On the adenosine receptor and adenosine inactivation at the rat diaphragm neuromuscular junction

Abstract

1 The effects of adenosine and adenosine analogues 2-chloroadenosine (CADO), L-N6-phenylisopropyladenosine (L-PIA), D-N6-phenylisopropyladenosine (D-PIA), N6-cyclohexyladenosine (CHA) and 5'-N-ethylcarboxamide adenosine (NECA) on evoked endplate potentials (e.p.ps) and on twitch tension were investigated in innervated diaphragms of the rat. 2 Adenosine and its analogues decreased, in a concentration-dependent manner, the amplitude of both the e.p.ps and the twitch responses evoked by nerve stimulation. The order of potency in decreasing the twitch tension was CHA, L-PIA, NECA > D-PIA > CADO > adenosine. L-PIA was about 8 times more potent than D-PIA. Neither adenosine nor the adenosine analogues affected the twitch responses of directly stimulated tubocurarine-paralysed muscles. 3 8-Phenyltheophylline (8-PT), theophylline and isobutylmethylxanthine (IBMX), in concentrations virtually devoid of effect on neuromuscular transmission, antagonized the inhibitory effect of 2-chloroadenosine. The order of potency of the alkylxanthines as antagonists of the adenosine receptor at the rat diaphragm neuromuscular junction was 8-PT > IBMX > theophylline. The antagonism by these xanthines was shown to be competitive, the pA2 value for 8-PT being 7.16. In concentrations slightly higher than those used to test its ability to antagonize the adenosine receptor, IBMX and 8-PT increased the amplitude of e.p.ps without modifying their decay phase or the resting membrane potential of the muscle fibre. 4 The adenosine uptake inhibitor, nitrobenzylthioinosine (NBI) and the adenosine deaminase inhibitor, erythro-9(2-hydroxy-3-nonyl)adenine (EHNA), in concentrations virtually devoid of effect on neuromuscular transmission, potentiated the inhibitory effect of adenosine at the rat diaphragm neuromuscular junction. The potentiation factors were about 2.6 for NBI (51M), 2.2 for EHNA (25 pM) and 4.6 for the combination of NBI (5 FM) and EHNA (25 FM). 5 It is concluded that both uptake and deamination contribute to the inactivation of adenosine at the rat diaphragm neuromuscular junction and that in this preparation the inhibitory effect of adenosine on transmission is mediated by a xanthine-sensitive adenosine receptor with an agonist profile which does not fit the criteria for its classification either as an A, or A2-adenosine receptor.

Inhibitory and excitatory effects of adenosine receptor agonists on evoked transmitter release from phrenic nerve endings of the rat

“…This order of potency of the adenosine analogues is similar to that found for the ability of R-PIA, NECA and CADO to inhibit nerve-evoked twitches in phrenic nerve-diaphragm preparations where the safety margin of neuromuscular transmission was decreased by high magnesium concentrations in the bath (Sebastiao & Ribeiro, 1988). The concentrations of R-PIA used in the present work to decrease the evoked release of [3H]-acetylcholine were of the same order as those needed to decrease the amplitude of e.p.ps recorded from tubocurarine-paralyzed phrenic nervediaphragm preparations.…”

“…The inhibitory effect of adenosine at the rat diaphragm neuromuscular junction is mediated by a xanthine-sensitive adenosine receptor with an agonist profile with R-N6-phenylisopropyladenosine (R-PIA) and 5'-N-ethylcarboxamide adenosine (NECA) about equipotent and more potent than 2-chloroadenosine (CADO) (Sebastiao & Ribeiro, 1988). This agonist profile was determined in preparations where the margin of safety of neuromuscular transmission was reduced with tubocurarine or with high magnesium concentrations in the bath.…”

“…Adenosine inhibits neuromuscular transmission at the rat diaphragm neuromuscular junction, decreasing the amplitude and the quantal content of evoked endplate potentials (Ginsborg & Hirst, 1972;Ribeiro & Walker, 1975) and the amplitude of nerve-evoked twitch responses (Sebastiao & Ribeiro, 1988).…”

Neuromuscular transmission modulation by adenosine upon aging

“…It is also known that in the same motor nerve terminal, both inhibitory adenosine A 1 receptors and excitatory adenosine A 2A receptors coexist, modulating the evoked release of acetylcholine (Correiade-Sá et al, 1991). The way in which adenosine is able to achieve a balance in the control of neurotransmission depends on the extracellular concentration of the nucleoside, which subsequently depends on extracellular adenosine generation and inactivation (via cellular uptake and/or extracellular deamination) (Sebastião and Ribeiro, 1988). At the rat neuromuscular junction extracellular adenosine can be originated by transport-mediated release (Cunha and Sebastião, 1993), in parallel with its formation from released adenosine triphosphate (ATP) (Smith, 1991).…”

Characterization of adenosine receptors in brush‐border membranes from pig kidney

“…Distinction between the two subtypes of adenosine receptors is usually made on the basis of the rank orders of potency of agonists and antagonists. Recently, the existence of another adenosine receptor subtype (A3), not related to the adenylate cyclase system, has been described (Ribeiro & Sebastiao, 1985;1986;Sebastiao & Ribeiro, 1988). The A3 adenosine receptor subtype is related to calcium channels and has a different order of potency of agonists and antagonists from Al or A2 subtypes.…”