On the type of receptor involved in the inhibitory action of adenosine at the neuromuscular junction

The effects of adenosine and adenosine analogues (L-N6-phenylisopropyladenosine (L-PIA), D-N6-phenylisopropyladenosine (D-PIA), N6-cyclohexyladenosine (CHA), N6-methyladenosine, 5'-N-ethylcarboxamide adenosine (NECA) and 2-chloroadenosine) on evoked endplate potentials (e.p.ps) and on twitch tension were investigated in innervated sartorius muscles of the frog. Adenosine and its analogues decreased, in a concentration-dependent manner, the amplitude of both the e.p.ps and the twitch responses evoked by indirect stimulation. The order of potencies in decreasing twitch tension was: L-PIA, CHA, NECA greater than 2-chloroadenosine greater than D-PIA greater than N6-methyladenosine, adenosine. L-PIA was about ten fold more potent than D-PIA. None of the adenosine analogues tested affected the twitch responses of directly stimulated tubocurarine-paralyzed muscles. In concentrations that did not modify neuromuscular transmission, theophylline and 8-phenyltheophylline (8-PT) but not isobutylmethylxanthine (IBMX), antagonized the inhibitory action of 2-chloroadenosine at the neuromuscular junction. 8-PT behaved as a competitive antagonist and was about forty fold more potent than theophylline. It is concluded that the R-type adenosine receptor at the neuromuscular junction should not be classified in the A1/A2 system. The possibility of calcium-linked adenosine receptors having pharmacological profiles distinct from those originally defined as modulating adenylate cyclase is discussed.