Effects of magnesium and calcium upon excitation-contraction coupling in the heart
Calcium (Ca2 ) availability to cardiac myofilaments is the primary factor determining the force of cardiac contraction. An intracellular organelle, the sarcoplasmic reticulum (SR), regulates the relaxation of the muscle as well as acting as a source of activating Ca2 during excitation-contraction coupling. Calcium metabolism in the heart undergoes developmental changes during early postnatal life and some of these changes may concern the activity of the SR. Using the technique of oxalate-supported Ca2 uptake into SR vesicles from rat hearts, the developmental changes in the activity of the SR Ca21 ATPase pump which regulates cardiac relaxation was monitored. It was observed that there occurred an increase in Call uptake with postnatal age in 3 strains of rat (Wistar, the Spontaneously Hypertensive Rat, SI-IR, and the Wistar- Kyoto, WKY, normotensive counterpart to the SHR). Additionally, age-related changes in the activity of an SR membrane protein, phospholamban (PLB), were investigated. Phospholamban phosphorylation increases the rate of Ca 2 uptake into the SR and it was observed that PLB phosphorylation in all 3 strains also increases with postnatal age. There seems then to be a coordinate relationship between the amount of PLB phosphorylated in the developing heart and the activity of the Ca 2 uptake mechanism immediately after birth in rats. When the SR is functionally immature in this very early postnatal period, the relative contribution to Ca 2 metabolism of another system, the sarcolemmal sodium/calcium exchanger, may be increased. In fact it was observed that when SR Ca 2 uptake is low the exchanger displays a heightened level of activity which may serve as a compensatory mechanism in maintaining intracellular Ca2 levels. Magnesium (Mg2 ) has, until recently, been thought to be present within the cell at levels which preclude it from having a regulatory role in cardiac function. New evidence suggests that Mg 2 may be present in cardiac cells at levels which would implicate it as a critical regulator of many myocardial cell systems. Preliminary experiments were undertaken using the isolated rat papillary muscle to observe the effect of changes in extracellular Mg 2 concentration on myocardial force development. It was observed that Mg 2 exerted concentration-dependent negative inotropic effects upon the rat heart. However, when the n—agonist, isoprenaline, (10 -7M-10 5M) was administered with solutions with varying Mg2l concentrations it was observed that an elevated Mg 2 concentration augmented the positive inotropic action of isoprenaline. In an effort to further understand the complex role of Mg 2' in myocardial cells, a cell culture model was set up of chick embryo ventricular cardiomyocytes. Using this system, the free intracellular Mg2 concentration of the heart cells was estimated using the fluorescent bioprobe, Magfura-2, under normal conditions and under conditions of energy depletion. It was found that the basal free intracellular Mg 2 concentration in these cells was approximately 0.5 mM and that mitochondrial inhibition resulted in rapid increases in intracellular free Mg 2 concentration which were associated with rapid decreases in ATP concentration. Using the SR Ca 2 uptake assay it was observed that around a narrow sub-millimolar range, changes in free Mg2 exerted strong inhibitory and stimulatory influences upon the Ca 2 handling capabilities of this intracellular organelle. The conclusion can be drawn that Mg2l plays a critical role in myocardial fimction, especially as a result of its regulation of cellular Ca 2 handling.
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