Calcium handling and cell contraction in rat cardiomyocytes depleted of intracellular magnesium

Objectives: Depressed levels of cardiac Mg have been found in patients with ischaemic heart disease or heart failure, but it is not known whether low intracellular free [Mg2+] ([Mg2+](i)) is a causal factor in such myocardial dysfunction, The aims for the present study were to develop a method of lo wering [Mg2+](i) in myocytes isolated from normal rat hearts, and so to det ermine whether a low [Mg2+](i) itself would cause abnormalities of intracel lular Ca2+ ([Ca2+](i)) homeostasis or myocyte contractile function in absen ce of any cardiac disease. Methods: Rat ventricular myocytes were loaded wi th mag-indo-1/AM or indo-1/AM for determination of total [Mg2+](i) and [Ca2 +](i), respectively. Mitochondrial [Ca2+] was determined by selective loadi ng of indo-1/AM into the mitochondria. Cell contraction was measured using an edge-tracking device. Myocytes were depleted of [Mg2+](i) by incubation in absence of external Mg2+. This resulted in a decrease in [Mg2+](i) from about 1.3 to 0.3 mM. In subsequent experiments, 1.7, mM MgCl2 was again pre sent in the superfusate. Results: Under basal conditions (low rate of stimu lation, 0.2 Hz, and 1 mM external [Ca2+])(i) the Mg-depleted cells showed v ery similar changes in [Ca2+] to control cells, despite an increase in the amplitude of cell contraction. But in presence of high external [Ca2+] (4 m M) and 5 Hz stimulation rate, the Mg-depleted cells showed defects in systo lic Ca2+ handling and in cell contraction; in particular, they were unable to increase systolic [Ca2+] in response to the stimulus, unlike control cel ls. Despite these alterations in total [Ca2+](i), mitochondrial Ca2+ uptake was unchanged in the Mg-depleted cells. Conclusions: A low [Mg2+](i) can i tself cause significant cardiomyocyte dysfunction in absence of any contrib uting disease slate. (C) 2000 Elsevier Science B.V. All rights reserved.