Atropine-resistant relaxation induced by high K+ in iris dilator muscle of the rat and pig.
1. The effects of high K+ ion concentration on the isometric tension in dilator muscle strips of the rat and porcine iris were examined. A high K+ solution, prepared by the replacement of Na+ in the medium with equimolar K+, was applied in the presence of 1 microM phentolamine, 1 microM propranolol and 1 or 10 microM atropine. High K+ (greater than 20 mM) induced a biphasic response; an initial phasic contraction followed by relaxation rather than tonic contraction. 2. An additional application of a Ca2+ antagonist, 1 microM nifedipine or nicardipine, almost completely blocked the K(+)-induced initial contraction and enhanced the following relaxation. The effect of K+ under these conditions was concentration-dependent in the range 20 to 80 mM. The maximum amplitude of the atropine-resistant relaxation induced by high K+ corresponds to 50-75% of that produced by acetylcholine in the absence of atropine. A similar K(+)-induced relaxation was observed in the porcine iris dilator. 3. The atropine-resistant relaxation in the rat iris dilator was not affected by pretreatment with 10 microM ouabain. The relaxation induced by 40 or 80 mM K+ in the porcine dilator was slightly enhanced or not affected, respectively, in the presence of 1 microM ouabain. Application of 10 microM ouabain per se induced relaxation in the porcine iris dilator. 4. The low Na+ ion concentration present in high K+ solutions was not responsible for the K(+)-induced relaxation since the complete replacement of Na in the medium with Tris did not affect significantly the relaxation produced by high K(+)-containing solutions. 5. Neither 1 microM tetrodotoxin, 10 microM indomethacin, 10 JM nordihydroguaiaretic acid nor hypoxic conditions affected the high K+-induced relaxation. 6. The inherent tone of the rat iris dilator was not affected by either 8-bromo cyclic GMP, dibutyryl cyclic GMP (0.1-0.3 mM) or nitroprusside (1-100 microM). 7. These results may suggest that the atropine-resistant relaxation induced by high K+ is not due to either activation of the Na-K pump or release of a relaxing factor produced by oxidative metabolism. Although the relaxation mechanism has not been elucidated, it is probably not mediated by an increase in cellular cyclic GMP levels.
Ryang
S
S
Department of Chemical Pharmacology, Faculty of Pharmaceutical Sciences, Nagoya City University, Japan.
Takei
S
S
Kawai
T
T
Imaizumi
Y
Y
Watanabe
M
M
D000818:Animals; D001285:Atropine / Q000494:pharmacology*; D004351:Drug Resistance; D066298:In Vitro Techniques; D007498:Iris / Q000187:drug effects; D007537:Isometric Contraction; D009126:Muscle Relaxation / Q000187:drug effects; D009130:Muscle, Smooth / Q000187:drug effects*; D009569:Nitric Oxide / Q000502:physiology; D011188:Potassium / Q000494:pharmacology*; D051381:Rats; D013045:Species Specificity; D013552:Swine
D009569:Nitric Oxide; D001285:Atropine; D011188:Potassium