DIETARY AND PHYSIOLOGICAL-STUDIES TO INVESTIGATE THE RELATIONSHIP BETWEEN CALCIUM AND MAGNESIUM SIGNALING IN THE MAMMALIAN MYOCARDIUM

This study employs both dietary and physiological studies to investiga te the relationship between calcium (Ca2+) and magnesium (Mg2+) signal ling 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 slowl y 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 contra st, 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%) a nd slightly decreased in the heart (88 +/- 6%) compared to CMD. Simila rly, sodium (Na+) contents were slightly higher in the heart and lower ed in the plasma of low Mg2+ diet rats compared to control Mg2+ diet r at. Perfusion of the isolated Langendorff's rat heart with a physiolog ical salt solution containing low concentrations (0-0.6 ms) of extrace llular magnesium [Mg2+](0) resulted in a small transient increase in t he amplitude of contraction compared to control [Mg2+](0) (1.2 ms). In contrast, elevated [Mg2+](0) (2-7.2 mM) caused a marked and progressi ve decrease in contractile force compared to control. In isolated vent ricular myocytes the L-type Ca2+ current (I-Ca,I-L) was significantly (p < 0.001) attenuated in cells dialysed with 7.1 mM Mg2+ compared to cells dialysed with 2.9 mu M Mg2+. The results indicate that hypomagne semia is associated with decreased levels of Mg2+ and elevated levels of Ca2+ in the heart and moreover, internal Mg2+ is able to modulate t he Ca2+ current through the L-type Ca2+ channel which in turn may be i nvolved with the regulation of contractile force in the heart.