Presynaptic A1-purinoceptor-mediated inhibitory effects of adenosine and its stable analogues on the mouse hemidiaphragm preparation

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Abstract
1. The effect of adenosine or its stable analogues (2-chloroadenosine, CADO; 5\t'-N-ethylcarboxamidoadenosine, NECA; and N6-cyclopentyladenosine, CPA) on the release of [3H]-acetylcholine ([3H]-ACh), and on the development of force of contraction evoked by electrical stimulation of the nerve, were studied in the mouse phrenic nerve-hemidiaphragm preparation. Evidence was obtained that the release of ACh is subject to presynaptic modulation through presynaptic A1(P1)-purinoceptors. 2. Adenosine or its stable analogues (CADO, NECA, CPA) inhibited, in a concentration-dependent manner, both the release of ACh and the force of the indirectly elicited contraction of hemidiaphragm preparation, provided in the latter case that the margin of safety was reduced by (\s+)-tubocurarine or magnesium. The order of potency in reducing ACh release was CPA > NECA > CADO > adenosine with IC50 values of 0.08 \+- 0.01, 0.74 \+- 0.05, 9.05 \+- 0.20, and 410.2 \+- 42.5 \gmmol/l, respectively. The order of potency in reducing twitch tension was CPA > NECA > CADO > adenosine with IC50 values of 0.11 \+- 0.02, 0.48 \+- 0.03, 2.07 \+- 0.49, and 240.4 \+- 20.0 \gmol/l, respectively. 3. 8-Phenyltheophylline (8-PT) antagonized the inhibitory effects of the adenosine receptor agonists on ACh release and twitch tension. The finding that -log KB and pA2 values estimated for 8-PT on ACh release and twitch tension were similar when adenosine or its stable analogues were applied as agonists suggest (i) that the motor nerve terminals are equipped with inhibitory P1-purinoceptors and (ii) that twitch responses of the partially curarized phrenic nerve-hemidiaphragm preparation to electrical stimulation can be used for studying the presynaptic inhibitory effect of P1-purinoceptor agonists. The findings that 8-PT and 8-cyclopentyl-1,3dipropylxanthine (DPCPX), a selective A1-purinoceptor antagonist, enhanced the release of [3H]-ACh in response to nerve stimulation and DPCPX antagonized the inhibitory effect of CADO on twitch tension (pA2 = 10.77 \+- 1.34) indicate that these receptors are of A1-subtype. 4. Dipyridamole, an adenosine uptake blocker, reduced the release of [3H]-ACh, and reinforced the effect of adenosine, but did not affect responses to adenosine analogues not taken up by the tissue. 5. It is concluded that the cholinergic axon terminals located at the neuromuscular junction are equipped with inhibitory A1-purinoceptors. Endogenous adenosine derived either from ATP or directly from the muscle or nerve terminals might thus activate these receptors to inhibit ACh release and thereby presynaptically influences chemical neurotransmission at the neuromuscular junction.