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.

Direct microcalorimetry as a technique in cell cultures

In vitro studies of energy metabolism in isolated cells contribute to improved knowledge of human energy metabolism under normal and pathological conditions. In every cellular system energy is taken up, metabolized and finally transformed into heat, which is dissipated into the environment. Thus, energy turnover of isolated cells can be assessed by microcalorimetric determination of their heat production. Microcalorimeters of the thermopile heat conduction type facilitate direct physical determination of thermogenesis with a sensitivity of 0.2 microW; 10(4)-10(5) cells being sufficient for one measurement. Peltier elements are sandwiched between the sample and a precisely thermostated heat sink, creating a detectable voltage proportional to the heat production. For adequate interpretation of the results, simultaneous biochemical investigations of relevant metabolic pathways are required. Up to now, numerous studies with blood cells, skeletal and heart muscle cells, hepatocytes, endothelial cells, fibroblasts and adipocytes have been performed in relation to various diseases and under the influence of certain hormones and pharmacological agents.

Modulation defect of sodium pump evidenced in diabetic patients by a microcalorimetric study

Sodium pump activity of intact erythrocytes in their own plasma was measured by microcalorimetry in 41 healthy subjects and 35 insulin-dependent diabetic patients. Results show that modulation of the sodium pump is altered in diabetic patients. Addition of insulin increases functioning of the Na(+)-K+ pump in controls but has no effect in diabetic patients. These subjects show a slower response of the Na(+)-K+ pump to the inhibitory effect of ouabain. Cross-incubation experiments suggest that these findings may be explained by the existence of a plasmatic factor that impairs the modulation of the sodium pump in diabetic patients.