pH sensitivity of the cardiac gap junction proteins,connexin 45 and 43

Intercellular communication through gap junction channels can be regulated by changes in intracellular pH (pH_i). This regulation may play an important role in ischemic heart tissue. Using the dual voltage-clamp technique, we compared the pH_i sensitivity of gap junction channels composed of connexin 43 (Cx43) and Cx45, two of the gap junction proteins that are expressed in heart. We made use of SKHep1 cells, endogenously expressing low levels of Cx45 and SKHep1 cells stably transfected with rat Cx43. To manipulate the pH_i we applied the NH₃/NH₄⁺ pH-clamp method. At pH_i 6.7 the g_j of Cx45 channels was reduced to 20% of control values (pH_i 7.0) and at pH_i 6.3 all channels closed. The g_j of Cx43 channels was 70% of control values at pH_i 6.7 and 40% at pH_i 6.3. Cx43 channels closed at pH_i 5.8. Single channel conductances were 17.8 pS for Cx45 and 40.8 pS for Cx43 at pH_i 7.0 and did not change significantly at lower pH_i. This suggests that the decrease in macroscopic conductance observed at low pH_i results from the decrease in open probability of gap junctional channels rather than from a decrease in single channel conductance. Our results demonstrate that gap junction channels built of Cx45 are far more pH sensitive than channels built of Cx43.