Dual effects of external magnesium on action potential duration in
guinea pig ventricular myocytes.
Zhang, Shetuan, Tohru Sawanobori, Hitoshi Adaniya, Yuji Hirano, and
Masayasu Hiraoka.
Department of Cardiovascular Diseases, Medical Research Institute,
Tokyo Medical and Dental University. 1-5-45 Yushima, Bunkyo-ku, Tokyo
113, Japan
APStracts 2:0031H, 1995.
Effects of extracellular magnesium (Mg2+) on action potential duration
and underlying membrane currents in guinea pig ventricular myocytes
were studied by using the whole-cell patch clamp method. Increasing
external Mg2+ concentration from 0.5 to 3 mM produced a prolongation
of action potential duration at 90% repolarization (APD90), whereas 5
and 10 mM Mg2+ shortened it. External Mg2+, at 3 mM or higher,
suppressed the delayed outward K+ current (Ik) and the inward
rectifier K+ current (Ik1). Increases in external Mg2+ depressed the
peak amplitude and delayed the decay time course of the Ca2+ current
(ICa), the latter effect probably being due to the decrease in Ca2+
-induced inactivation. Thus, 3 mM Mg2+ suppressed the peak ICa but
increased the late ICa amplitude at the end of 200 ms depolarization
pulse, while 10 mM Mg2+ suppressed the both components. A high
concentration of Mg2+ (10 mM) shifted the voltage dependent
activation and inactivation by about 10 mV to more positive voltages
with screening the membrane surface charges. Application of manganese
(1-5 mM) also showed 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 action potential duration 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 the
suppressive effects on K+ currents.
Received 12 November 1993; accepted in final form 10 January
1995.
APS Manuscript Number H1005-3.
Article publication pending Am. J. Physiol. (Heart Circ. Physiology).
ISSN 1080-4757 Copyright 1995 The American Physiological Society.
Published in APStracts on 24 February 1995.