pH Dependence of transmission at electronic synapses of the crayfish septate axon

Gap junctions between segments of the crayfish septate axon mediate electronic transmission of impulses propagating along the length of the nerve cord. We simultaneously measured intracellular pH (pH_i) and gap junctional conductance (g_j) while axons were exposed to saline equilibrated with CO₂, weak acids, and the weak base ammonium chloride. Normal pH_i is about 7.1. When pH_i is elevated, g_j is unaffected. When pH_i is reduced, g_j declines with an apparent pK of about 6.7 and a Hill coefficient of about 2.7. We also measured effects of pH_i on non-junctional conductance (g_nj) and on the coupling coefficient, k. Over the pH_i range 6.2-8, g_nj increases approximately linearly with pH_i. Since k is a function of g_j and g_nj, it reached a maximum at about pH_i 7.1, decreasing at higher and lower pH_i. The pH_i dependence of g_j in crayfish septate axon is less steep and has a lower apparent pK than the g_j-pH_i relation in two vertebrate embryos previously examined. This finding illustrates a difference in gating among analoguos and possibly homologous membrane channels.