Relative deficiency of acidic isoforms of osteopontin from stone former urine

We have tested the relative electrophoretic mobility of osteopontin (OPN) isolated from urine obtained from normal individuals (NU) against similar samples derived from the urine of stone formers (SFU) using high-resolution isoelectric focusing (isoelectric point, pI range 3.5-4.5) in 2D electrophoresis, with Western blot detection. We also report the results from competitive ELISA analyses of these samples. We demonstrated that human urinary OPN has a discrete four band separation pattern that conforms to four previously documented OPN isoforms. The lower two M(r) isoforms migrate to a greater degree toward the acidic end of the gel than do the higher two M(r) isoforms. Densitometry of the signal reveals significant difference in the migration pattern of OPN from SFU as compared to that from NU based on an analysis of the spot intensities grouped in 0.1 pI unit increments. A novel method for the calculation of a weight-averaged pI based on the relative signal strength in an OPN 2D Western blot was developed. The analysis revealed a significantly increased weight-averaged pI values for the higher M(r) forms of OPN in the stone former compared to normal population. Additionally, alkaline phosphatase-treated NU samples resulted in a significant average pI shift of 0.05 units in the alkaline direction, suggesting that a decrease in the average degree of phosphorylation could be responsible for the difference between NU and SFU pI.

Li+ protects nerve cells against destabilization of Ca2+ homeostasis and delayed death caused by removal of external Na+

In experiments with fura-2 loaded cultured rat cerebellar granule cells we have compared the changes in [Ca2+]i homeostasis produced by replacement of external Na+ with the organic cation N-methyl-D-glucamine (NMDG) or Li+. The Na+/NMDG replacement caused an increase in baseline [Ca2+]i and a considerable delay in [Ca2+]i recovery following a glutamate (Glu) pulse in almost all the cells. In contrast Na+/Li+ replacement usually did not change baseline [Ca2+]i and produced only a small (if any) delay in the post-glutamate [Ca2+]i recovery. Previously [Storozhevykh et al. (1998) FEBS Lett. 431, 215-218] we revealed that perturbation of [Ca2+]i homeostasis caused by Na+/NMDG replacement cannot be explained by a reversal of the Na+/Ca2+ exchange but is mainly due to Ca2+ influx through NMDA channels activated by Na+ dependent release of endogenous excitatory amino acids ('reversed Glu uptake'). In the present work we confirmed this conclusion and obtained evidence suggesting that in contrast to NMDG Li+ interferes with the 'reversed Glu uptake' triggered by removal of external Na+. Thus it has been shown that the addition of Li+ (20 mM) to a Na+-free NMDG-containing solution suppressed both the perturbation of [Ca2+]i homeostasis and delayed neuronal death caused by Na+/NMDG replacement. Li+ is also able to abolish the [Ca2+]i response induced by PDC which at high concentrations (> 200 microM) is shown to stimulate the release of endogenous Glu. In contrast to Na+/Li+, Na+/NMDG replacement greatly enhances [Ca2+]i increase caused by PDC. Control experiments showed that Na+/Li+ replacement does not decrease the [Ca2+]i response to the Glu pulse. Therefore we concluded that a considerable quantitative difference between the effects of Na+/NMDG and Na+/Li+ replacements on both [Ca2+]i homeostasis and cell viability resulted mainly from the ability of Li+ to attenuate the release of endogenous Glu in response to the removal of external Na+.

Role of Na+/Ca2+ exchange in regulation of neuronal Ca2+ homeostasis requires re-evaluation

In cultured rat cerebellar granule cells an inhibition of plasma membrane Na+/Ca2+ exchange by removal of external Na+ (replacement with NMDG) caused an increase in [Ca2+]i at rest and a considerable delay in [Ca2+]i recovery from Glu-imposed [Ca2+]i load. These effects did not result from Ca2+ influx through reversed Na+/Ca2+ exchange since they were readily abolished or prevented by using the NMDA receptor inhibitor AP-5 (100 microM) or the NMDA channel blocker memantine (25-50 microM). The effect of Na+/NMDG replacement could be enhanced by: (1) an increase in cytoplasmic Na+ concentration by monensin pretreatment of neurons; (2) external alkalinity, pH 8.5; (3) blockade of the mitochondrial Ca2+ uptake with antimycin plus oligomycin. Analysis of the data obtained led us to conclude that all the changes in [Ca2+]i caused by Na+/NMDG replacement are mainly due to a release of endogenous Glu (reversed Glu uptake) and a subsequent Ca2+ influx through NMDA receptor-mediated channels.