PKC activation inhibits Ca2+ signaling in tracheal epithelial cells kept in simulated microgravity

Microgravity has been shown to alter protein kinase C (PKC) activity; there fore, we investigated whether microgravity influences mechanically stimulat ed Ca2+ signaling and ATP-induced Ca2+ oscillations, both of which are modu lated by PKC. Rabbit tracheal epithelial outgrowth cultures or suspended ep ithelial sheets were rotated in bioreactors to simulate microgravity. Mecha nical stimulation of a single cell increased the cytosolic Ca2+ concentrati on in 35-55 cells of both outgrowth cultures and epithelial sheets kept at unit gravity (G) or in simulated microgravity (s mu G). In outgrowth cultur es, 12-O-tetradecanoylphorbol-13-acetate (TPA; 80 nM), a PKC activator, res tricted Ca2+ "waves" to about 10 cells in unit G and to significantly fewer cells in s mu G. TPA only slightly reduced the spread of Ca2+ waves in epi thelial sheets kept in s mu G but did not inhibit Ca2+ waves of sheets kept in unit G. In both cell preparations from both conditions, TPA inhibited A TP-induced Ca2+ oscillations; however, the effect was more pronounced in ce lls kept in s mu G. These results suggest that PKC activation is more robus t in cells subjected to s mu G.