Plots of the SpIH activation time versus junctional conductance for Cx43 (•), Cx26 (▾), and Cx40 (○) in the absence (A) and presence (B) of phosphodiesterase (IBMX) and adenylate cyclase inhibitors (2’,5′ dideoxyadenosine). The SpIH activation times were reduced in IBMX and 2’5′dideoxyadenosine-treated cells and were reciprocally proportional to junctional conductance. The solid lines correspond to the first order regression and the dashed lines are the 95% confidence intervals. The arrows pointing to 15 and 20 nS indicate the threshold for detection of cAMP in recipient cell for Cx26 and Cx40, respectively.