Decreased erythrocyte (Ca2+ + Mg2+)-ATPase activity in hemodialyzed uremic patients

Ca2+-Mg2+-dependent ATP-ase activity and calcium homeostasis in children with chronic kidney disease

Extracellular calcium concentrations in humans are thousands times higher than within cells. Maintenance of such gradient requires specific regulation including intracellular stores, Ca binding proteins and transmembrane protein systems. The aim of the study was to estimate PMCA (plasma membrane Ca-transporting adenosine triphosphatase; ATPase 3.6.1.38) activity and calcium homeostasis in erythrocytes of children with chronic kidney disease (CKD). Twenty-one children wth CKD stages 1-3 (group I) and 18 healthy children (group II) were examined. Group I was divided into two subgroups: Ia (8 patients with normal intact parathyroid hormone, iPTH, serum levels) and Ib (13 patients with increased iPTH). iPTH, urea, creatinine, inorganic phosphorus, cytosolic Ca2+ in red blood cells (R-Ca), and PMCA were determined. Significantly elevated R-Ca levels were observed in children from subgroup Ib in comparison with group II and subgroup Ia. The lowest activity of PMCA was found in subgroup Ia and Ib in comparison with group II. There was a negative correlation between PMCA and R-Ca in group Ia and Ib (r=-0.8, r=-0.9, respectively). In children with CKD treated conservatively, activity of PMCA in erythrocytes is disturbed. An increase in R-Ca and decrease in PMCA activity are also observed.

Carbamylation of erythrocyte membrane proteins: an in vitro and in vivo study

OBJECTIVES

To establish the degree of erythrocyte membrane protein carbamylation in uremic and nonuremic patients, and to characterize the in vitro binding of cyanate to the individual proteins of the cytoskeletal matrix.

DESIGN AND METHODS

For in vivo studies, erythrocyte ghosts were digested with proteinase K and the released peptides colorimetrically assayed for carbamylation, using the diacetyl monoxime reagent, and quantitated using homocitrulline. For in vitro studies, erythrocyte ghosts were incubated with [14C] cyanate, and the membrane proteins separated by SDS-PAGE. Cyanate incorporation was quantitated by liquid scintillation counting and imaging densitometry of the excised bands.

RESULTS

Erythrocytes from uremic patients were found to have a greater level of carbamylation than those from nonuremic patients (47.09 +/- 7.80 and 25.89 +/- 6.92 nmol homocitrulline/mg proteolyzed protein released, respectively). In vitro incorporation of [14C] cyanate into membrane protein followed the sequence: spectrin > ankyrin > Band 4.1 > Band 3 > actin > Band 7.

CONCLUSIONS

The increased level of erythrocyte membrane protein carbamylation in uremic compared to nonuremic patients may lead to membrane destabilization and contribute to the decreased erythrocyte survival time observed in uremia.