FACILITATION OF CALCIUM-DEPENDENT POTASSIUM CURRENT

The activation of Ca-dependent K+ current, I-c, was studied in macropa tches on the cell bodies of molluscan neurons. When a depolarizing vol tage-clamp pulse was applied repeatedly, I-c facilitated in a manner t hat resembled the facilitation of synaptic transmitter release. Facili tation was characterized by an increase in I-c amplitude, a progressiv e increase in instantaneous outward current, and a decrease in utiliza tion time. Experiments were done to investigate the mechanism responsi ble for I-c facilitation. Facilitation was reduced by microinjection o f an exogenous Ca2+ buffer into the cytoplasm, indicating that facilit ation is a Ca2+-dependent process. It was also reduced at elevated tem peratures. Conversely, facilitation was greatly potentiated by blockin g the Na/Ca exchange mechanism. It is concluded that the facilitation of Ca-dependent K+ current results from the accumulation of Ca2+ at th e inner face of the membrane during the repeated activation of Ca2+ ch annels by depolarization. The Ca2+ indicator fluo-3 was used in fluore scence imaging experiments to measure changes in [Ca](i) near the cell membrane during repeated depolarizing pulses and the interpretation o f these results was aided by numerical simulations of Ca2+ accumulatio n, diffusion, and buffering in the peripheral cytoplasm. These experim ents showed that the time course of I-c facilitation matches the time course of Ca2+ accumulation at the membrane. It was found that the str ength of I-c facilitation varies among patches on the same neuron, sug gesting that the accumulation of Ca2+ is not uniform along the inner s urface of the membrane and that gradients in [Ca](i) develop and are m aintained during trains of depolarizing pulses. Potential mechanisms t hat may lead to local differences in Ca2+ accumulation and I-c facilit ation are discussed.