Atriopeptin, sodium azide and cyclic GNP reduce secretion of aqueous humour and inhibit intracellular calcium release in bovine cultured ciliary epithelium

1 This study examined the involvement of cyclic GMP, protein kinase G and i ntracellular Ca2+ movements in the modulation of aqueous humour formation. 2 Using the bovine arterially-perfused eye preparation, drug effects on int raocular pressure and aqueous humour formation rate were measured by manome try and fluorescein dilution, respectively. Drug effects on intracellular [ Ca2+] were determined by fura-2 fluorescence ratio technique in non-transfo rmed, cultured ciliary epithelium. 3 Intra-arterial injection of atriopeptin (50 pmol) or sodium azide (10 nmo l) produced significant reduction in aqueous humour formation (>38%). This was blocked by selective inhibition (KT-5823) of protein kinase G, but not by selective inhibition (KT-5720) of protein kinase A. Reductions of intrao cular pressure produced by atriopeptin or azide were almost completely bloc ked by KT-5823. 4 ATP (100 mu M) caused rapid, transient increase in intracellular Ca2+ fol lowed by a slow decline and prolonged plateau. This response showed concent ration-dependent inhibition by atriopeptin, azide or 8-bromo cyclic GMP, an d this inhibition of the rapid (peak) Ca2+ increase was enhanced by zaprina st (100 mu M; phosphodiesterase inhibitor). KT-5823 blocked the suppression of the peak Ca2+ response but not suppression of the plateau. 5 Arterial perfusion of ATP (0.1-100 mu M) produced a concentration-depende nt decrease in aqueous humour formation. 6 Aqueous humour formation in the bovine eye can be manipulated through cyc lic GMP, operating via protein kinase G. Close parallels appear when Ca2+ m ovements are modified by similar manipulations of cyclic GMP, suggesting th at Ca2+ transients may play an important role in aqueous humour formation a nd that interplay occurs between cyclic GMP and Ca2+.