title: Differential release of [3H]acetylcholine from the rat phrenic nerve-hemidiaphragm preparation by electrical nerve stimulation and by high potassium.
abstract: Neuronal transmitter stores of the phrenic nerve were labelled under different conditions. Subsequently, transmitter release evoked by electrical nerve stimulation and by a high potassium-low sodium solution was studied. Incubation of the end-plate preparation with [3H]choline at rest led to the synthesis of [3H]acetylcholine which could not be released by electrical nerve stimulation but it was released by high potassium-low sodium solution, independent of the presence of extracellular calcium. When the end-plate preparation was labelled during stimulation at 1 Hz, prolonged periods of electrical nerve stimulation released 83% of the total releasable [3H]transmitter pool in a completely calcium-dependent manner. After exhaustion of the electrically releasable pool, high potassium-low sodium solution still caused a significant outflow. Without a preceding exhaustion of the [3H]acetylcholine pool, high potassium-low sodium solution released a similar amount in the absence of extracellular calcium or after pretreatment with the intracellular calcium chelating substance, Quin-2. When evoked transmitter release was studied at different temperatures (36, 26 and 16 degrees C) Q 10 values of 1.6 and 1.0 were found for the release caused by electrical nerve stimulation and high potassium-low sodium solution (calcium-independent effect), respectively. After labelling during a short interval (2 min) but at a high stimulation rate (50 Hz), only 72% of the releasable [3H]transmitter could be released by electrical nerve stimulation, whereas the outflow due to the calcium-independent effect of high potassium-low sodium solution increased from 17 (labelling during stimulation at 1 Hz) to 28%. It is suggested that the calcium-independent effect of high potassium-low sodium solution reflects the release of acetylcholine from the cytoplasmic compartment, as this outflow occurred after labelling at rest and increased when cytoplasmic synthesis was enhanced by a high loading stimulation. In contrast to high potassium-low sodium solution, propagated nerve activity cannot release acetylcholine synthesized at rest (presumed to be cytoplasmic), but only [3H]acetylcholine synthesized during quantal release (presumed to be vesicular). The absolute requirement of extracellular calcium for electrically stimulated release suggests an exocytotic release mechanism. The low Q 10 value of 1.6 does not fit into the concept of a carrier- or channel-operated release mechanism.(ABSTRACT TRUNCATED AT 400 WORDS)
mesh_terms: D000109:Acetylcholine / Q000378:metabolism*; D000818:Animals; D002118:Calcium / Q000502:physiology; D003593:Cytoplasm / Q000502:physiology; D004558:Electric Stimulation; D007473:Ion Channels / Q000502:physiology; D008297:Male; D009045:Motor Endplate / Q000502:physiology*; D009469:Neuromuscular Junction / Q000502:physiology*; D010791:Phrenic Nerve / Q000502:physiology*; D011189:Potassium Chloride / Q000494:pharmacology*; D051381:Rats; D011919:Rats, Inbred Strains; D009435:Synaptic Transmission / Q000187:drug effects; D013572:Synaptic Vesicles / Q000502:physiology
chemical_list: D007473:Ion Channels; D011189:Potassium Chloride; D000109:Acetylcholine; D002118:Calcium