CALCIUM-ACTIVATED POTASSIUM CHANNELS IN RAT DISSOCIATED VENTROMEDIAL HYPOTHALAMIC NEURONS

A potassium-selective channel, characterized by a single channel conductance of 160 PS was demonstrated to be present in rat freshly dispersed ventromedial hypothalamic nucleus neurons. The single channel activity was shown to be dependent, using inside-out membrane patches, upon the presence of intracellular calcium ions, with maximal sensitivity between 10(8) and 10(-6) M (Ca2+], and to be modulated by membrane voltage, depolarization causing an increase in open-state probability in the presence of an activating concentration of calcium. Therefore these properties place this channel into the category of a large conductance (maxi-K+) calcium-activated potassium (Ca2+-K+) channel. This channel is active in cell-attached recordings from glucoreceptive cells when depolarized by glucose or tolbutamide with openings often associated with action current repolarization. These openings were shown to be abolished in the presence of extracellular Cd2+ and La3+ ions, which block calcium channels, suggesting that extracellular calcium entry upon cell depolarization is responsible for their activation. On a few occasions, a larger conductance (250 pS) Ca2+-K+ channel was observed in inside-out membrane patches isolated from ventromedial hypothalamic nucleus neurons. In contrast to the 160 pS channel, the presence of intracellularly-applied ATP caused a concentration-dependent, reversible inhibition of its open-state probability.