Cl − channel blockers inhibit transition of quiescent (G 0 ) fibroblasts into the cell cycle

Modulation of ion permeability during the cell cycle is one of the key events in cell cycle progression. We have compared the effects of K + and Cl − channel blockers on the cell cycle in synchronous and asynchronous NIH3T3 cells. The Cl − channel blocker 5‐ N ‐2‐(3‐phenylpropylamino) benzoic acid (NPPB; 0.2 mM) inhibited entry into S phase in synchronous cells but not in asynchronous cells, while the K + channel blocker 4‐aminopyridine (4‐AP) showed similar inhibitory effects in both conditions. In NIH3T3 cells synchronized by serum deprivation/replenishment, G 0 ‐to‐G 1 transition occurred within 8 h after serum addition, and the G 1 /S checkpoint at 10–14 h. NPPB applied only at 0–8 or 8–14 h after serum addition inhibited entry into S phase. Cl − permeability measured as 125 I efflux increased at 4 and 10 h after serum addition. Ki‐67‐negative cells, which represent quiescent G 0 phase cells, progressively decreased in number until 8 h after serum addition. The Cl − channel blockers (NPPB and 4,4′‐diisothiocyanatostilbene‐2,2′‐disulfonic acid [DIDS]) but not the K + channel blocker (4‐AP) significantly decreased the rate of reduction in number of Ki‐67‐negative cells. These data indicate that an increase in Cl − permeability plays an important role in reentry of quiescent cells into the proliferating phase, in addition to the known effects on passage through the G 1 /S checkpoint. © 2003 Wiley‐Liss, Inc.