Intracellular pH regulation of SV-40 virus transformed human MRC-5 fibroblasts and cell membrane cholesterol

Human cytomegalovirus infection enhances osmotic stimulation of Na+/H+ exchange in human fibroblasts

Infection with human cytomegalovirus (HCMV) causes an enlargement (cytomegaly) of human fibroblasts (MRC-5). As a first step toward determining whether solute uptake, mediated in part by Na+/H+ exchange, is responsible for the development of cytomegaly, we studied the effects of HCMV infection on intracellular pH (pHi) regulation (nominal CO2/ HCO3- concn = 0) by comparing cytomegalic cells with mock-infected cells. Seventy-two hours after HCMV infection of MRC-5 cells we observed the following changes relative to mock-infected cells: resting pHi is 0.1-0.2 pH unit more alkaline; the intrinsic buffering power of the cytoplasm was reduced by approximately 40-50%; acid-loading H(+)-equivalent fluxes were reduced; and there were alterations of Na+/H+ exchanger (NHE) properties, including an alkaline shift of the pHi dependence of activity, a reduction of the apparent affinity for extracellular Na+, and an increase of the apparent maximum velocity and a large increase in stimulation by a hyperosmotic challenge. These results indicate that HCMV infection exerts a profound effect on functional properties of the NHE, on acid-loading mechanisms, and on intrinsic cellular buffering power. These effects are consistent with a role for the NHE in the development of cytomegaly.

Oxidized-LDL induced changes in membrane physico-chemical properties and [Ca2+]i of bovine aortic endothelial cells. Influence of vitamin E

The effects of 3 days' exposure to native and oxidatively modified human low density lipoprotein (LDL and Ox-LDL) on cultured bovine aortic endothelial cell cholesterol content, membrane microviscosity and intracellular free calcium concentration ([Ca2+]i) were studied. Free cholesterol content increased by 35% and 100% in LDL and Ox-LDL treated cells, respectively, these effects being reversed by vitamin E; esterified cholesterol, which rose by 110% in the Ox-LDL group only, was not affected by vitamin E. Membrane microviscosity, measured as the fluorescence polarization of the trimethylammonium derivative of diphenyl-hexatriene, increased by 9% in Ox-LDL treated cells only. This effect was also reversed by vitamin E. Using the calcium sensitive fluorescent dye fura 2-AM, increases in basal [Ca2+]i of 36% in LDL and 81% in Ox-LDL treated cells were observed. The bradykinin mediated increase in [Ca2+]i was enhanced in both the LDL and, to a greater extent, the Ox-LDL group. Vitamin E reversed the effects of LDL on [Ca2+]i but had no influence in the Ox-LDL group. The lipoproteins affected all parameters measured in this study. Oxidized LDL produced reversible and irreversible alterations to the membrane and the [Ca2+]i. All changes associated with LDL were abolished by vitamin E. Such modifications in the physicochemical properties of the membrane and [Ca2+]i could be involved in the initiation of the atherosclerotic process.

Unusual low proton permeability of liposomes prepared from the endogenous myelin lipids

In contrast with most other lipid substrates, in this article we show that liposomes prepared from the total myelin lipids exhibited a negligible proton permeability. Neither the generation of valinomycin-induced potassium diffusion potentials as high as -177 mV nor the imposition of large pH gradients (up to three units) was able to produce a substantial flux of protons through liposomal membranes, as determined by the distribution of [14C]-methylamine, or the changes in the fluorescence of the probes 9-aminoacridine, acridine orange, and pyranine. The presence of cations (Na+, K+, Ca2+) did not alter this behavior. Voltage clamping did not increase the transmembrane delta pH-driven proton permeability. However, liposome diameter was found to be critical because small unilamellar vesicles displayed a much higher proton permeability than large unilamellar or multilamellar vesicles. This abnormally low proton permeability is interpreted by virtue of the characteristic biochemical composition of myelin lipid matrix, with a high content of cholesterol and sphingolipids and a very low level of free fatty acids. These results could be important for elucidating the role of myelin in the regulation of pH in the brain. In addition, the myelin lipid extract could be useful for reconstituting proteins that participate in the transport of H+ through the membrane.