In order to investigate the effects of magnesium ion on intracellular potaential of cardiac muscle, a guinea-pig papillary muscle preparation was excised out to a in vitro contraction chamber and the isometric contraction of the preparation was recorded in normal Tyrode´s solution by a control stimuli(0.5Hz) and then intracellualr potenital was depolarized by potassimu 19mM/L Tyrode´s
:Y t
solution.
After partially depolarization to-55-6OmV with potassium ion, magnesium ion were applied from normal(ImM/L) to 16 mM/L.
The following results were obtained;
1
I. In normal Tyrode´s solution, addition of magnessium ion had no significant effect on the action potential paramenters except upstroke veolcities and contractility.
t, 2. In potassium 19mM/L Tyrode´s solution, upstroke velocties and duration of action potential of papillary muscle were changed by addition of magnesium ion. And upstroke velocities seperated with an initial first peak that manifect as a sharp and blunt second peak.
kt 3. By increase of concentration of magnesium, first peak of upstroke velocities increased but second peak decreased.
t 4. With 14mM/L magnesium ion in partially depolarized preparation, second peak was disappeared by change of calcium ion concentration from normal(2mM/L) to OAmM/L.
5, According to change of concentration of calcium ion from 0.4mM/L to 2mM/L by 0.4mM/L step, first peak of upstroke velocities changed 14.3 + 1.8V/Sec(mean + 1.9V/Sec, 19.5 + 1,5V/Sec, 22.0 + 1.3V/Sec, 24.4 1.5V/Sec, and second peak changed OV/Sec, 1.3 + 0.1 ´/Sec; 2.5 0.IV/ Sec 3.4 0.1 IV/Sec, and 4.2 0.1 V/Sec.
6. With treatrnent of TTX(tetrodotxin: O. lmg/L), first peak changed with concentration of calcium ion, but upstroke velocities were declined to values of 5.7 2.OV/Sec., 9.7 + 2.0V/Sec, 10.8 1.2V/Sec, 13.6 L8V/Sec., and 16.7 2.OV/Sec. But second peak had no significant change.
Above result suggest that magnesium ion induced first peak was affected by sodium ion in partially depolarized cardiac muscle.
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