Effects of magnesium, ouabain and bumetanide on 86rubidium uptake in a human atrial cell line

Extracellular Mg2+ regulates the intracellular Na+ concentration in rat sublingual acini

The intracellular free Na+ concentration ([Na+]i) increases during muscarinic stimulation in salivary acinar cells. The present study examined in rat sublingual acini the role of extracellular Mg2+ in the regulation of the stimulated [Na+]i increase using the fluorescent sodium indicator benzofuran isophthalate (SBFI). The muscarinic induced rise in [Na+]i was approximately 4-fold greater in the absence of extracellular Mg2+. When Na+ efflux was blocked by the Na+,K+-ATPase inhibitor ouabain, the stimulated [Na+]i increase was comparable to that seen in an Mg2+-free medium. Moreover, ouabain did not add further to the stimulated [Na+]i increase in an Mg2+-free medium suggesting that removal of extracellular Mg2+ may inhibit the Na+ pump. In agreement with this assumption, ouabain-sensitive Na+ efflux and rubidium uptake were reduced by extracellular Mg2+ depletion. Our results suggest that extracellular Mg2+ may regulate [Na+]i in sublingual salivary acinar cells by modulating Na+ pump activity.

Myocardial rubidium-82 tissue kinetics assessed by dynamic positron emission tomography as a marker of myocardial cell membrane integrity and viability

BACKGROUND

Recent reports have demonstrated the clinical use of rubidium-82 chloride (Rb-82) in combination with positron emission tomography (PET) not only as a tracer of myocardial blood flow but also as a marker of cell membrane integrity using static imaging early and late after tracer injection. The purpose of this study was to compare myocardial Rb-82 kinetics assessed by dynamic PET imaging as a marker for tissue viability with regional fluorine-18 fluorodeoxyglucose (FDG) uptake in patients with coronary artery disease.

METHODS AND RESULTS

Twenty-seven patients with angiographically proven coronary artery disease and 5 subjects with a low likelihood for coronary artery disease underwent dynamic PET imaging under resting conditions using Rb-82 and FDG. Both image sequences served as input data for a semiautomated regional analysis program. This program generated polar maps representing Rb-82 tissue half-life and FDG utilization assessed by Patlak's approach. Myocardial tissue viability was visually determined from static Rb-82 and FDG images. Regions were categorized as normal, ischemically compromised, and scar tissue. Their coordinates were subsequently copied to the functional polar maps for further analyses. In normal subjects, Rb-82 tissue half-life was homogeneous throughout the left ventricle (90 +/- 11 seconds). In coronary patients, differences between Rb-82 tissue half-lives in normal and scar tissue were highly significant (95 +/- 10 and 57 +/- 15 seconds, respectively; P < .0001). FDG uptake in these two tissue groups was 78 +/- 12% and 40 +/- 13%, respectively (P < .0001). Ischemically compromised tissue with reduced perfusion but maintained FDG uptake displayed an Rb-82 half-life of 75 +/- 9 seconds, indicating active cellular tracer retention, which was significantly different from scar tissue. Overall agreement of tissue categorization as either viable or scar was 86% between Rb-82 kinetics and FDG utilization. In a subgroup of 11 patients with all three tissue types within one image set, Rb-82 tissue half-life discriminated between normal, ischemic, and scar tissue (97 +/- 9, 75 +/- 9, and 60 +/- 15 seconds, respectively; P < .01).

CONCLUSIONS

This study demonstrated a significant relationship between cell membrane integrity as assessed by dynamic Rb-82 PET imaging and myocardial glucose utilization as a marker for tissue viability. In regions with reduced perfusion, Rb-82 kinetics was different in compromised but metabolically active and irreversibly injured myocardium. The predictive value of this approach must be evaluated in follow-up studies.

NMR studies of the effect of Mg2+ on post-ischemic recovery of ATP and intracellular sodium in perfused kidney

Ischemia is often implicated as a cause of acute renal failure. We have investigated the effect of various concentrations of extracellular Mg2+ on the post-ischemic recovery of ATP and low intracellular Na+ in the isolated perfused rat kidney using 31P and triple-quantum filtered (TQ) 23Na-NMR spectroscopy. Following a 1 h period of stopped flow ischemia, the kidneys exposed to 0.3 mM Mg2+ throughout the experiment exhibited a significantly (p < 0.05) decreased post-ischemic fractional recovery of ATP (56 +/- 7%) as well as a significantly (p < 0.05) increased accumulation of P(i) (250 +/- 30%) as compared to kidneys exposed to 1.2 mM Mg2+ throughout (88 +/- 5% recovery of [ATP] and 158 +/- 8% accumulation of [P(i)]). Kidneys exposed to 0.3 mM Mg2+ during the pre-ischemic and ischemic periods but to 1.2 mM Mg2+ during reperfusion also showed better recovery of ATP (83 +/- 6%) and lower accumulation of P(i) (143 +/- 8%) compared to kidneys exposed to low Mg2+ (0.3 mM) throughout the experiment. Measurements of the 23Na TQ signal following ischemia-reperfusion revealed that kidneys exposed to 1.2 mM Mg2+ exhibited significantly improved maintenance of low intracellular Na+ as compared to those exposed to 0.3 mM Mg2+ ([Na+]i = 107 +/- 7% in 1.2 mM Mg2+ vs. 152 +/- 3% in 0.3 mM Mg2+). No significant difference was found in the pre-ischemic basal intracellular free Ca2+ level (as measured by 19F-NMR in combination with 5 FBAPTA) between kidneys perfused with 1.2 mM and 0.3 mM Mg2+, and comparable depletion of ATP occurred during ischemia under both experimental conditions. These data indicate that increased extracellular Mg2+ has a protective effect against post-ischemic damage, probably related to its role in resynthesis of ATP during post-ischemic reperfusion. Our results would imply a greater vulnerability of the kidney to ischemic damage in hypomagnesemic clinical conditions such as alcoholism and diabetes.

Magnesium homeostasis and clinical disorders of magnesium deficiency

OBJECTIVE

To survey the causes of clinical hypomagnesemia and Mg deficiency. The relationship of hypomagnesemia to digitalis toxicity, congestive heart failure, arrhythmias, and acute myocardial infarction is discussed, as is the clinical interrelationship of Mg and K concentrations, the principal intracellular cations.

DATA SOURCES

A MEDLINE search and retrieval was used to identify relevant references.

STUDY SELECTION

Clinical reports, as well as studies, were selected for this review.

DATA EXTRACTION

There were very few placebo-controlled clinical studies. Clinical observations were related primarily to compilation of series in which Mg was administered and clinical results reported. In addition, conclusions derived from review articles on the subject of clinical Mg depletion were used.

DATA SYNTHESIS

Clinical diagnosis of Mg deficiency is ascertained most expeditiously by estimating serum Mg concentrations. Although available on order by physicians, the lack of routine serum Mg analysis as part of the "electrolyte panel" impedes the diagnosis of clinical Mg deficiency. Renal loss of Mg resulting from the widespread use of loop diuretics is responsible for significant numbers of patients with Mg deficiency and hypomagnesemia. Life-threatening cardiac arrhythmias and seizures represent the most serious manifestations of clinical hypomagnesemia and Mg depletion. In the most critically ill patients, treatment with intravenous Mg is recommended. Oral repletion of Mg is reserved for the less critically ill hospitalized patients and ambulatory patients. Close attention must be paid to optimizing K replenishment in hypokalemic patients by concurrent treatment of any accompanying hypomagnesemia to avoid the problem of refractory K repletion.

CONCLUSIONS

Hypomagnesemia is one of the most frequent serum electrolyte abnormalities in current clinical practice. Routine inclusion of serum Mg analysis in the electrolyte panel will enhance the clinical recognition and treatment of hypomagnesemic Mg-depleted patients. Failure to respond to treatment of recurrent ventricular tachycardia/fibrillation to usual antiarrhythmic therapy in patients with acute myocardial infarction, idiopathic dilated cardiomyopathy, and congestive heart failure should alert the clinician to consider administering intravenous Mg. Repair of coexisting hypomagnesemia in hypokalemic patients is essential to avoid the problem of refractory K repletion caused by coexisting Mg depletion. More controlled clinical studies of Mg deficiency are necessary to ascertain the cost-effectiveness of Mg replacement therapy.

Inhibition of Na(+) -K+ pump activity by divalent cations in intact peritoneal mast cells of the rat

1. The inhibition by the divalent cations magnesium, barium and strontium and the trivalent ion lanthanum of the Na(+) -K+ pump in the plasma membrane of rat peritoneal mast cells was studied in pure mast cell populations by measurement of the ouabain-sensitive uptake of the radioactive potassium analogue, 86rubidium (86Rb+). 2. Exposure of the cells to magnesium induced a time- and concentration-dependent decrease in the ouabain-sensitive K+(86Rb+)-uptake of the cells without influencing the ouabain-resistant uptake. The time-dependent decrease was apparent after incubation of the cells for 10 min or more, but no decrease was observed after 2 min incubation when the cells are supposed to be loaded with sodium due to the cell isolation procedure. 3. Barium and strontium caused concentration-dependent decreases in the ouabain-sensitive K(+) -(86Rb+) -uptake of the cells but the ouabain-resistant uptake was not changed. Half maximum decrease in the ouabain-sensitive K+(86Rb+)-uptake was observed with 1.8 mM magnesium, 1.2mM barium and 0.7 mM strontium. 4. The trivalent ion lanthanum blocked almost completely the ouabain-sensitive K+(86Rb+)-uptake at a concentration of 1 microM as does 1 mM calcium. Combining either of these ions with magnesium had no further inhibitory effect on the ouabain-sensitive uptake. 5. In conclusion, in addition to the previously suggested modulation by calcium of the activity of the Na+ (-)K+ pump, evidence is provided in this investigation that the modulation may be a more general effect of divalent and polyvalent cations present in the extracellular space through their influence on the sodium permeability of the plasma membrane.