Abstract
OBJECTIVES
To critically evaluate whether an altered calcium homeostasis in erythrocytes could be contributing to the symptomatology of the Tarui's disease, which is an inherited phosphofructokinase (PFK) deficiency of the muscle isoenzyme. PFK is a tetrameric enzyme with three different isoenzymes, muscle (M), liver (L), and platelet (P). Erythrocytes contain a 50 : 50 hybrid of M and L type. The deficiency of the muscle isoenzyme displays a symptomatology which is mainly characterized by myopathy, and a compensated haemolytic anaemia.
DESIGN
Erythrocyte deformability was assessed before and after autoincubation. Energy related metabolites and energy charge was determined in erythrocytes under various experimental conditions.
SETTING
The clinical part of the study was performed at the Departments of Cardiology and Clinical Chemistry, Umeå University Hospital, and the experimental investigation was carried out at the Department of Clinical Chemistry of the University Hospital of Uppsala, Sweden.
SUBJECTS
Four family members with Tarui's disease participated in the study: the proband (patient 1), a 39-year-old male and two siblings, patient 2 (male, aged 46 years) and patient 3 (female, 30 years). Patient 4 (male, 16 years) was the son of the patient 2. Five healthy persons served as controls (controls 1-5).
INTERVENTIONS
None.
MAIN OUTCOME MEASURES
Cell-physiological variables were determined after autoincubation of erythrocytes (i.e. incubation in their own plasma at 37 degrees C) and after incubation in a composite buffered medium.
RESULTS
Erythrocyte deformability as assessed by the erythrocyte fluidity was substantially decreased in patients compared to the moderate decrease in the control after 24 h of autoincubation, in presence of endogenous Ca2+ (heparin plasma). Moreover, autoincubation of erythrocytes shows that the patient's erythrocytes, although being moderately deficient in PFK activity, exhibit a normal (or slightly increased) lactate production compared to controls. Despite this, we show an increased ATP turnover with an Ca2+-induced AMP deaminase (and 5'-nucleotidase) activation leading to an increase in hypoxanthine content in patients' erythrocytes of about 100% after 24 h of autoincubation in heparin plasma, when compared to controls. A loss of volume in patient's erythrocytes after 24 h of autoincubation (in presence of Ca2+), as revealed by a diminished MCV, was consistent with an increased metabolic pool of intracellular calcium ions with a selective loss of K+ due to the activation of the K+ channel by intracellular Ca2+ (Gardos-effect).
CONCLUSION
We conclude that the different calcium ion-induced effects on energy metabolism, structure and function of patients' erythrocytes are due to an augmented membrane leakage of Ca2+ and therefore an accumulated intracellular Ca2+ pool. This will result in an increased energy demand by the Ca2+-stimulated ATPase (calcium pump) to compensate for the dissipated Ca2+ gradient across the plasma membrane. The concomitant haemolysis may be explained by a diminished erythrocyte deformability due to Ca2+ overload.
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