Effects of divalent cations on biphasic potassium contractures and on contractile inactivation in low calcium solutions in frog single twitch muscle fibers.

Abstract

When the concentration of external Ca2+ was reduced for 30 sec in a single twitch muscle fiber of a frog, the peak tension of the initial component of biphasic 80 mM K+ contractures was potentiated,

Effects of conditioning depolarization, repriming, and external calcium reduction on the potassium contractures in frog single twitch muscle fibers.

“…On the other hand, when the concentration of external Cat + was reduced, the peak tension of the secondary component of 80 K contracture was rapidly and strongly inhibited in parallel with the drastic shortening of its time course (Fig. 7), confirming our previous findings TAKAUJI et al, 1984). As illustrated in a diagramatic way (Fig.…”

“…7). This result is consistent with our previously reported data TAKAUJI et al, 1984) and may also be explained in a similar diagramatic way as the foregoing: the reduction of external Ca2+ would cause much earlier onset of the inactivation process of the secondary component at an increased rate (i) (Fig. 9, middle tracings), and thus, lead to a rapid shortening of its time course, an increase in its spontaneous relaxation rate, and an eventual decrease in its peak tension (Fig.…”

“…7 b, c, and d). In 2 of 11 cases, the peak tension of the secondary component of 80 K contractures was slightly potentiated initially (not shown here; see also TAKAUJI et al, 1984, Fig. 4).…”

“…Thus, when the time course and the peak tension of test K contractures are examined, it would be much better to make experiments in the wide range of K+ concentrations of test contractures. In view of these considerations and the fact that the initial component is considered not to be accompanied by the inactivation process (see below; also TAKAUJI et al, 1984), the peak tension and the time course of the secondary component after prolonged conditioning depolarization and during repriming may be explained by a model illustrated above (Fig. 8) and therefore they may be determined by a balance between the activation process and the inactivation process of the secondary component.…”