ISOTOPIC STUDIES ON CALCIUM IONS RELEASING FROM THE MUSCLE FIBRE SURFACE AT THE TIME OF OUABAIN APPLICATION
1971 年 18 巻 3 号 p. 117-126
詳細
1971 年 18 巻 3 号 p. 117-126
In isolated skeletal amphibian muscles, significance of the transient increase in 45Ca dissociation occurring by ouabain application was investigated in connection with a detailed explanation of the mechanism of inhibiting effect of ouabain on active sodium efflux. Washout experiments of 22Na and 45Ca ions were made for determining the kinetic behaviors of these ions. 1. By changing experimental conditions, the locations of Ca or Ca++ in the muscle fibre could be divided into surface component, membrane protein, hydrophilic membrane lipid, tubular surface, and intracellular storage. 2. A previous report by the present author disclosed that the nature of ouabain action was not decalcification from the membrane. An increase in 45Ca dissociation inevitably observable following ouabain treatment was presumably brought about through a transient increase in simple diffusion of 45Ca ions released from protein anionic sites of the membrane. The increase had no relation to pump activity nor to alteration in cellular Ca++, and it was closely relating to an application of ouabain. 3. An increase in 45Ca release occurring simultaneously with a reduction in ouabain-sensitive, Ca-independent 22Na efflux may be explained in terms that covalent binding of unsaturated lactone rings of ouabain to protein portion renders Ca ions on the sites of membrane protein releaseable indirectly by altering the conformation of protein including transport enzymes. The portions of protein which are available for this type of binding may not be original Ca sites. 4. Substitution experiments of divalent ions indicated that either membrane structure or enzymatic properties in the membrane, which are maintained with help of Ca ions, not with other divalent ions, may be a prerequisite for expressing ouabain action. When Ca ions in the resting membrane are biophysically considered some properties other than charge number should be taken into account.
