A B STRA CT In vivo and in vitro studies were carried out to characterize the exchangeable bone magnesium pool and determine what effect age and magnesium depletion has on bone magnesium. A highly significant correlation was found between the size of the in vitro elutable and in vivo exchangeable bone magnesium (r = 0.97). To show that the exchangeable bone magnesium was the surface-limited bone magnesium, elution studies were performed 4 h after the in vivo administration of radiomagnesium. Specific activity in the eluant was 85% of that found in the serum at time of death, suggesting that the elutable and exchangeable bone magnesium pools were largely the same pool.Bone magnesium concentration fell with increasing age. The entire fall in bone magnesium was a result of a decrease in the surface-limited fraction. Since bone crystals have been shown to enlarge with aging with resulting contraction of the surface area, this would be the most apparent explanation for this finding. During magnesium depletion, magnesium concentration in both the exchangeable and nonexchangeable pools decreased. The fractional change in the exchangeable pool was much larger than the change in total or nonexchangeable bone magnesium, suggesting that the surface-limited magnesium pool is available during magnesium depletion. The change in size of the nonexchangeable bone magnesium pool appeared to be more related to the duration of magnesium depletion than the change in serum magnesium levels. The fall in magnesium concentration in this pool is probably a consequence of continuing formation of low magnesium bone during the depletion period. maintained on a magnesium-deficient diet (1). Since that time, numerous studies in a variety of different animal species have confirmed this finding (2-4). However, Heaton (5) and MacIntyre and Davidsson (6) have shown that, although bone magnesium concentration falls, total bone magnesium does not change during magnesium depletion. This finding has prompted a number of investigators to suggest, that bone magnesium concentration falls during magnesium depletion as a consequence of formation of low magnesium bone and that bone magnesium stores are neither available nor utilized during the depletion period. This would seem to explain why bone magnesium has been found to decrease much less in adults as compared to immature animals when both have been placed on a magnesium-deficient diet, since the latter animals would have a more rapid bone formation rate.In vitro studies using fresh human bone have shown that approximately 30% of bone magnesium fulfills the criteria for being surface limited, in that it is freely exchangeable and rapidly elutable and it readily equilibrates with the magnesium present in an incubating media (7). It would be expected that this pool should behave similarly in vivo and be available to replace other body deficits during magnesium depletion. In contrast, the larger bone magnesium pool appears to be intimately associated with the apatite crystals and would not be expected t...
Noninvasive techniques employing external counting of radiolabeled protein have the potential for measuring pulmonary vascular protein permeability, but their specificity and sensitivity remain unclear. We tested the specificity and sensitivity of a double-radioisotope method by injecting radiolabeled albumin (131I) and erythrocytes (99mTc) into anesthetized dogs and measuring the counts of each isotope for 150 min after injection with an external gamma probe fixed over the lung. We calculated the rate of increase of albumin counts measured by the probe (which reflects the rate at which protein leaks into the extravascular space). To assess permeability we normalized the rate of increase in albumin counts for changes in labeled erythrocyte signal to minimize influence of changes in vascular surface area and thus derived an albumin leak index. We measured the albumin leak index and gravimetric lung water during hydrostatic edema (acutely elevating left atrial pressure by left atrial balloon inflation: mean pulmonary arterial wedge pressure = 22.6 Torr) and in lung injury edema induced by high- (1.0 g/kg) and low-dose (0.25 g/kg) intravenous thiourea. To test specificity we compared hydrostatic and high-dose thiourea edema. The albumin leak index increased nearly fourfold from control after thiourea injury (27.2 +/- 2.3 X 10-4 vs. 7.6 +/- 0.9 X 10-4 min-1) but did not change from control levels after elevating left atrial pressure (8.9 +/- 1.2 X 10-4 min-1) despite comparable increases in gravimetric lung water. To test sensitivity we compared low-dose thiourea with controls. Following low-dose thiourea, the albumin leak index nearly doubled despite the absence of a measurable increase in lung water. We conclude that a noninvasive double radioisotope measurement of pulmonary vascular protein leak, employing external counting techniques and a simplified method of calculation, is specific for lung injury and is also sensitive enough to detect lung injury insufficient to produce detectable pulmonary edema.
A simple, safe method for bedside measurement of left ventricular function and pulmonary blood volume has been developed. A single scintillation probe positioned over the mid-left ventricle records the passage of bolus of radionuclide (ll3mIn) injected into the superior vena cava, from which the left ventricular ejection fraction can be determined after proper background correction. ll3mIn, half-life I-7 hours, easily prepared from a commercially available generator, rapidly binds to plasma transferrin and can be used to determine blood volume and cardiac output. With cardiac output and left ventricular ejection fraction determined simultaneously, left ventricular end-diastolic volume can be calculated and pulmonary blood volume can be determined from pulmonary transit time and cardiac output.With this method, we have demonstrated stable and changing left ventricular function and pulmonary blood volume in a variety of clinical situations, e.g. myocardial infarction, pulmonary embolism, and severe pulmonary parenchymal disease. Blood volume and cardiac output so determined correlate well with standard techniques. The left ventricular ejection fraction determined by this method correlates well with this measurement obtained from left ventricular angiography. Loss of the pulmonary valley between the right and left ventricles has been demonstrated in severe pulmonary vascular obstruction (embolism), in severe bronchial obstruction, and as a result of massive right ventricular dilatation in the presence of normal pulmonary blood volume. This method, which can be frequently performed, has proved useful in serial evaluation of cardiac function at the bedside.The ability to quantitate left ventricular function at the bedside would be an important aid for the care of the seriously ill patient, especially if the measurement could be repeated frequently. Not only would such information be of value in the management of the individual patient, but it would provide the basis for evaluation of various forms of therapy. External monitoring of the passage of a bolus of radioactive tracer through the central circulation has been developed over the past 25 years (Prinzmetal et al., 1948) in an attempt to fulfil the need for a safe, easily repeatable method of quantitating cardiac function. Earlier attempts to use simple scintillation probes and low frequency data for evaluation of the left ventricle after a right-sided
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