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PURPOSE Apolipoprotein A1 (ApoA1) has been shown to inhibit myopia development in chicks, but the underlying biological mechanism remains unknown. Because ApoA1 interacts with cyclic adenosine monophosphate (cAMP) in many cellular systems, we examined whether this interaction is important in myopia development. METHODS The nonmetabolizable cAMP analogue 8-Bromo-cAMP (8-Br-cAMP) was administered intravitreally to the right eyes of 8-day old chicks for 4 consecutive days. Control eyes received vehicle. Chicks in group 1 received 8-Br-cAMP (0.1 mM or 1 mM) and were fitted with -10 diopter (D) lenses on both eyes, whereas chicks in group 2 (0.1 mM 8-Br-cAMP) wore plano lenses over both eyes. The levels of retinal cAMP and ApoA1 were examined in another two groups of chicks wearing -10 D (group 3) and +10 D lenses (group 4) over their right eyes for 3 days, respectively (plano over left eyes). RESULTS The 8-Br-cAMP significantly inhibited development of lens-induced myopia (group 1: 0.1 mM versus vehicle: +1.71 ± 1.22 D versus -8.00 ± 2.19 D; 1 mM versus vehicle: +1.38 ± 1.34 D versus -9.96 ± 1.14 D, mean ± SEM, P < 0.01 for both); 1 mM, but not 0.1 mM 8-Br-cAMP increased expression of retinal ApoA1 levels in right eyes (P < 0.01). The 8-Br-cAMP had minimal effect on normal eye growth. Both retinal cAMP and ApoA1 levels were significantly increased only in hyperopic eyes (group 4). CONCLUSIONS The 8-Br-cAMP robustly inhibited development of lens-induced myopia. The increase in retinal ApoA1 observed in cAMP-treated and hyperopic eyes suggested a possible interplay between ApoA1 and cAMP in regulating eye growth.

Although glaucoma treatments alter aqueous humor (AH) dynamics to lower intraocular pressure, the regulatory mechanisms of AH circulation and their contributions to the pathogenesis of ocular hypertension and glaucoma remain unclear. We hypothesized that gadolinium-enhanced magnetic resonance imaging (Gd-MRI) can visualize and assess AH dynamics upon sustained intraocular pressure elevation and pharmacologic interventions.Gadolinium contrast agent was systemically administered to adult rats to mimic soluble AH components entering the anterior chamber (AC) via blood-aqueous barrier. Dynamic Gd-MRI was applied to examine the signal enhancement in AC and vitreous body upon microbead-induced ocular hypertension and unilateral topical applications of latanoprost, timolol maleate, and brimonidine tartrate to healthy eyes.Gadolinium signal time courses in microbead-induced hypertensive eyes possessed faster initial gadolinium uptake and higher peak signals in AC than control eyes, reflective of reduced gadolinium clearance upon microbead occlusion. Opposite trends were observed in latanoprost- and timolol-treated eyes, indicative of their respective drug actions on increased uveoscleral outflow and reduced AH production. The slowest initial gadolinium uptake but strongest peak signals were found in AC of both brimonidine-treated and untreated fellow eyes. These findings drew attention to the systemic effects of topical hypotensive drug treatment. Gadolinium leaked into the vitreous of microbead-induced hypertensive eyes and brimonidine-treated and untreated fellow eyes, suggestive of a compromise of aqueous-vitreous or blood-ocular barrier integrity.Gadolinium-enhanced MRI allows spatiotemporal and quantitative evaluation of altered AH dynamics and ocular tissue permeability for better understanding the physiological mechanisms of ocular hypertension and the efficacy of antiglaucoma drug treatments.

The mouse is a useful laboratory animal model for studying glaucoma and human aqueous humor dynamics, thereby facilitating evaluation of drugs1–3 and genetic manipulations.4,5 Lowering intraocular pressure (IOP) is the only clinical intervention documented to delay the onset and slow the rate of progression of irreversible blindness in glaucoma,6–8 so that accurate and precise measurement of IOP in the small mouse eye is central to studying glaucoma mechanisms in mice. IOP is fundamentally dependent on arterial delivery of solute and water to and venous drainage from the eye.9 When the perfusion pressure (mean arterial pressure minus IOP) falls below a critical level, the rate of aqueous humor formation becomes dependent on ciliary blood flow,10,11 so relating cardiac to aqueous dynamics is also highly pertinent. Several approaches have been applied to measure the IOP of the small mouse eye. The most conventional method is to estimate IOP manometrically by cannulating the eye with a microneedle 45 to 50 μm in diameter,12,13 but leakage around the relatively large-bore microneedle can occur12 and probably leads to underestimates and variability of the measured values.3 The servo-null micropipette system (SNMS) is an electrophysiologic approach that has been fully validated for measuring mouse IOP,3 but it is also invasive and requires expensive equipment and specialized training for performing the measurements. Applanation tonometry has also been adapted to the mouse,14 but the technique requires a specially designed prism and has been criticized because of the subjective endpoint of measurement.15 Recently, an induction-impact tonometer has been adapted.15 This device is sensitive to changes in IOP at low-normal ranges, but is relatively insensitive to changes in the high range and shows significant variance. As a result, the readout changes very little over the range of 25 to 35 mm Hg,15 limiting its usefulness in the study of glaucomatous mice. Most recently, a handheld tonometer (Tonopen; Mentor, Norwell, MA) has been applied to the mouse for measuring IOP and validated through comparisons with the SNMS and detection of the ocular hypotensive effect of brimonidine.16 However, as pointed out by the investigators, the time resolution is limited, precluding measurement of pulsatility or rapid responses of IOP to drugs. A further consideration is the substantial variance in single measurements of a single eye.16,17 In the current study, we assessed whether an adaptation of pneumotonometry might be useful in measuring IOP in mice.

PURPOSE To investigate the potential significance of cAMP in the regulation of Cl(-) transport across the bovine ciliary body/epithelium (CBE). METHODS Fresh native bovine CBE preparation was mounted in a modified Ussing chamber. The effects of cAMP-stimulating agents on short-circuit current (I(sc)) and net (36)Cl(-) secretion were determined. RESULTS Addition of cAMP-stimulating agents inhibited net Cl(-) secretion. Forskolin, when added bilaterally, reduced Cl(-) secretion by 60%. Similarly, bilateral isoproterenol or vasoactive intestinal peptide inhibited Cl(-) transport by 15% and 37%, respectively, suggesting a cAMP-sensitive Cl(-) transport across the ciliary epithelium. This notion was supported by the exogenous application of 8-bromo-cAMP (8-Br-cAMP) or 3-isobutyl-1-methylxanthine (IBMX), which reduced the net Cl(-) secretion by 49% and 85%, respectively. In unstimulated preparations, addition of 5-nitro-2-(3-phenylpropylamino)-benzoic acid (NPPB) to the blood side had no effects on I(sc) and net Cl(-) transport, indicating that Cl(-) reabsorption was negligible under baseline conditions. Also, pretreatment with NPPB from the blood side did not prevent forskolin-induced I(sc) inhibition, suggesting that the inhibition of Cl(-) transport did not result from the facilitation of Cl(-) reabsorption. However, pretreatment with heptanol from both sides completely blocked the forskolin-induced I(sc) inhibition, suggesting that cAMP may reduce Cl(-) transport by uncoupling the intercellular gap junctions. CONCLUSIONS The results suggest that cAMP plays a crucial role in modulating Cl(-) secretion across the ciliary epithelium. The effect is possibly mediated, at least in part, by the regulation of the permeability of gap junctions between pigmented and nonpigmented ciliary epithelial cells.