Effects of caffeine and ryanodine on low pHi-induced changes in gap junction conductance and calcium concentration in crayfish septate axons.
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 (10-30 mM) 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 microM) 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.
Peracchia
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Department of Physiology, University of Rochester, New York 14642.
D000470:Alkaloids / Q000494:pharmacology*; D000818:Animals; D003400:Astacoidea; D001369:Axons / Q000378:metabolism* / Q000502:physiology; D002104:Cadmium / Q000494:pharmacology; D002110:Caffeine / Q000494:pharmacology*; D002118:Calcium / Q000378:metabolism*; D002121:Calcium Channel Blockers / Q000494:pharmacology; D004553:Electric Conductivity; D006863:Hydrogen-Ion Concentration; D007365:Intercellular Junctions / Q000378:metabolism* / Q000502:physiology; D007811:Lanthanum / Q000494:pharmacology; D008839:Microelectrodes; D015737:Nisoldipine / Q000494:pharmacology; D012433:Ryanodine / Q000494:pharmacology*
D000470:Alkaloids; D002121:Calcium Channel Blockers; D002104:Cadmium; D012433:Ryanodine; D002110:Caffeine; D015737:Nisoldipine; D007811:Lanthanum; D002118:Calcium