pH dependence of transmission at electrotonic synapses of the crayfish septate axon

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