Effects of the ionophore gramicidin D on energy metabolism in human erythrocytes

Stressed cybrids model demyelinated axons in multiple sclerosis

Multiple sclerosis is likely caused by a complex interaction of multiple genes and environmental factors. The contribution of mitochondrial DNA genetic backgrounds has been frequently reported. To evaluate the effect of mitochondrial DNA haplogroups in the same genetic and environmental circumstances, we have built human transmitochondrial cell lines and simulated the effect of axon demyelination, one of the hallmarks of multiple sclerosis pathology, by altering the ionic gradients through the plasmalemma and increasing ATP consumption. In this model, mitochondrial biogenesis is observed. This process is larger in Uk cybrids, which mirrors their lower oxidative phosphorylation capacity in basal conditions. This model replicates a process occurring in both patients suffering from multiple sclerosis and several animal models of axon demyelination. Therefore, it can be used to analyze the contribution of various mitochondrial DNA genotypes to multiple sclerosis. In this sense, a longer or stronger energy stress, such as that associated with demyelinated axons in multiple sclerosis, could make Uk individuals more susceptible to this pathology. Finally, pharmacologic compounds targeted to mitochondrial biogenesis could be a potential therapy for multiple sclerosis.

Imbalance of plasma membrane ion leak and pump relationship as a new aetiological basis of certain disease states

The basis for life is the ability of the cell to maintain ion gradients across biological membranes. Such gradients are created by specific membrane-bound ion pumps [adenosine triphosphatases (ATPases)]. According to physicochemical rules passive forces equilibrate (dissipate) ion gradients. The cholesterol/phospholipid ratio of the membrane and the degree of saturation of phospholipid fatty acids are important factors for membrane molecular order and herewith a determinant of the degree of non-specific membrane leakiness. Other operative principles, i.e. specific ion channels can be opened and closed according to mechanisms that are specific to the cell. Certain compounds called ionophores can be integrated in the plasma membrane and permit specific inorganic ions to pass. Irrespective of which mechanism ions leak across the plasma membrane the homeostasis may be kept by increasing ion pumping (ATPase activity) in an attempt to restore the physiological ion gradient. The energy source for this work seems to be glycolytically derived ATP formation. Thus an increase in ion pumping is reflected by increased ATP hydrolysis and rate of glycolysis. This can be measured as an accumulation of breakdown products of ATP and end-products of anaerobic glycolysis (lactate). In certain disease entities, the balance between ATP formation and ion pumping may be disordered resulting in a decrease in inter alia (i.a.) cellular energy charge, and an increase in lactate formation and catabolites of adenylates. Cardiac syndrome X is proposed to be due to an excessive leakage of potassium ions, leading to electrocardiographic (ECG) changes, abnormal Tl-scintigraphy of the heart and anginal pain (induced by adenosine). Cocksackie B3 infections, a common agent in myocarditis might also induce an ionophore-like effect. Moreover, Alzheimer's disease is characterized by the formation of extracellular amyloid deposits in the brain of patients. Perturbation of cellular membranes by the amyloid peptide during the development of Alzheimer's disease is one of several mechanisms proposed to account for the toxicity of this peptide on neuronal membranes. We have studied the effects of the peptide and fragments thereof on 45Ca2+-uptake in human erythrocytes and the energetic consequences. Treatment of erythrocytes with the beta 1-40 peptide, results in qualitatively similar nucleotide pattern and decrease of energy charge as the treatment with Ca2+-ionophore A23187. Finally, in recent studies we have revealed and published in this journal that a rare condition, Tarui's disease or glycogenosis type VII, primarily associated with a defect M-subunit of phosphofructokinase, demonstrates as a cophenomenon an increased leak of Ca2+ into erythrocytes.

Postpolio muscular dysfunction: relationships between muscle energy metabolism, subjective symptoms, magnetic resonance imaging, electromyography, and muscle strength

Eleven patients with previous polio were studied. The concentration of energy-related metabolites and energy charge was measured from the vastus lateralis muscle, as was isometric muscle strength of knee extension. Cross-sectional area of the quadriceps femoris muscle was calculated from magnetic resonance imaging. Reinnervation was studied using macroelectromyography. Muscle weakness, pain, and newly acquired muscle weakness in the legs was estimated by the patients. The findings in the legs in which the patients experienced new loss of muscle function were compared with the stable legs. There were no significant differences between these groups in any of the objectively measured variables. Only hip pain correlated with new loss of muscle function. Creatine phosphate was decreased in 5 patients. The symptoms and subjective muscle strength did not correlate with any of the objective measurements. There were no significant relationships between energy-related metabolites and postpolio symptoms.

Activation of AMP deaminase in human erythrocytes by calcium ions

We examined 5'-AMP aminohydrolase (AMP deaminase, EC 3.5.4.6. AMPD) together with metabolite changes in intact red blood cells from normal human adults under different incubation conditions. Moderate changes occurred in erythrocyte adenylates upon incubation for 1 h at 37 degrees C in the absence of glucose and the concentration of IMP was negligible. Calcium in the incubation medium, at concentrations up to 1.50 mmol l-1 did not influence the nucleotide pattern. Inclusion of the ionophore A 23187 in the incubation medium resulted in a sharp decrease of ATP content at 60 min and increases in AMP and ADP concentrations. A maximally low level of ATP was observed at 60 min of incubation in the presence of both the ionophore and 0.25 mmol l-1 of external calcium. With increasing concentrations of calcium up to 1.50 mmol l-1 in the presence of the ionophore, a significant rise occurred in both IMP and ATP concentrations, whether glucose was present or not, and significant correlations existed between the concentrations of calcium and IMP. The presence of the ionophore together with glucose but without extra calcium in the incubation medium produced the highest concentrations of lactate, indicative of an enhanced glycolytic flux under these conditions. The stimulatory effect of intracellular calcium ions on AMPD activity is discussed.

Alzheimer amyloid beta-peptides exhibit ionophore-like properties in human erythrocytes

There is growing evidence that the amyloid beta-peptide (beta 1-40) is involved in the aetiology of Alzheimer's disease also implicating an altered calcium homeostasis of affected cells. Beta 1-40 has been proposed to form calcium channels in synthetic bilayer membranes [1]. We wanted to investigate in the present study whether beta 1-40 (or fragments thereof) could act as ionophores in a biological membrane like the one in human erythrocytes. Incubation of the cells for 2 h and 4 h at 37 degrees C together with 6 mumol L-1 of beta 1-40 or of fragments beta 1-28 and beta 25-35, resulted in a significantly decreased energy charge qualitatively similar to the one obtained by a known calcium ionophore (A 23187, 0.05 mumol L-1). Moreover, beta 1-40 and its two fragments induced a significant alteration of 45Ca permeability in human red blood cells of the same type as the one achieved by the calcium ionophore. The ionophoric action of beta 1-40 and its two fragments may lead to an increase of the intracellular calcium ion concentration, in turn resulting in enhanced Ca(2+)-ATPase activity and a decrease in energy charge. This may be valid also for neuronal plasma membranes and could, therefore, be a possible aetiological mechanism in Alzheimer's disease.

Dissipation of the calcium gradient in human erythrocytes results in increased heat production

Heat production rates were measured by a microcalorimetric method in suspended human erythrocytes in the absence and presence of different concentrations of the divalent cationophore A23187. Determinations were carried out during 60 min under static conditions on erythrocytes incubated in various isotonic media at 37 degrees C, pH 7.35. In incubations containing the ionophore, time-power curves showed an early peak followed by a descending slope levelling off at a steady state after 30-60 min. In contrast, the controls lacked the early peak, showing hyperbolic ascending curve profiles before reaching steady state. The appearance of the early peak in the presence of ionophore was dependent upon the composition of the medium, both Mg2+ ions and glucose being decisive. Likewise, dose-response relationships concerning heat production at 60 min depended on the composition of the media. In a basic incubation medium lacking Mg2+ and glucose, no effect was seen on heat production by the ionophore (1-3 mumol/l). Rather modest effects were obtained by the ionophore at 2 and 3 mumol/l when Mg2+ was present. A clear-cut dose-response relationship was observed in a Mg2+ and glucose enriched medium for the ionophore from 1-3 mumol/l. The significant increase in heat production observed at 60 min with 2 mumol/l of A23187 in the Mg2+ and glucose enriched medium was abolished by 1 mmol/l EGTA. Calmidazolium, a calmodulin antagonist, could only marginally reduce the ionophoric effect on heat production. It was concluded that the appearance of the early peak was not the result of an increase in glycolytic rate but rather a consequence of the ionophoric action on the Ca2+ gradient.