CALCIUM INDUCED CALCIUM RELEASE IS INVOLVED IN THE AFTERHYPERPOLARIZATION IN ONE CLASS OF GUINEA-PIG SYMPATHETIC NEURON

The mechanisms underlying two potassium conductances which are activated by Ca2+ influx during the action potential in sympathetic prevertebral neurones of guinea pigs have been investigated pharmacologically. One Ca-activated K+ conductance, which is present in all mammalian sympathetic postganglionic neurones, is maximal after the action potential and decays exponentially with a time constant of about 130 ms; this conductance was inhibited by apamin (50-100 nM) consistent with the involvement of SK channels. A second Ca-activated K+ conductance with much slower kinetics is present in a large subpopulation of coeliac neurones. This conductance was resistant to apamin but markedly inhibited by application of ryanodine (5-20 muM), suggesting that Ca2+ influx during the action potential triggers release of Ca2+ from intracellular stores which in turn activates a different class of K+ channel. Noradrenaline (100 muM) depressed the second K+ conductance selectively.