Effects of adenosinergic agents on the vascular resistance and on the optic nerve response in the perfused cat eye

The function of A1- and A2a-adenosine receptors in the control of vascular resistance and in the modulation of light-evoked neuronal activity was investigated in the isolated perfused cat eye. The A1 agonist CCPA, the A1 antagonist CPT, the A2a agonist CGS 21680 and the A2 antagonist DMPX were used. The agents were applied intra-arterially at concentrations in the low nanomolar to micromolar range during rod-selective photic stimulation. The flow rate of perfusate, reflecting vascular resistance and the light-evoked optic nerve response (ONR) were recorded. Our results show a vasodilating effect of both A1 and A2 agonists and a vasoconstricting effect of the respective antagonists. The dose-effect relationships are suggestive, however, of an A2a receptor-mediated mechanism. The amplitude of the ONR-ON component was decreased during application of both adenosine-agonists. Analysis of the dose-effect relationships and the blockade of the CCPA-induced decrease by CPT suggests that inhibition is mediated by A1 receptors. However, CGS 21680-mediated inhibition cannot be explained by unspecific binding at A1 receptors alone and suggests the involvement of inhibitory A2a receptors.