Adenosine A1 receptor-mediated inhibition of adenylate cyclase in rabbit retina

The purine adenosine has been postulated as playing a role in CNS neurotransmission or modulation. Evidence is now provided that inhibition of adenylate cyclase in rabbit retinal homogenates is mediated via adenosine A1 receptors. Nanomolar concentrations of the A1 receptor agonists, cyclohexyladenosine (CHA) and phenylisopropyladenosine (PIA), significantly inhibited the activity of forskolin-stimulated adenylate cyclase in preparations in which endogenous adenosine was destroyed by pretreatment with adenosine deaminase. With increasing concentrations of either agonist, biphasic effects on enzyme activity were observed. The effect of the mixed A1-A2 agonist, N-ethyl-carboxamidoadenosine (NECA), on forskolin-stimulated, as well as basal adenylate cyclase, was also investigated. At micromolar concentrations, NECA significantly increased the activity of adenylate cyclase. Isobutylmethylxanthine, a potent antagonist at extracellular adenosine receptors, blocked the effects observed with PIA, CHA, and NECA. The uptake of both 3H-CHA and 3H-adenosine into retinal cells was demonstrated autoradiographically. Both agonists labeled ganglion cell bodies and certain cell bodies located in the proximal portion of the inner nuclear layer.