SUMMARY1. The process by which adenosine receptor agonists inhibit the evoked release of acetylcholine (ACh) was studied at motor nerve endings to frog skeletal muscle. Adenosine and 2-chloroadenosine were employed as agonists.2. Each agonist reduced the mean number of ACh packets released synchronously in response to a nerve impulse (im). Adenosine was from one to two orders of magnitude less potent than 2-chloroadenosine as an inhibitor of this release.3. Focally recorded nerve terminal action potentials were unaffected by either adenosine receptor agonist.4. In normal Ca (1P8 mm), addition of sufficient Mg to reduce m to less than half the control value did not alter the degree ofinhibition produced by adenosine receptor agonists.5. ACh release evoked by methods that do not require Ca entry through Ca channels (Ca-containing liposomes, La) was inhibited by either 2-chloroadenosine or adenosine.6. In Ca-free solutions containing Ba, the magnitude of neurally evoked asynchronous ACh release (miniature end-plate potential frequency = m.e.p.p.f) was depressed by either adenosine receptor agonist without change in the rate constant of decay of m.e.p.p.q; the m.e.p.p.f decay is thought to reflect the rate of clearance of Ba from regions of ACh release.7. Agents which displace Ca from storage sites and also inhibit phosphodiesterases increased m.e.p.p. in the virtual absence of extracellular Ca and increased the level of inhibition produced by adenosine receptor agonists.8. RMI 12,330A (7 x 10-6 to 7 x 10-5 M), an adenylate cyclase inhibitor, occluded the effects of adenosine receptor agonists on ACh release. 9. The results are consistent with the hypothesis that activation of extracellular adenosine receptors on adenylate cyclase inhibits evoked ACh release by reducing the affinity for Ca of an intracellular component of the secretary apparatus.

SUMMARY1. Conventional electrophysiological techniques were used to record from isolated rat phrenic nerve-hemidiaphragm preparations. After periods of rest (20 min) or nerve stimulation (7/sec for 20 min) the bathing medium of the preparation was removed and assayed for adenosine triphosphate (ATP) and adenosine diphosphate (ADP) using a sensitive modification of the firefly luciferase method (Silinsky, 1974 6. These results in conjunction with experiments on the hydrolysis of exogenous ATP suggest that ATP is released from the motor nerve ending and is subsequently degraded by enzymatic activity. It is also suggested that the released nucleotide may be derived from the cholinergic vesicle.

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SUMMARY1. The effects of adenosine (50 fIM) and 2-chloroadenosine (1-25 /M) were studied on Ca2+ currents in frog motor nerve endings.2. Ca2+ currents associated with the synchronous, neurally evoked release of acetylcholine (ACh) were measured using either perineural or patch recording methods. Tetraethylammonium and/or 3,4-diaminopyridine were employed to block K+ currents.3. Ca2+ currents were depressed by wo-conotoxin (1-5-2-5 gM), Cd21 (100 fM-2 mM), Co2+ (500 /M-5 mM) or by a reduction of the extracellular calcium concentration. Such currents were also observed when Sr2+ was substituted for Ca2+. Both ACh release and Ca2+ currents at motor nerve endings have been reported to be insensitive to 1,4-dihydropyridine antagonists in this species.4. Adenosine receptor agonists did not affect Ca2+ currents at concentrations that produced maximal inhibition of ACh release. 5. The effects of adenosine receptor agonists were examined on asynchronous K+-dependent ACh release under conditions in which the Ca2+ concentration gradient is likely to be reversed (Ca2+-free Ringer solution containing 1 mm EGTA). ACh release was measured by monitoring the frequency of occurrence of miniature endplate potentials (MEPPs). In Ca2+-free solutions containing 1 mm EGTA, high K+ depolarization caused a decrease in MEPP frequency, presumably because it elicits the efflux of Ca2+ from the nerve ending via membrane Ca2+channels in a reverse Ca2+

This review focuses on the effects of phorbol esters and the role of phorbol ester receptors in the secretion of neurotransmitter substances. We begin with a brief background on the historical use of phorbol esters as tools to decipher the role of the enzyme protein kinase C in signal transduction cascades. Next, we illustrate the structural differences between active and inactive phorbol esters and the mechanism by which the binding of phorbol to its recognition sites (C1 domains) on a particular protein acts to translocate that protein to the membrane. We then discuss the evidence that the most important nerve terminal receptor for phorbol esters (and their endogenous counterpart diacylglycerol) is likely to be Munc13. Indeed, Munc13 and its invertebrate homologues are the main players in priming the secretory apparatus for its critical function in the exocytosis process.