Unusual Slow Gating of Gap Junction Channels in Oocytes Expressing Connexin32 or Its COOH-Terminus Truncated Mutant.

Gap junction channels are gated by a chemical gate and two transjunctional voltage ( V j)-sensitive gates: fast and slow. Slow V j gate and chemical gate are believed to be the same. The slow gate closes at the negative side of V j and is mostly inactive without uncouplers or connexin (Cx) mutations. In contrast, our present data indicate otherwise. Oocytes expressing Cx32 were subjected to series of −100 m V V j pulses (12-s duration, 30-s intervals). Both peak (PK) and steady-state (SS) junctional conductances ( G j), measured at each pulse, decreased exponentially by 50−60% (tau = ∼1.2 min). G jPK dropped more dramatically, such that G jSS/ G jPK increased from 0.4 to 0.6, indicating a drop in V j sensitivity. Less striking effects were obtained with –60 m V pulses. During recovery, G j, measured by applying 20 m V pulses (2-s duration, 30-s intervals), slowly returned to initial values (tau = ∼7 min). With reversal of V j polarity, G jPK briefly increased and G jSS/ G jPK decreased, suggesting that V j-dependent hemichannel reopening is faster than hemichannel closing. Similar yet more dramatic results were obtained with COOH-terminus truncated Cx32 (Cx32-D225), a mutant believed to lack fast V j gating. The data indicate that the slow gate of Cx32 is active in the absence of uncouplers or mutations and displays unusual V j behavior. Based on previous evidence for direct calmodulin (CaM) involvement in chemical/slow gating, this may also be CaM-mediated. [ABSTRACT FROM AUTHOR]