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Magnesium and intracellular sodium activity in cardiac Purkinje fibers
1Department of Physiology, State University of New York, Brooklyn.
Abstract:
The actions of magnesium on intracellular sodium activity (aiNa), transmembrane potentials and contractile force were studied in sheep cardiac Purkinje fibers perfused in vitro. Increasing [Mg]o from 1.05 to 2-8 mM shortens the action potential, decreases force and increases aiNa in active (and quiescent) fibers. High Ca (8.1 mM) increases force and decreases aiNa: in its presence, 8 mM Mg ('high Mg') decreases force as well as the action potential duration and still increases aiNa. Manganese decreases force and aiNa: the effects of high Mg are not changed by Mn-induced blockade of the slow channel. Even in the presence of 0.25 mM lidocaine (which decreases action potential duration, force and aiNa), high Mg increases aiNa, has little effect on the action potential duration and increases the threshold requirements. In the absence of [Ca]o, 1.35 mM Sr increases the action potential duration as well as aiNa and reduces force: high Mg markedly decreases aiNa, force and action potential duration. Thus, Mg decreases the duration of the action potential (as Ca does) but (in contrast to Ca) decreases contractile force (possibly by acting on intracellular structures) and increases intracellular Na activity. These effects are not dependent on Ca or Na influx and show that the increase in aiNa by Mg is the net result of opposing actions.
Insights
Magnesium (Mg) affects heart cells by shortening action potential duration and increasing intracellular sodium activity (aiNa). However, unlike calcium, Mg decreases contractile force, indicating complex interactions within cardiac Purkinje fibers.
Area of Science:
- Cardiology
- Electrophysiology
- Pharmacology
Background:
- Magnesium plays a crucial role in cardiac function, influencing ion transport and cellular electrical activity.
- Understanding magnesium's effects on cardiac Purkinje fibers is vital for comprehending its impact on heart rhythm and contractility.
Purpose of the Study:
- To investigate the specific actions of extracellular magnesium on intracellular sodium activity (aiNa), transmembrane potentials, and contractile force in sheep cardiac Purkinje fibers.
- To elucidate the mechanisms underlying magnesium's effects, particularly in the presence of varying calcium, manganese, and lidocaine concentrations.
Main Methods:
- In vitro perfusion of sheep cardiac Purkinje fibers.
- Manipulation of extracellular magnesium ([Mg]o) concentrations (1.05 to 8 mM).
- Simultaneous measurement of intracellular sodium activity (aiNa), transmembrane potentials, and contractile force under various ionic conditions (Ca, Mn, Sr, lidocaine).
Main Results:
- Increasing extracellular magnesium shortens action potential duration, decreases contractile force, and increases intracellular sodium activity (aiNa).
- These effects persist even when calcium influx is altered or blocked by manganese or lidocaine.
- Magnesium's impact on contractile force is opposite to that of calcium, suggesting intracellular mechanisms are involved.
Conclusions:
- Magnesium decreases action potential duration and contractile force while increasing intracellular sodium activity in cardiac Purkinje fibers.
- The observed effects of magnesium are largely independent of calcium and sodium influx, pointing to complex intracellular actions.
- Magnesium's net effect on intracellular sodium activity results from a balance of opposing actions.