抄録
Membrane potentials of mouse parathyroid cells were measured by means of the intracellular micro electrode method. Effects of temperature on membrane potential were observed. The membrane potential in a low calcium concentration (1.0mM) showed the maximum value (-60mV) at 36°C and a high temperature (over 40°C) caused depolarization of the membrane potential to -30mV at 44°C. Increase of Ca ion concentration caused sharp depolarization in a sigmoid curve, that is, the depolarization mechanism was cooperative. The membrane potential depended on the concentration of Mg ion and the temperature of the external medium. As the temperature of the external solution containing 1.0 mM of Mg ion was raised from 36°C to 40°C, the membrane potential decreased linearly. The enthalpies of Ca-binding reaction were obtained from the Van't Hoff plot as 2.5 kcal at 36-40°C and 16.0 kcal at 40-44°C. The activation energies of the K+ permeability increase reaction were obtained from the Arrhenius plots as 10 kcal for proceeding reaction and 13 kcal for reverse reaction at 36-40°C. The difference, 3 kcal, corresponded to the enthalpy change of the total reaction, and was close to that of Ca-binding reaction. The Hill constant of Ca ion depended on the temperature; n=20 at 36°C, n=8 at 40°C and n≅1 at 44°C. These values showed that the heat of a Ca-binding reaction was affected by the temperature. By means of the Gibbs-Helmholz equation, the enthalpy of the electrical-cell of Ca++ and Mg++ were calculated from the membrane potential as a function of temperature.
The cooperative phenomenon in mouse parathyroid cell was caused by the change of K permeability which was effected by Ca-binding reaction. There were at least two reactions involved in the process. One was the accelerative cooperative reaction of Ca binding and the another was the decrease of K permeability which was caused from the aggregation of molecules of channel protein or carrier protein. The cooperativity was produced with these mechanisms working together, and the Hill constant showed large value.
