Dual effects of external magnesium on action potential duration in guinea pig ventricular myocytes.
Effects of extracellular magnesium (Mg2+) on action potential duration (APD) and underlying membrane currents in guinea pig ventricular myocytes were studied by using the whole cell patch-clamp method. Increasing external Mg2+ concentration [Mg2+]o) from 0.5 to 3 mM produced a prolongation of APD at 90% repolarization (APD90), whereas 5 and 10 mM Mg2+ shortened it. [Mg2+]o, at 3 mM or higher, suppressed the delayed outward K+ current and the inward rectifier K+ current. Increases in [Mg2+]o depressed the peak amplitude and delayed the decay time course of the Ca2+ current (ICa), the latter effect is probably due to the decrease in Ca(2+)-induced inactivation. Thus 3 mM Mg2+ suppressed the peak ICa but increased the late ICa amplitude at the end of a 200-ms depolarization pulse, whereas 10 mM Mg2+ suppressed both components. Application of 10 mM Mg2+ shifted the voltage-dependent activation and inactivation by approximately 10 mV to more positive voltage due to screening the membrane surface charges. Application of manganese (1-5 mM) also caused dual effects on APD90, similar to those of Mg2+, and suppressed the peak ICa with slowed decay. These results suggest that the dual effects of Mg2+ on APD in guinea pig ventricular myocytes can be, at least in part, explained by its action on ICa with slowed decay time course in addition to suppressive effects on K+ currents.
Zhang
S
S
Department of Cardiovascular Diseases, Tokyo Medical and Dental University, Japan.
Sawanobori
T
T
Adaniya
H
H
Hirano
Y
Y
Hiraoka
M
M
D000200:Action Potentials / Q000187:drug effects*; D000465:Algorithms; D000818:Animals; D001464:Barium / Q000494:pharmacology; D015220:Calcium Channels / Q000502:physiology; D002478:Cells, Cultured; D004533:Egtazic Acid / Q000494:pharmacology; D006168:Guinea Pigs; D006321:Heart / Q000187:drug effects / Q000502:physiology*; D006352:Heart Ventricles; D016018:Least-Squares Analysis; D008274:Magnesium / Q000494:pharmacology*; D008345:Manganese / Q000494:pharmacology; D008564:Membrane Potentials / Q000187:drug effects; D013997:Time Factors
D015220:Calcium Channels; D001464:Barium; D008345:Manganese; D004533:Egtazic Acid; D008274:Magnesium