You could try 18β-glycyrrhetinic acid, heptanol, or 2-aminoethoxydiphenyl borate.

You can have a read to the paper in the link, where they studied their effects on hypoxic pulmonary vasoconstriction.

http://www.ncbi.nlm.nih.gov/pubmed/23708740

It should be noted that GA derivatives and APB have significant non junctional effects. 

Just  CO2 to the medium

Nat Methods. 2007 Apr;4(4):353-8.

Unitary permeability of gap junction channels to second messengers measured by FRET microscopy.

Hernandez VH1, Bortolozzi M, Pertegato V, Beltramello M, Giarin M, Zaccolo M, Pantano S, Mammano F.

Nat Cell Biol. 2005 Jan;7(1):63-9. 

Impaired permeability to Ins(1,4,5)P3 in a mutant connexin underlies recessive hereditary deafness.

Beltramello M1, Piazza V, Bukauskas FF, Pozzan T, Mammano F.

Thank you everyone for your input.

Simon