Torao Nagai

Potassium contracture of anterior byssus retractor muscle of Mytilus edulis

Abstract 1. 1. The potassium contracture in the anterior byssus retractor muscle of Mytilus edulis relaxed spontaneously, and the relaxation was accelerated by 5-HT (10 −6 M), but the contractile activation and inactivation was not affected significantly. 2. 2. By reducing [Ca] 0 , the “activation curve” was shifted downward at higher [K] 0 , and the “inactivation curve” was shifted toward lower [K] 0 and the rate of inactivation was increased. 3. 3. The steady-state inactivation was maintained for at least 2 hr without complete inactivation. 4. 4. The half-inactivation time was dependent on [K] 0 , while the half-relaxation time in the contracture induced by conditioning K solutions was not.

Relation between the inhibitory action of native tropomyosin and the dual effect of magnesium on the ATPase activity of actomyosin.

Abstract The effects of various concentrations of Mg2+ on the ATPase activities of native tropomyosin-free synthetic actomyosin, myosin B, and synthetic actomyosin to which native tropomyosin was added, and on the superprecipitation of myosin B, were investigated in the presence of 6.2 × 10−8 m Ca2+ and 1.7 × 10−5 m Ca2+. At 6.2 × 10−8 m Ca2+, ATPase activity of synthetic actomyosin was enhanced by 5 × 10−6 to 1 × 10−4 m Mg2+. Even with supramaximal amounts of Mg2+, the ATPase activity was not inhibited below the original level attained without added Mg2+. At 1.7 × 10−5 m Ca2+, the ATPase was markedly activated by Mg2+ in concentrations lower than 10−5 m . Further addition of Mg2+ decreased the ATPase activity toward, but not below, the original level. Unlike synthetic actomyosin, Mg2+-activated ATPase activity of myosin B and of synthetic actomyosin to which native tropomyosin was added falls below the original level at high Mg2+ concentrations if Ca2+ level is very low. This inhibitory effect of Mg2+ is abolished by high concentrations of Ca2+. On the basis of these and related findings, we suggest mechanisms for the inhibitory action of native tropomyosin and for the dual effect of Mg2+.