A slight acidification of the cytoplasm blocks gap junctions. You can achieve this by reducing extracellular pH (Brain Res 1036:42-49, 2005; J Neurophysiol. 88:1893-902, 2002), or by reducing pH of the electrode solution (I'm not sure that has been done, though).

Have a look at this publication

good luck

You might want to try 2-APB: http://jpet.aspetjournals.org/content/319/3/1452.long

Not a specific gap junction channel blocker, of course.

Thank you very much everyone for your ideas.

Please note that 2-APB was applied in the bathing medium, not intracellularly. This paper contains relevant information on the inhibitory effect of low intracellular pH on gap junctions.