Voltage clamped depolarization contraction in frog twitch fibers immersed in extremely low calcium medium
Fujino, M.; Sato, Y.
Boei Ika Daigakko Zasshi 12(4): 233-237
The voltage clamped depolarization concentration (VDC) was recorded from the frog (Xenopus laevis) skeletal muscle fibers isolated singly and effects of an extremely low calcium (Ca) Ringer (ER) containing EGTA at high concentrations were examined. The preliminary experiments showed that a reduction of resting potentials in ER was protected by the addition of 20-25 mM Mg to ER using either TRIS or MOPS buffer whose concentrations were less than 5 mM. In the optimum composition of ER (70 mM EGTA, 25 mM Mg, 2.5 KCl, and 2.5 TRIS), the concentrations of free Ca and Mg ions were estimated 2.5 .times. 10-10 M and 6.7 mM (20.degree.C), respectively, assuming contaminative Ca=0.1mM. The VDC in this medium revealed a mechanical threshold at -40 mV and the maximal height (92% of tetanus height) at -30 mV (20.degree.C). The potential-dependent activation curve in the ER was similar to that in a gluconate Ringer (GR) with both 100 mM Na-gluconate and 1.8 mM CaCl2 instead of EGTA, but shifted to the higher potentials of about 20 mV compared with that in a normal Ringer (NR). This shift was not accounted for the effect of extremely low Ca but accounted for the effect of high Mg since Ca-free Ringer containing 10-80 mM Mg evoked similar shift. On the other hand, the width of VDC in ER was smaller than that in either GR or NR. This is probably related to facilitation of inactivation process. In conclusion, the medium of extemely low Ca does not influence the activation process of excitation-contraction coupling.