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Terbutaline, Forskolin and cAMP Reduce Secretion of Aqueous Humor and Inhibit Intracellular Calcium Release in Bovine Cultured Ciliary Epithelium

In order to elucidate the involvement of cyclic AMP and intracellular Ca2+ concentration [Ca2+]i, in the modulation of aqueous humor (AH) secretion, we studied the effects of terbutaline, forskolin and 8‐Br‐cAMP on AH secretion in the isolated intact bovine eye and on ATP‐induced increase in [Ca2+]i in cultured ciliary epithelial (CE) cells. Drug effects on AH secretion were measured by fluorescein dilution. Drug effects on [Ca2+]i were studied by the Fura‐2 fluorescence ratiometric measurement. Terbutaline (100 nmol–100 μM), forskolin (30 nM–100 μM) or 8‐Br‐cAMP (100 nM – 10 μM), administered at fixed concentrations in the arterial perfusate of the intact eye, produced significant reductions in AH secretion. The AH reducing effect of terbutaline was blocked by a selective inhibitor of protein kinase A (KT‐5720). ATP (100 μM) caused a rapid, transient (peak) increase in [Ca2+]i followed by a small but sustained plateau phase lasting more than 5 minutes. Preincubation of the cells (6 min) with terbutaline, forskolin or 8‐Br‐cAMP significantly reduced the peak calcium response to ATP. Each of the drugs, on the other hand, augmented the sustained plateau phase of the calcium response. KT‐5720 partially reversed the inhibitory effect of terbutaline on the peak and totally inhibited its effect on the plateau phase. Taken together, our data support the notion that AH secretion in the bovine eye can be manipulated through cyclic AMP, operating through protein kinase A and that interaction of [Ca2+]i and cAMP may play a role in modulating AH secretion.