Effects of calcium blockers and Mn2+ on the response of isolated rat iris sphincter and dilator muscles to agonists and Ca2+.

Effects of calcium blockers and Mn on the mechanical response of the isolated rat iris sphincter and the dilator were investigated. The contraction of the sphincter in response to acetylcholine, the contraction of the dilator in response to norepinephrine and the relaxation-contraction of the dilator in response to acetylcholine and also resting tone of dilator muscle were strongly dependent to external Ca concentration to different extents, while these responses and the tone were scarcely affected by calcium blockers, D-600, diltiazem (all 1 micro M), or nitroprusside Na (0.1 nM). In depolarized sphincter muscle (in the presence of atropine and phentolamine), the contraction induced by externally added Ca was reduced moderately by the Ca-blockers but this effect was far weaker than that expected from their effect on other smooth muscles. Mn (0.3 mM) moderately reduced the calcium contraction of depolarized sphincter. In depolarized dilator muscle, the contraction induced by Ca was not inhibited effectively by any of these Ca blockers but was moderately reduced by Mn. It is suggested that the rat iris muscles are resistant to "organic Ca-blockers" but sensitive to Mn, an "inorganic Ca-blocker".