Depolarization-induced release of adenosine 5'-triphosphate from isolated varicosities derived from the myenteric plexus of the guinea pig intestine

In order to study the release of ATP from a neuronal preparation thought to be 'purinergic', isolated varicosities were preppared from myenteric plexus by micing and homogenizing the longitudinal muscle of guinea pig ileum in 0.32 M sucrose. The presence of varicosities within the crude preparation (P₂) isolated by differential centrifugation, was confirmed by electron microscopy and by the presence of occluded lactate dehydrogenase as a cytoplasmic marker. Varicosities were purified further from the P₂ fraction on a discontinuous sucrose density gradient and characterized morphologically. Varicosities resembling cholinergic, 'purinergic', and adrenergic axonal terminals were identified. The release of ATP from both crude and purified varicosities was detected by monitoring the light produced when the released ATP reacted with firefly luciferin-luciferase which was present in the incubation medium. Elevated extracellular K⁺ or Rb⁺ caused the release of ATP, whereas elevated Na⁺ and Li⁺ did not. The release of ATP by either elevated K⁺ or by the depolarizing drug, veratridine, was Ca²⁺ dependent. This requirement of extracellular Ca²⁺ for veratridine-induced release of ATP from isolated myenteric varicosities contrasted strikingly with that for brain synaptosomes, where the release of ATP induced by veratridine was increased rather than decreased in a Ca²⁺-free medium. The characteristics of ATP release from myenteric varicosities are consistent with a possible neurotransmitter function for ATP in myenteric plexus, although it is also possible that ATP is co-released with some other neurotransmitter.