Autoreceptor‐mediated purinergic and cholinergic inhibition of motor nerve terminal calcium currents in the rat.

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Abstract
1. After blocking K+ currents with 10 mM‐tetraethylammonium (TEA) or TEA plus 250 microM‐3,4‐diaminopyridine (3,4‐DAP). motor nerve terminal Ca2+ currents were recorded using focal extracellular electrodes. Two transmitters released from the terminal. ATP and acetylcholine (ACh), were then applied, and the effects on the nerve terminal Ca2+ current were measured. 2. ATP (50 microM) reduced the Ca2+ current by 34%, but this action is prevented when hydrolysis to adenosine is blocked by alpha,beta‐methyladenosine 5'‐diphosphate (200 microM). Thus, inhibition by ATP presumably occurs subsequent to ATP hydrolysis to adenosine. 3. Adenosine (50 microM) inhibited the terminal Ca2+ current by 29%. This was mimicked by the adenosine analogue L‐phenylisopropyl adenosine (L‐PIA) and blocked by theophylline (100 microM), which antagonizes adenosine receptors at micromolar concentrations. 4. ACh (100 microM) or the anticholinesterase methane sulphonyl fluoride (MSF; 1 mM) also depressed the terminal Ca2+ current. This response was mimicked by muscarine (100 microM) and antagonized by atropine (100 microM) or pirenzipine (4 microM), which is generally specific for M1 receptors. 5. Addition of Ba2+, which blocks adenosine‐mediated K+ currents, had no effect on the inhibitory effects of either adenosine or ACh; similarly, neither adenosine nor ACh in the bath affected K+ current records obtained after blocking all inward currents with 10 mM‐Co2+ and focal application of tetrodotoxin. 6. Incubation of the muscle for 4 h in pertussis toxin (10(‐5) g ml‐1) eliminated both adenosine‐ and ACh‐induced inhibition of the terminal Ca2+ current. This result indicates the possible involvement of a G protein in the transduction of the feedback pathway. 7. Neither cyclic AMP analogues, the adenylate cyclase activator forskolin (10 microM), the phorbol ester phorbol 12‐myristate 13‐acetate (PMA; 3 microM) nor the diacylglycerol analogue 1,2‐oleoylacetylglycerol (OAG; 3 microM) had any effect on adenosine‐ or ACh‐induced depression of the terminal Ca2+ current. Therefore, pathways involving these particular second messengers are most probably not involved. 8. The effects of adenosine and ACh are non‐additive. 9. These results indicate that ATP and ACh, which are released during exocytosis, may inhibit their own release through attenuation of the terminal Ca2+ current via autoreceptors coupled to a G protein.