Regulation of intracellular pH by cell-cell adhesive interactions.

As was shown in our previous work, the intracellular pH (pHi) of cultured human fibroblasts depends on cell density. The pHi is low in single cells, higher in cells, forming small groups and maximal in a sparse monolayer. On the other hand, the pHi is low in areas of confluent monolayers. In the present work, we show that the effects of inhibitors of various pH-controlling mechanisms as well as inhibitors of key enzymes in signal transduction pathways depend on the local cell density. We have found that N-ethylmaleimide and 7-chloro-4-nitrobenz-2-oxa-1,3-diazole, known as inhibitors of V-type H+ ATPase, inhibit the elevation of pHi induced by cell-cell contact interactions; meanwhile Cd2+ ions, which inhibit H+ conductive pathway, cause an increase of pHi in a confluent monolayer. Our data revealed also that the Na+/H+ antiporter does not play an essential role in the pHi regulation by intercellular contacts. Inhibitors of phospholipase A2 (4-bromophenacyl-bromide), phospholipase C (neomycin) and protein kinase C (H-7) dramatically change the way the pHi is modulated by local cell density. It is suggested that cell-cell interactions regulate cell activities via modulation of pHi, which is under positive control from phospholipase A2 and under negative control from protein kinase C.

[Cell growth and motility in culture (in vitro) under microgravity conditions. The Fibroblast Experiment]. / Rost i podvizhnost' kletok v kul'ture (in vitro) v usloviiakh mikrogravitatsii. Eksperiment "Fibroblast".

The experiment "Fibroblast" was performed in 1992 on biosatellite "Cosmos-2229" in onboard device "Biobox" designed by the order of European Space Agency. The main objective was elucidation of the mechanisms responsible for the effect of space flight factors, mostly microgravity, on cell culture. We studied time-related changes in growth, motility and some morphological characteristics of the cells in monolayer cultures on a solid substrate and in three-dimensional cultures supported by sponge gels. Studies were carried out on connective tissue cells isolated from the mouse embryos. Comparative after-flight analysis of the cell cultures exposed to space flight and of those under the normal gravity conditions (1 g) on the Earth has revealed some differences. The space flight conditions, mainly microgravity, induced marked changes in morphological characteristics and functional activity of the cultured fibroblasts: changes in the nucleus size and shape, retardation of cell growth and division rate. We believe that these changes may be due to weakening of intercellular contacts and cell adhesion to the substrate. These findings are important both for general biology and space medicine, specifically for the problems of tissue regeneration and wound healing under the conditions of long-term space flight.