Release of calcium ions linked to the activation of potassium conductance in a caffeine-treated sympathetic neurone.

Abstract

1. The mechanism of spontaneous and rhythmic hyperpolarizations which occur in bullfrog sympathetic ganglion cells under the effect of caffeine (2--10 mM) were further analysed. 2. Intracellular injection of EGTA blocked generation of caffeine hyperpolarizations (C-hyperpolarizations): this confirmed the previous conclusion (Kuba & Nishi, 1976) that these hyperpolarizations are caused by rhythmic increases in the K+ conductance (GK) of the membrane as a result of rises in free intracellular Ca2+. 3. The amplitude and duration of a C-hyperpolarization induced by an action potential was a function of the time since the previous one; the longer the interval, greater the area. 4. The relationship between the product of the amplitude and duration of a C-hyperpolarization and the preceding interval depended on external Ca2+; when this was low the relationship shifted, so as to indicate an involvement of a Ca2+ accumulating process in the generation mechanism of C-hyperpolarizations. 5. A rapid lowering of temperature triggered the generation of a C-hyperpolarization before appearance of a rhythmic one. There seemed to be no threshold temperature for the effect of such a cold shock; cooling from any temperature within a certain range (10--25 degrees C) by more than a few degrees was effective. 6. The rapid cooling effect was observed even in a Ca2+-free Mg2+ solution. 7. Dantrolene Na increased the interval between rhythmic C-hyperpolarizations or blocked them, but affected less those triggered by an action potential or cold shock. 8. Intracellular injection of Ca2+ triggered the generation of a C-hyperpolarization before the appearance of a rhythmic one. 9. The latency of the generation of an action potential-evoked C-hyperpolarization was dependent on the preceding interval; the shorter the interval, the longer the latency. There was a refractory period for induction of an action potential-induced C-hyperpolarization. 10. The interval between rhythmic C-hyperpolarizations was increased by a small or moderate membrane hyperpolarization (5--20 mV) and decreased by a larger hyperpolarization; the refractory period of an action potential-induced C-hyperpolarization was similarly increased and then decreased by progressive membrane hyperpolarization. 11. These results suggest that rhythmic increases in the GK under the effect of caffeine are due to oscillations of the intracellular Ca2+ concentration and that there may be Ca storage sites in the bullfrog sympathetic ganglion cell which are comparable to the sarcoplasmic reticulum in the skeletal muscle fibre.