Figure 4: P2 Receptor Expression in Smooth Muscle Cells after 24 h under Normal Gravity and Simulated Microgravity. The experiments for SMC were performed similarly to those for the endothelial cells above. All cells on the surface of flasks were isolated for RT-PCR. P2X4, P2X7 and P2Y2 were up-regulated whereas P2X2, P2Y1 and P2Y14 were down-regulated in the SMCs after 24 h clinorotation (a). Cells grown within 6 mm of the center had the optimal simulated microgravity condition and were thus isolated to confirm the above P2 receptor alteration on both, RNA (b) and protein (c) level after 24 h clinorotation with and without EC-conditioned medium from normal gravity. P2X2 and P2X4 showed no significantly changed. P2X7 and P2Y2 was up-regulated, P2Y1 and P2Y14 were down-regulated. The EC-conditioned medium can compensate for the increase of P2X7 and P2Y2 expression, but no significant effect was observed on P2Y1 and P2Y14 (d).