AJP - Heart and Circulatory Physiology, Vol 253, Issue 2 248-H255, Copyright © 1987 by American Physiological Society
Mg inhibits voltage and tension oscillation but potentiates twitch in depolarized myocardium
T. Ito and T. Ehara
In the presence of 1-5 X 10(-8) M isoproterenol, guinea pig ventricular
muscle, depolarized by K-rich (27 mM) solution, exhibits a biphasic twitch
tension, after contraction (AC), and oscillatory afterpotentials (OAP) in
response to 1-Hz stimulation. We obtained data suggesting that OAP and AC
are due to a spontaneous oscillatory release of Ca from the sarcoplasmic
reticulum (SR) and that the initial component (C1) of the biphasic twitch,
which produces a large and early peak in tension, reflects the
excitation-coupled Ca release from SR. Elevation of extracellular Mg
concentration ([Mg]o) to 3-6 mM suppressed AC and OAP. However, C1 was
markedly potentiated by high [Mg]o. When single extrasystoles with various
coupling intervals were interposed during a regular stimulation at 0.2 Hz,
C1 in the extrasystole, which was large with a short coupling interval,
gradually decreased with lengthening of the coupling interval. Elevation of
[Mg]o markedly reduced the rate of this decrease suggesting that the
decrease in available Ca in the SR during diastole was slowed by high
[Mg]o. Thus, in depolarized ventricle, high Mg solution seems to depress
the spontaneous oscillatory Ca release from SR and subsequently enhances
the Ca release coupled to membrane excitation. The latter enhancement may
be related to stabilization of the Ca in SR during diastole.
