Magnesium affects excitation, conduction, and contraction of isolated mammalian cardiac muscle.
An increase of extracellular Mg concentration, [Mg]o, reduced myocardial excitability and conduction without affecting the resting membrane potential or action potential configuration in ventricular myocytes and papillary muscles from a number of mammalian species. Although there was a small increase of specific membrane resistance and no change to intracellular resistivity, the threshold voltage was shifted to depolarized potentials. Thus loss of excitability can be explained by a shift of the activation of inward currents to depolarized potentials, and reduced conduction velocity is due solely to a diminution of local circuit currents. Mgo also was negatively inotropic, the magnitude of this effect being species dependent. Raised [Mg]o caused a small increase of intracellular [Mg] with a small decrease of intracellular [Na+], did not affect intracellular pH, and attenuated the intracellular Ca2+ transient associated with cell shortening in rat (but not rabbit) myocytes. An increase of [Mg]o reduced the magnitude of the voltage-dependent inward Ca2+ current, ICa, in rat and rabbit myocytes, and the activation curve of ICa was shifted to more depolarized potentials. A scheme to account for the negative inotropic effect of Mg is presented.
Hall
S K
SK
Department of Physiology, United Medical School, St. Thomas Hospital, London, United Kingdom.
Fry
C H
CH
D000818:Animals; D002118:Calcium / Q000502:physiology; D004056:Differential Threshold; D004594:Electrophysiology; D005289:Ferrets; D006329:Heart Conduction System / Q000187:drug effects* / Q000502:physiology; D007425:Intracellular Membranes / Q000378:metabolism; D007477:Ions; D007537:Isometric Contraction; D008274:Magnesium / Q000494:pharmacology*; D008564:Membrane Potentials; D009200:Myocardial Contraction / Q000187:drug effects*; D009206:Myocardium / Q000166:cytology / Q000378:metabolism; D009994:Osmolar Concentration; D010210:Papillary Muscles / Q000187:drug effects* / Q000378:metabolism / Q000502:physiology; D011817:Rabbits; D051381:Rats; D012146:Rest
D007477:Ions; D008274:Magnesium; D002118:Calcium