Intracellular Ca2+ during metabolic activation of KATP channels in spontaneously active dorsal vagal neurons in medullary slices.
Intracellular Ca2+ ([Ca2+]i) and membrane properties were measured in fura-2 dialysed dorsal vagal neurons (DVN) spontaneously active at a frequency of 0.5-5 Hz. [Ca2+]i increased by about 30 nm upon rising spike frequency by more than 200% due to 20-50 pA current pulses or 10 micrometer serotonin. It fell by 30 nm upon block of spiking by current-injection, tetrodotoxin or Ni2+ and also during hyperpolarization due to gamma-aminobutyric acid or opening of adenosine triphosphate (ATP) -sensitive K+ (KATP) channels with diazoxide. KATP channel-mediated hyperpolarizations during anoxia or cyanide produced an initial [Ca2+]i decrease which reversed into a secondary Ca2+ rise by less than 100 nm. Similar moderate rises of [Ca2+]i were observed during block of aerobic metabolism under voltage-clamp as well as in intact cells, loaded with fura-2 AM. The magnitude of the metabolism-related [Ca2+]i transients did not correlate with the amplitude of the KATP channel-mediated outward current. [Ca2+]i did not change during diazoxide-induced or spontaneous activation of KATP outward current observed in 10% of cells after establishing whole-cell recording. Increasing [Ca2+]i with cyclopiazonic acid did not activate KATP channels. [Ca2+]i was not affected upon block of outward current with sulphonylureas, but these KATP channel blockers were effective to reverse inhibition of spike discharge and, thus, the initial [Ca2+]i fall upon spontaneous or diazoxide-, anoxia- and cyanide-induced KATP channel activation. A sulphonylurea-sensitive hyperpolarization and [Ca2+]i fall was also revealed in the early phase of iodoacetate-induced metabolic arrest, whereas after about 20 min, occurrence of a progressive depolarization led to an irreversible rise of [Ca2+]i to more than 1 micrometer. The results indicate that KATP channel activity in DVN is not affected by physiological changes of intracellular Ca2+ and the lack of a major perturbance of Ca2+ homeostasis contributes to their high tolerance to anoxia.
II. Physiologisches Institut, Universität Göttingen, Humboldtallee 23, D-37073 Göttingen, Germany. kb@neuro-physiol.med.uni-goettingen.de
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D000200:Action Potentials / Q000502:physiology; D000818:Animals; D002118:Calcium / Q000378:metabolism*; D015220:Calcium Channels / Q000187:drug effects / Q000378:metabolism; D000252:Calcium-Transporting ATPases / Q000037:antagonists & inhibitors; D015687:Cell Hypoxia / Q000502:physiology; D003486:Cyanides / Q000494:pharmacology; D003981:Diazoxide / Q000494:pharmacology; D005260:Female; D006019:Glycolysis / Q000187:drug effects; D066298:In Vitro Techniques; D007211:Indoles / Q000494:pharmacology; D007424:Intracellular Fluid / Q000378:metabolism*; D008297:Male; D008526:Medulla Oblongata / Q000378:metabolism*; D009532:Nickel / Q000494:pharmacology; D018408:Patch-Clamp Techniques; D015221:Potassium Channels / Q000378:metabolism*; D051381:Rats; D012701:Serotonin / Q000494:pharmacology; D013779:Tetrodotoxin / Q000494:pharmacology; D014630:Vagus Nerve / Q000378:metabolism* / Q000502:physiology; D005680:gamma-Aminobutyric Acid / Q000494:pharmacology
D015220:Calcium Channels; D003486:Cyanides; D007211:Indoles; D015221:Potassium Channels; D012701:Serotonin; D013779:Tetrodotoxin; D005680:gamma-Aminobutyric Acid; D009532:Nickel; D000252:Calcium-Transporting ATPases; D003981:Diazoxide; D002118:Calcium; C000543:cyclopiazonic acid