Generation of plasma membrane potential by the Na+-pump coupled to proton extrusion

Glucose induces a Na(+),K(+)-ATPase-dependent transient hyperpolarization in human sperm. I. Induction of changes in plasma membrane potential by the proton ionophore CCCP

When human sperm was incubated in medium deprived of glucose, glucose restoration caused a transient hyperpolarization of the plasma membrane. This hyperpolarization was also induced by fructose but not by 2-deoxyglucose, a substrate that cannot be metabolized. The hyperpolarization was inhibited by NaF, a glycolysis inhibitor, but not by mitochondrial inhibitors (cyanide, rotenone and antimycin), suggesting that it depended on glycolysis. Furthermore, the hyperpolarization was still induced in medium containing a high concentration of KCl and was insensitive to the K(+) channel blocker TEA and the Cl(-) channel blocker niflumic acid, but it was blocked by ouabain. This suggested that upon glucose addition, there was an increase in the concentration of ATP, that in turns increased the Na(+),K(+)-ATPase activity. Since this pump is electrogenic (2K(+)/3Na(+)) the plasma membrane hyperpolarized. On the other hand, CCCP, a proton ionophore, inhibited the hyperpolarization induced by glucose. When CCCP was added to glucose-treated hyperpolarized sperm, it caused a depolarization that triggered a Ca(2+) influx sensitive to nickel, an inhibitor of voltage-dependent calcium channels. Moreover, CCCP caused hyperpolarization in sperm incubated in medium without calcium, a known condition that depolarizes sperm. This indicated that CCCP induced proton permeability in the plasma membrane that was able to change the membrane potential to a value corresponding to the E(H) and that was also able to clamp it, so that it prevented the hyperpolarization induced by glucose.

Plasma membrane potential of lymphocytes from ataxia telangiectasia patients

The plasma membrane potential of lymphocytes prepared from ataxia telangiectasia (AT) patients and normal subjects was assessed using the optical indicator bis-(3-phenyl-5-oxoisoxazol-4-yl) pentamethineoxonol (oxonol-V). AT lymphocytes had a potential of -46 +/- 9 mV and normal lymphocytes had a potential of -63 +/- 4 mV. The intracellular cation content (Na+ and K+) of AT and normal lymphocytes was similar. AT and normal lymphocytes were both depolarized by extracellular K+ and to a similar extent. This study indicates that one feature characterizing ataxia telangiectasia is a modification of the ability of the lymphocyte cell membrane to sustain a normal membrane potential.

Dynamic changes in plasma membrane properties of Semliki Forest virus infected cells related to cell fusion

The mechanism of the processes leading to membrane fusion is as yet unknown. In this report we demonstrate that changes in membrane potential and potassium fluxes correlate with Semliki Forest virus induced cell-cell fusion at mildly acidic pH. The changes observed occur only at pH's below 6.2 corresponding to values required to trigger the fusion process. A possible role of these alterations of the plasma membrane related to membrane fusion phenomena is discussed.

Some novel aspects of the relationship between the amino acid gradient and the sodium electrochemical gradient in mouse ascites tumour cells

Accumulation of 2-aminoisobutyrate by mouse ascites tumour cells was studied in circumstances where nigericin reversed the normal direction of the Na+ concentration gradient. The membrane potential (delta psi) was assayed using oxonol V as a voltage-sensitive probe. The amino acid gradient (delta mu A) that formed did not significantly exceed the likely magnitude of the Na+ electrochemical gradient when this was in the range 2-6 kJ mol-1. When delta-Na mu increased up to 11 kJ mol-1, delta mu A was almost constant at 7-8 kJ mol-1. The observations indicate that when delta psi is large changes in cellular [Na+] in the range 16-80 mM scarcely affect delta mu A.

Can viral envelope proteins act as or induce proton channels?

The mechanism of the process leading to cell-cell fusion induced by enveloped viruses at a mildly acidic pH is as yet unknown. In this report we demonstrate that the fusion events induced by three viruses of different families, namely Semliki Forest (togavirus), vesicular stomatitis (rhabdovirus) and influenza (orthomyxovirus), share common features. In all three systems a sudden drop of the intracellular pH--below the critical extracellular pH required to trigger "fusion from within" (FFWI)--is observed. This influx of protons is specific and not due to a general leakiness of the plasma membrane, and therefore might be caused by the opening of a proton channel.

Na+ currents generated by the purified (Na+ + K+)-ATPase on planar lipid membranes

Purified (Na+ + K+)-ATPase from pig kidney was attached to black lipid membranes and ATP-induced electric currents were measured as described previously by Fendler et al. ((1985) EMBO J. 4, 3079-3085). An ATP concentration jump was produced by an ultraviolet-light flash converting non-hydrolysable caged ATP to ATP. In the presence of Na+ and Mg2+ this resulted in a transient current signal. The pump current was not only ATP dependent, but also was influenced by the ATP/caged ATP ratio. It was concluded that caged ATP binds to the enzyme (and hence inhibits the signal) with a Ki of approx. 30 microM, which was confirmed by enzymatic activity studies. An ATP affinity of approx. 2 microM was determined. The addition of the protonophore 1799 and the Me+/H+ exchanger monensin made the bilayer conductive leading to a stationary pump current. The stationary current was strongly increased by the addition of K+ with a K0.5 of 700 microM. Even in the absence of K+ a stationary current could be measured, which showed two Na+-affinities: a high-affinity (K0.5 less than or equal to 1 mM) and a low-affinity (K0.5 greater than or equal to 0.2 M). In order to explain the sustained electrogenic Na+ transport during the Na+-ATPase activity, it is proposed, that Na+ can replace K+ in dephosphorylating the enzyme, but binds about 1000-times weaker than K+. The ATP requirement of the Na+-ATPase was the same (K0.5 = 2 microM) with regard to the peak currents and the stationary currents. However, for the (Na+ + K+)-ATPase the stationary currents required more ATP. The results are discussed on the basis of the Albers-Post scheme.