PSS. 3. 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. 4. In 2 mM EGTA containing Ca 2+-free PSS, NE increased the intracellular Ca 2+ ([Ca 2+] i) and muscle tension. Ryanodine abolished the increase in [Ca 2+] i induced by NE but slightly inhibited the tension. 5. 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 Ca 2+ and partly due to the Ca 2+-independent processes.
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