Activation by intracellular ATP of a potassium channel in neurones from rat basomedial hypothalamus

Abstract

1. Cell-attached recordings from isolated glucose-sensitive hypothalamic neurones show that on removal of extracellular glucose there is an increased action current frequency concomitant with decreased single-channel activity. Conversely activation of single K+ channels was observed when extracellular glucose was increased. Isolation of membrane patches into the inside-out configuration following cell-attached recording demonstrated the presence of an ATP-activated K+ channel. 2. The ATP-activated K+ channel was characterized by a mean single-channel conductance of 132 pS in symmetrical 140 mM KCl solutions. Single-channel open-state probability (Po) was not calcium dependent, and the presence of calcium did not prevent activation of the channel by ATP. 3. Activation of the channel by ATP was concentration dependent and the Po of the ATP-activated channel was unaffected by membrane voltage, regardless of the degree of activation elicited by ATP. 4. Open and closed time histograms were constructed from inside-out and cell-attached recordings and were consistent with a single open and two closed states. Channel openings were grouped in bursts. Application of ATP, in isolated patches, and glucose, in cell-attached patches, increased the burst duration and number of bursts per second and decreased the slow closed-state time constant. In neither case was there a significant change in the fast closed-state time constant nor the open-state time constant. 5. The non-hydrolysable ATP analogue adenylylimidodiphosphate (AMP(PNP)) and 'Mg2(+)-free' ATP produced little change in the Po of the ATP-activated K+ channel when applied to the intracellular surface of excised patches. These results suggest that activation of this channel is via an enzymic mechanism. 6. ADP, GTP and GDP also activated the channel in a Mg(2+)-dependent manner. ADP and ATP activated the channel in an additive manner and neither GTP nor GDP inhibited channel activity induced by ATP. 7. It is concluded that the ATP-activated K+ channel observed in isolated inside-out patches from hypothalamic neurones is the same as the channel activated by an increase in the concentration of extracellular glucose in cell-attached recordings from glucose-sensitive neurones.