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

Microgravity has been shown to alter protein kinase C (PKC) activity; therefore, we investigated whether microgravity influences mechanically stimulated Ca2+ signaling and ATP-induced Ca2+ oscillations, both of which are modulated by PKC. Rabbit tracheal epithelial outgrowth cultures or suspended epithelial sheets were rotated in bioreactors to simulate microgravity. Mechanical stimulation of a single cell increased the cytosolic Ca2+ concentration 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 cultures, 12-O-tetradecanoylphorbol-13-acetate (TPA; 80 nM), a PKC activator, restricted 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 epithelial 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 ATP-induced Ca2+ oscillations; however, the effect was more pronounced in cells kept in s mu G. These results suggest that PKC activation is more robust in cells subjected to s mu G.