Endothelin-evoked contractions in bovine ciliary muscle and trabecular meshwork: Interaction with calcium, nifedipine and nickel

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Abstract
In the present study, we compared contractile responses of isolated bovine ciliary muscle and trabecular mesh- work strips to endothelin-1 and carbachol. 1. Endothelin-1 is a potent contracting agent for ciliary muscle and trabecular meshwork. The EC50 was 5×10-−9 mol/1 for both tissues. The maximal force evoked by endothelin was 73% of the maximal carbachol response in trabecular meshwork and 52% in ciliary muscle. 2. Carbachol contracted both tissues with an EC50 of 2×0-−7 mol/1. 3. In ciliary muscle, the tension was completely dependent on extracellular calcium. 4. In trabecular meshwork, 23 ± 4% of the endothelin-and 42 ± 10% of the carbachol-induced force response remained after removal of extracellular calcium. 5. Nifedipine (10-−5 mol/1) had only a slight relaxing effect in both tissues. 6. Nickel (10-−3 mol/1) inhibited the development of force in both tissues. The relaxation induced by nickel was more pronounced in endothelinthan in carbachol-induced contractions. Different intracellular mechanisms mediating the action of endothelin and carbachol can be described: i) a calcium-and a nickel-sensitive pathway in both tissues and ii) an additional mechanism independent of external calcium in trabecular meshwork. These results indicate functional differences between the contractile region of trabecular meshwork and ciliary muscle. Endothelin may participate in accommodation and regulation of the intraocular pressure.

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