Increase in gap junction resistance with acidification in crayfish septate axons is closely related to changes in intracellular calcium but not hydrogen ion concentration

Abstract

Neutral-carrier pH-and Ca-sensitive microelectrodes were used to investigate the relationship between junctional electrical resistance and either pH_i or [Ca²⁺]_i in crayfish septate axons uncoupled by acidification. For measuring [Ca²⁺]_i a new neutral carrier sensor sensitive to picomolar [Ca²⁺] and virtually insensitive to other ions was used. Uncoupling was induced by superfusing the axons with Na-acetate solutions (pH 6.3). With acetate, the time course of changes in junctional resistance differed markedly from that of pH_i or [H⁺]_i peaked 40–90 sec before junctional resistance. The difference in shape and peak time between pH_i and junctional resistance curves caused significant hysteresis in the pH_i versus junctional resistance relationship. In addition, junctional resistance maxima reached with slow acidification rates were 3–4 times greater than those with fast acidifications of similar magnitude. With acetate, [Ca²⁺]_i, increased by approximately one order of magnitude from basal values of 0.1–0.3 μm. The curves describing the time course of changes in [Ca²⁺]_i and junctional resistance matched well with each other in shape, peak time and magnitude. Both junctional resistance and [Ca²⁺]_i recovered following a single exponential decay with a time constant of ∼2 min. Different rates of acidification caused increases in [Ca²⁺]_i and junctional resistance comparable in magnitude. The data indicate that the increase in junctional resistance induced by acidification is more closely related to [Ca²⁺]_i than to [H⁺]_i.