Effect of external ATP on the plasma membrane permeability and (Na+ +K+)-ATPase activity of mouse neuroblastoma cells

P2X and P2Y receptors—role in the pathophysiology of the nervous system

Purinergic signalling plays a crucial role in proper functioning of the nervous system. Mechanisms depending on extracellular nucleotides and their P2 receptors also underlie a number of nervous system dysfunctions. This review aims to present the role of purinergic signalling, with particular focus devoted to role of P2 family receptors, in epilepsy, depression, neuropathic pain, nervous system neoplasms, such as glioma and neuroblastoma, neurodegenerative diseases like Parkinson’s disease, Alzheimer’s disease and multiple sclerosis. The above-mentioned conditions are associated with changes in expression of extracellular ectonucleotidases, P2X and P2Y receptors in neurons and glial cells, as well as releasing considerable amounts of nucleotides from activated or damaged nervous tissue cells into the extracellular space, which contributes to disturbance in purinergic signalling. The numerous studies indicate a potential possibility of using synthetic agonists/antagonists of P2 receptors in treatment of selected nervous system diseases. This is of particular significance, since numerous available agents reveal a low effectiveness and often produce side effects.

Purinergic signalling and cancer

Receptors for extracellular nucleotides are widely expressed by mammalian cells. They mediate a large array of responses ranging from growth stimulation to apoptosis, from chemotaxis to cell differentiation and from nociception to cytokine release, as well as neurotransmission. Pharma industry is involved in the development and clinical testing of drugs selectively targeting the different P1 nucleoside and P2 nucleotide receptor subtypes. As described in detail in the present review, P2 receptors are expressed by all tumours, in some cases to a very high level. Activation or inhibition of selected P2 receptor subtypes brings about cancer cell death or growth inhibition. The field has been largely neglected by current research in oncology, yet the evidence presented in this review, most of which is based on in vitro studies, although with a limited amount from in vivo experiments and human studies, warrants further efforts to explore the therapeutic potential of purinoceptor targeting in cancer.

Potassium-induced reverse transformation of cells infected with a temperature-sensitive transformation mutant virus

High potassium concentrations altered the morphology and the ability to grow in soft agar in 6m2 cells, a clone of rat kidney cells infected with a temperature-sensitive mutant of Moloney sarcoma virus. Approximately 60% of cells exhibited normal morphology in the presence of 94.8 mM potassium in isotonic medium at the temperature permissive for transformation, whereas 100% were normal at 72 mM potassium in hypertonic media. A significant reduction of growth in soft agar was also induced with these conditions. However, the synthesis ratio of virus-specified transforming protein to marker viral protein was not altered. Na+K+-ATPase might play a role in this reverse-transformation process.

Membrane regulation of the Na+,K+-ATPase during the neuroblastoma cell cycle: correlation with protein lateral mobility

The pumping activity of the plasma membrane-bound Na+,K+-ATPase shows considerable variation during the cell cycle of mouse neuroblastoma Neuro-2A cells. Addition of external ATP at millimolar concentrations, which selectively enhances the plasma membrane permeability of Neuro-2A cells for sodium ions, stimulates the Na+,K+-ATPase pumping activity at all phases of the cell cycle from a factor of 1.05 in mitosis up to 2.2 in G1 phase. Determination of the number of Na+,K+-ATPase copies per cell by direct 3H-ouabain binding studies in the presence of external ATP shows a gradual increase in the number of pump sites on passing from mitosis to the late S/G2-phase by approximately a factor of 2. From these data the pumping activity per copy of Na+,K+-ATPase, optimally stimulated with respect to its various substrate ions, has been determined during the various phases of the cell cycle. This optimally stimulated pumping activity per enzyme copy, which is a reflection of the physicochemical state of the plasma membrane, is high in mitosis, almost twofold lower in early G1 phase, and increases gradually again during the other phases of the cell cycle. This shows that the observed regulation of Na+,K+-ATPase activity during the cell cycle is caused by a combination of three independent factors--namely variation in intracellular substrate availability (Na+), changes in number of enzyme copies per cell, and modulation of the plasma membrane environment of the protein molecules. The modulation of the optimal pumping activity per enzyme copy shows a good correlation (rho = 0.96) with the known modulation of protein lateral mobility during the cell cycle, such that a high protein lateral mobility correlates with a low enzyme activity. It is concluded that changes in plasma membrane properties take place during the Neuro-2A cell cycle that result in changes in the rate of protein lateral diffusion and Na+,K+-ATPase activity in directly correlated way.

Effect of fatty acids on plasma membrane lipid dynamics and cation permeability in neuroblastoma cells

In this study the effects of experimental modifications of plasma membrane lipid lateral mobility on the electrical membrane properties and cation transport of mouse neuroblastoma cells, clone Neuro-2A, have been studied. Short-term supplementation of a chemically defined growth medium with oleic acid or linoleic acid resulted in an increase in the lateral mobility of lipids as inferred from fluorescence recovery after photobleaching of the lipid probe 3,3'-dioctadecylindocarbocyanide iodide. These changes were accompanied by a marked depolarization of the membrane potential from -51 mV to -36 mV, 1.5 h after addition, followed by a slow repolarization. Tracer flux studies, using 86Rb+ as a radioactive tracer for K+, demonstrated that the depolarization was not caused by changes in (Na+ + K+)-ATPase-mediated K+ influx or in the transmembrane K+ gradient. The permeability ratio (PNa/PK), determined from electrophysiological measurements, however, increased from 0.10 to 0.27 upon supplementation with oleic acid or linoleic acid. This transient rise of PNa/PK was shown by 24Na+ and 86Rb+ flux measurements to be due to both an increase of the Na+ permeability and a decrease of the K+ permeability. None of these effects occurred upon supplementation of the growth medium with stearic acid.