Effects of caffeine and raynodine on low pHi-induced changes in gap junction conductance and calcium concentration in crayfish septate axons

Summary Electrical uncoupling of crayfish septate axons with acidification has been shown to cause a substantial increase in Ca²⁺]_i which closely matches in percent the increase in junctional resistance. To determine the origin of Ca²⁺]_i increase, septate axons have been exposed either to drugs that influence Ca²⁺ release from internal stores, caffeine and ryanodine, or to treatments that affect Ca²⁺ entry. A large increase in junctional resistance and Ca²⁺]_i maxima above controls resulted from addition of caffeine (10–30mm) to acetate solutions, while a substantial decrease in both parameters was observed when exposure to acetate-caffeine was preceded by caffeine pretreatment. In contrast, ryanodine (1–10 m) always caused a significant decrease in junctional resistance and Ca²⁺]_i maxima when applied either together with acetate or both before and with acetate. Calcium channel blockers such as La³⁺, Cd²⁺ and nisoldipine had no effect, while an increase in the Ca²⁺] of acetate solutions either decreased junctional resistance and Ca²⁺]_i maxima or had no effect. The data suggest that cytoplasmic acidification causes an increase in Ca²⁺]_i by releasing Ca²⁺ from caffeine and ryanodine-sensitive Ca²⁺ stores. The increase in Ca²⁺]_i results in a decrease in gap junction conductance.