Functional fiber types of single muscle fibers from the depressor mandibulae muscle of R. japonica and their input mechanism of excitation-contraction (E-C) coupling

II. Fiber types classified according to their K⁺-sensitivity

Abstract

Observations on the potassium-induced contraction of single muscle fibers from depressor mandibulae muscle showed them to be fast fibers. This confirms the hypothesis formulated earlier by electron microscopy. The K⁺ concentration versus tension curve showed that each fiber could be classified into one of 3 groups according to the K⁺ concentrations (20, 30 and 50 mM) required for one half-maximum tension development and the threshold of tension. Their contractile activation and inactivation response to K⁺-depolarization varied according to this classification; thus, the contractile response of the more K⁺-sensitive groups was more rapid. Thus, the fibers were distributed as follows: high K⁺ sensitivity group, 51%(n=19), moderate group, 38%(n=14), and low group, 11%(n=4). Further, under the condition of Ringer's fluid at pH 5.6, the K⁺ concentration versus tension curve shifted to the right.
Caffeine-induced contractions that do not depend on a change of membrane potential were the same in the 3 groups. This result indicates that there exists no difference in the contraction process among the three groups after the release of Ca²⁺ from terminal cisternae. In addition, there is no difference in the diffusion of K⁺ Ringer among the three groups, because they have the same shape, including the T-tubule. These results suggest that the mechanism that causes the difference in K⁺ sensitivity exists between the T-tubule and terminal cisternae, which is the site of E-C coupling.
Electron microscopy, however, did not reveal any morphologic difference that would explain the difference in K⁺-sensitivity among the 3 groups. Therefore, the variation in K⁺-sensitivity is due to a molecular difference in the mechanism of E-C coupling between T-tubules and terminal cisternae.
It is reported that the digastric muscle of mammals, which is homologous to the depressor mandibulae muscle of the present animal, R. japonica, has characteristically few muscle spindles. It follows, then, that movement control by the digastric muscle should proceed without the muscle spindle. This is probably the case also in the frog depressor mandibulae muscle, and the described differences in K⁺-sensitivity may help regulate movement of the depressor mandibulae muscle.