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

Abstract

The mechanism of spontaneous and rhythmic hyperpolarizations which occur in bullfrog [Rana catesbeiana] sympathetic ganglion cells under the effect of caffeine (2-10 mM) were further analysed. Intracellular injection of EGTA [ethylene glycol bis-(.beta.-aminoethyl ether)tetraacetic acid] blocked generation of caffeine hyperpolarizations (C-hyperpolarizations); this confirmed the previous conclusion (Kuba and 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+. The amplitude and duration of a C-hyperpolarization induced by an action potential were a function of the time since the previous one. 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. 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.degree. C) by more than a few degrees was effective. The rapid cooling effect was observed even in a Ca2+-free Mg2+ solution. Dantrolene Na increased the interval between rhythmic C-hyperpolarizations or blocked them, but affected less those triggered by an action potential or cold shock. Intracellular injection of Ca2+ triggered the generation of a C-hyperpolarization before the appearance of a rhythmic one. 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. 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. Rhythmic increases in the GK under the effect of caffeine are due to oscillations of the intracellular Ca2+ concentration, and there may be Ca storage sites in the bullfrog sympathetic ganglion cell which are comparable to the sarcoplasmic reticulum in the skeletal muscle fiber.