Effect of specific ion channel blockers on cultured Schwann cell proliferation

Mitogenesis in a variety of tissues is known to be inhibited by K+ channel blockers such as tetraethylammonium (TEA) and 4-aminopyridine (4-AP). Using radiolabeled thymidine as a proliferation index we have examined what role, if any, specific K+ channels have in cultured Schwann cells that have been induced to proliferate by pre-exposure to mitogens. TEA and 4-AP are "broad-spectrum" in that they block a variety of different types of K+ channel. In contrast, we found that alpha-dendrotoxin (alpha-DTX), a specific blocker of the type 1 fast delayed rectifier current (the largest component of Schwann cell K+ current) does not affect proliferation, suggesting that type 1 current may not be involved in mitogenesis. This suggestion is supported by our finding that the values of the KD for the mitogenic effect (722 nM, 4-AP; 13 mM, TEA) are much larger than the corresponding electrophysiological values for type 1 channels (0.1 mM, 4-AP; 0.2 mM, TEA). Charybdotoxin (200 nM) and iberiotoxin (100 nM), inhibitors of Ca2+-activated K+ channels, cesium (5 mM), an inhibitor of inward rectifier channels, and furosemide (100 pM), which blocks Na+/K+/Cl- cotransport, all had no effect on proliferation. Interestingly, 4,4'-diisothiocyanatostilbene 2,2'-disulphonate (DIDS), which blocks voltage-gated Cl- channels, reduced proliferation. In summary, broad-spectrum K+ channel blockers inhibit Schwann cell proliferation, but inhibitors specific for type 1, Ca2+-activated, and inward rectifier K+ channels do not. Whether the inhibition is mediated by type 2 K- channels, by an as yet unidentified Schwann cell K+ channel, or by another mechanism remains unclear.