Relation of acetylcholine release to Ca2+ uptake and intraterminal Ca2+ concentration in guinea-pig cortex synaptosomes.
[14C]Acetylcholine (ACh) release and parallel alterations in 45Ca2+ uptake and intrasynaptosomal free CA2+ concentration ([Ca2+]i) were measured in guinea-pig brain cortex synaptosomes. Depolarization by high K+ concentrations caused a rapid transient increase in Ca2+ uptake, terminating within 60 s (rate constant = 0.060 s-1; t1/2 = 11.6 s). This resulted in a rapid increase (within 1 s) in [Ca2+1]i, which then fell to a maintained but still-elevated plateau level (t1/2 for the decline was 15 s). Peaks of [Ca2+]i showed a sigmoidal dependence on depolarization, contrasting with the simple linear dependence of plateau levels of [Ca2+]i. The K+-evoked ACh release also had two phases: a fast initial increase (t1/2 = 11.3 s), which terminated within 60 s, was followed by a slow additional increase during sustained depolarizations of up to 10 min. Depolarization by veratridine led to a slow gradual increase in Ca2+ uptake (t1/2 = 130 s) over a 10-min incubation period, whereas an elevated plateau level of [Ca2+]i was achieved within 2 min (without a rapid peak elevation). The Ca2+-dependent fraction of the veratridine-evoked ACh release correlated with the increase in [Ca2+]i rather than with Ca2+ uptake. Using two different methods of depolarization partially circumvented the time limitations imposed by a buffering Ca2+ indicator and we suggest that, in the main, ACh is released in bursts associated with [Ca2+]i transients.
D000109:Acetylcholine / Q000378:metabolism*; D000634:Aminoquinolines; D000818:Animals; D002118:Calcium / Q000378:metabolism*; D002540:Cerebral Cortex / Q000378:metabolism*; D004305:Dose-Response Relationship, Drug; D005456:Fluorescent Dyes; D006168:Guinea Pigs; D007700:Kinetics; D011188:Potassium / Q000494:pharmacology; D013050:Spectrometry, Fluorescence; D013574:Synaptosomes / Q000187:drug effects / Q000378:metabolism*; D014701:Veratridine / Q000494:pharmacology
D000634:Aminoquinolines; D005456:Fluorescent Dyes; D014701:Veratridine; D000109:Acetylcholine; C035739:Quin2; D011188:Potassium; D002118:Calcium