19F NMR magnetization transfer between 5-FBAPTA and its complexes. An alternative means for measuring free Ca2+ concentration, and detection of complexes with protein in erythrocytes

The 19F NMR Ca(2+)-indicator molecule 5,5'-difluoro-1,2-bis(o- aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (5-FBAPTA) was used in a procedure that enhances its range of applications to measuring free Ca2+ concentrations in buffer solutions and human erythrocytes. Even if the signal from the Ca-5-FBAPTA complex was not visible, the concentration of the complex could be calculated from saturation transfer spectra. This was demonstrated with well characterized buffer solutions in vitro and shown to also apply to concentrated haemolysates. The analysis required a precise estimate of the dissociation rate constant of the complex; this was found to be 295/s at 37 degrees C and the corresponding association rate constant was 4.1 x 10(8) L/mol/s. These values differ from those obtained previously in different buffer conditions and by two different NMR methods. A series of spectra were acquired from haemolysates containing 5-FBAPTA, in which saturating irradiation was applied at a frequency that was progressively off-set from the carrier frequency. Saturation transfer to the free 5-FBAPTA was seen from irradiation at frequencies different from that of Ca-5-FBAPTA, thus suggesting the presence of complexes with proteins.

Calcium homeostasis of human erythrocytes and its pathophysiological implications

In human red cells, Ca is mainly bound to the inner side of the plasma membrane. A smaller part may be present within intracellular Ca storing vesicles, while only a few percent of total red cell Ca is in ionized form. In some hemolytic anemias (sickle cell anemia, beta-thalassemia), an increased number of endocytotic vesicles storing Ca is probably responsible for the elevation of total red cell Ca content. Red cell Ca inward transport, which is partially susceptible to inhibition by Ca entry blockers, has been reported to be enhanced by physiological shear stress and enrichment in membrane cholesterol, as well as in some hemolytic anemias. Normal intracellular ionized Ca levels have been assessed in several diseases where elevated Ca inward transport rates or decreased Ca efflux through the Ca pump (hemolytic anemias, cystic fibrosis, essential hypertension) had been observed previously. Thus, red cell Ca homeostasis is apparently capable of keeping ionized Ca levels within the physiological range of 20-60 nM under most pathological conditions investigated so far. Conceptually, changes in red cell Ca homeostasis (or also in other red cell membrane parameters) may be of pathophysiological importance in two respects: 1) A disturbance may be directly responsible for some of the symptoms associated with a disease. This is the case in sickle cell anemia, where red cell dehydration is possibly caused by transient elevations of intracellular ionized calcium, which may activate K efflux through the Ca-activated K channel. The presence of dehydrated red cells will, in turn, lead to microvascular occlusion, a pathophysiologically important phenomenon in sickle cell anemia.(ABSTRACT TRUNCATED AT 250 WORDS)