K channel activation by nucleotide diphosphates and its inhibition by glibenclamide in vascular smooth muscle cells

<jats:p><jats:list list-type="explicit-label"> <jats:list-item><jats:p>Whole‐cell and inside‐out patch recordings were made from single smooth muscle cells that had been isolated enzymatically and mechanically from the rabbit portal vein.</jats:p></jats:list-item> <jats:list-item><jats:p>In whole‐cells the inclusion in the recording pipette solution of nucleotide diphosphates (NDPs), but not tri‐ or monophosphates, induced a K‐current that developed gradually over 5 to 15 min. Intracellular 1 m<jats:sc>m</jats:sc> guanosine 5′‐diphosphate (GDP) induced a slowly developing outward K‐current at − 37 mV that reached a maximum on average of 72 ± 4 pA (<jats:italic>n</jats:italic> = 40). Half maximal effect was estimated to occur with about 0.2 m<jats:sc>m</jats:sc> GDP. Except for ADP, other NDPs had comparable effects. At 0.1 m<jats:sc>m</jats:sc>, ADP was equivalent to GDP but at higher concentration ADP was less effective. ADP induced its maximum effect at 1 m<jats:sc>m</jats:sc> but had almost no effect at 10 m<jats:sc>m</jats:sc>.</jats:p></jats:list-item> <jats:list-item><jats:p>In 14% of inside‐out patches exposed to 1 m<jats:sc>m</jats:sc> GDP at the intracellular surface, characteristic K channel activity was observed which showed long (> 1 s) bursts of openings separated by longer closed periods. The current‐voltage relationship for the channel was linear in a 60 m<jats:sc>m</jats:sc>:130 m<jats:sc>m</jats:sc> K‐gradient and the unitary conductance was 24 pS.</jats:p></jats:list-item> <jats:list-item><jats:p>Glibenclamide applied via the extracellular solution was found to be a potent inhibitor of GDP‐induced K‐current (<jats:italic>I</jats:italic><jats:sub>K(GDP)</jats:sub>) in the whole‐cell. The <jats:italic>K</jats:italic><jats:sub>d</jats:sub> was 25 n<jats:sc>m</jats:sc> and the inhibition was fully reversible on wash‐out.</jats:p></jats:list-item> <jats:list-item><jats:p>(<jats:italic>I</jats:italic><jats:sub>K(GDP)</jats:sub>) was not evoked if Mg ions were absent from the pipette solution. In contrast the omission of extracellular Mg ions had no effect on outward or inward (<jats:italic>I</jats:italic><jats:sub>K(GDP)</jats:sub>).</jats:p></jats:list-item> <jats:list-item><jats:p>Inclusion of 1 m<jats:sc>m</jats:sc> ATP in the recording pipette solution reduced (<jats:italic>I</jats:italic><jats:sub>K(GDP)</jats:sub>) and also attenuated its decline during long (25 min) recordings.</jats:p></jats:list-item> <jats:list-item><jats:p>When perforated‐patch whole‐cell recording was used, metabolic poisoning with cyanide and 2‐deoxy‐<jats:sc>d</jats:sc>‐glucose induced a glibenclamide‐sensitive K‐current. This current was not observed when conventional whole‐cell recording was used. Possible reasons for this difference are discussed.</jats:p></jats:list-item> <jats:list-item><jats:p>These K channels appear similar to ATP‐sensitive K channels but we refer to them as nucleotide diphosphate‐dependent K channels (K<jats:sub>NDP</jats:sub>) to emphasise what seems to be a primary role for nucleotide diphosphates in their regulation.</jats:p></jats:list-item> </jats:list></jats:p>