EFFECTS OF ANION SUBSTITUTIONS ON POTASSIUM-INDUCED PHASIC AND TONIC CONTRACTION IN TAENIA COLI

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Abstract
The anions (Br-, NO3-, I- or SCN"), having the potentiating effect on skeletal muscle contracture were used to substitute 50% or 100% of Cl- of NaCl in the medium. The anion substitutions caused increase in tension of the 1st peak of K-induced contracture, except in the case of 100% substitution by SCN", and decrease in tension of tonic response. The anion substitutions caused a potentiation of phasic contraction with high-K at room temperature as well as increase in tension of the 1st peak in the K-induced contracture, and degrees of increase in maximum tension of the phasic contraction were approximately in the order Cl- < Br-< NO3-<I- in the 50% substitution and Cl-< Br-<NO3-< I-in the 100% substitution. There was a difference between the type of potentiating action of SCN" on the phasic contraction and ones of the others (Br-, I- or NO3-), that is, 100% SCN- substitution caused prolongation of the duration of the contraction, although a shortening is caused by the other anions. The Ca-contracture having no phasic response was induced by the application of 2.5 mM CaClo to the depolarized smooth muscle kept in the high-K, Ca-free medium at 37[degree]C. The substitution with 100% NO3" decreased tension of the Ca-contracture to a similar degree as the decrease in tonic tension of the K-induced contracture preceded by the phasic one. A similarity between, the potentiating effects of anion substitutions, on K-induced phasic contraction in taenia coli, and K contracture in the fast skeletal muscle was discussed. The anion substitutions (Br-, NOq", I" or SCN-) had different effects on K-induced phasic and tonic contraction in taenia coli.