Regulation of Aqueous Humor Secretion by Melatonin in Porcine Ciliary Epithelium

Secretion of melatonin, a natural hormone whose receptors are present in the ciliary epithelium, displays diurnal variation in the aqueous humor (AH), potentially contributing to the regulation of intraocular pressure. This study aimed to determine the effects of melatonin on AH secretion in porcine ciliary epithelium. The addition of 100 µM melatonin to both sides of the epithelium significantly increased the short-circuit current (Isc) by ~40%. Stromal administration alone had no effect on the Isc, but aqueous application triggered a 40% increase in Isc, similar to that of bilateral application without additive effect. Pre-treatment with niflumic acid abolished melatonin-induced Isc stimulation. More importantly, melatonin stimulated the fluid secretion across the intact ciliary epithelium by ~80% and elicited a sustained increase (~50-60%) in gap junctional permeability between pigmented ciliary epithelial (PE) cells and non-pigmented ciliary epithelial (NPE) cells. The expression of MT3 receptor was found to be >10-fold higher than that of MT1 and MT2 in porcine ciliary epithelium. Aqueous pre-treatment with MT1/MT2 antagonist luzindole failed to inhibit the melatonin-induced Isc response, while MT3 antagonist prazosin pre-treatment abolished the Isc stimulation. We conclude that melatonin facilitates Cl- and fluid movement from PE to NPE cells, thereby stimulating AH secretion via NPE-cell MT3 receptors.

Regulation of gap junction coupling in bovine ciliary epithelium

Aqueous humor is formed by fluid transfer from the ciliary stroma sequentially across the pigmented ciliary epithelial (PE) cells, gap junctions, and nonpigmented ciliary epithelial (NPE) cells. Which connexins (Cx) contribute to PE-NPE gap junctional formation appears species specific. We tested whether small interfering RNA (siRNA) against Cx43 (siCx43) affects bovine PE-NPE communication and whether cAMP affects communication. Native bovine ciliary epithelial cells were studied by dual-cell patch clamping, Lucifer Yellow (LY) transfer, quantitative polymerase chain reaction with reverse transcription (qRT-PCR), and Western immunoblot. qRT-PCR revealed at least 100-fold greater expression for Cx43 than Cx40. siCx43 knocked down target mRNA expression by 55 +/- 7% after 24 h, compared with nontargeting control siRNA (NTC1) transfection. After 48 h, siCx43 reduced Cx43 protein expression and LY transfer. The ratio of fluorescence intensity (R(f)) in recipient to donor cell was 0.47 +/- 0.09 (n = 11) 10 min after whole cell patch formation in couplets transfected with NTC1. siCx43 decreased R(f) by approximately 60% to 0.20 +/- 0.07 (n = 13, P < 0.02). Dibutyryl-cAMP (500 microM) also reduced LY dye transfer by approximately 60%, reducing R(f) from 0.41 +/- 0.05 (n = 15) to 0.17 +/- 0.05 (n = 20) after 10 min. Junctional currents were lowered by approximately 50% (n = 6) after 10-min perfusion with 500 microM dibutyryl-cAMP (n = 6); thereafter, heptanol abolished the currents (n = 5). Preincubation with the PKA inhibitor H-89 (2 microM) prevented cAMP-triggered current reduction (n = 6). We conclude that 1) Cx43, but not Cx40, is a major functional component of bovine PE-NPE gap junctions; and 2) under certain conditions, cAMP may act through PKA to inhibit bovine PE-NPE gap junctional communication.