Dietary and physiological studies to investigate the relationship between calcium and magnesium signalling in the mammalian myocardium

This study employs both dietary and physiological studies to investigate the relationship between calcium (Ca²⁺) and magnesium (Mg²⁺) signalling in the mammalian myocardium. Rats maintained on a low Mg²⁺ diet (LMD; 39 mg Kg^-1 Mg²⁺ in food) consumed less food and grew more slowly than control rats fed on a control Mg²⁺ diet (CMD; 500 mg Kg^-1 Mg²⁺ in food). The Mg²⁺ contents of the heart and plasma were 85 ± 3% and 34 ± 6.5%, respectively relative to the control group. In contrast, Ca²⁺ contents in the heart and plasma were 177 ± 5% and 95 ± 3%. The levels of potassium (K⁺) was raised in the plasma (129 ± 16%) and slightly decreased in the heart (88 ± 6%) compared to CMD. Similarly, sodium (Na⁺) contents were slightly higher in the heart and lowered in the plasma of low Mg²⁺ diet rats compared to control Mg²⁺ diet rat. Perfusion of the isolated Langendorff's rat heart with a physiological salt solution containing low concentrations (0-9.6 mM) of extracellular magnesium [Mg²⁺](o) resulted in a small transient increase in the amplitude of contraction compared to control [Mg²⁺](o) (1.2 mM). In contrast, elevated [Mg²⁺](o) (2-7.2 mM) caused a marked and progressive decrease in contractile force compared to control. In isolated ventricular myocytes the L-type Ca²⁺ current (I(Ca,L)) was significantly (p < 0.001) attenuated in cells dialysed with 7.1 mM Mg²⁺ compared to cells dialysed with 2.9 μM Mg²⁺. The results indicate that hypomagnesemia is associated with decreased levels of Mg²⁺ and elevated levels of Ca²⁺ in the heart and moreover, internal Mg²⁺ is able to modulate the Ca²⁺ current through the L-type Ca²⁺ channel which in turn may be involved with the regulation of contractile force in the heart.