Mechanisms underlying the neurogenic relaxation of isolated porcine sphincter pupillae

Ghrelin in ocular pathophysiology: from the anterior to the posterior segment

Ghrelin is a 28 amino acid acylated peptide produced in several organs that binds the growth hormone secretagogues receptor type 1a (GHSR-1a). It acts over a wide range of systems, e.g. the endocrine, cardiovascular, musculoskeletal and immune systems and the eye. The aim of this work is to review the physiologic and pathologic implications of the ghrelin-GHSR-1a in the eye. A systematic revision of studies published between 2000 and 2013 in English, Spanish or Portuguese in MEDLINE, EMBASE and Scopus was performed. Search words used included: ghrelin, GHSR-1a, ocular production, iris muscular kinetics, ciliary body, glaucoma, retinopathy and uvea. The production of ghrelin by the ocular tissue has been detected both in the anterior and posterior segments, as well as the presence of GHSR-1a. This peptide promotes the relaxation of the iris sphincter and dilator muscles, being this effect independent from GHSR-1a and dependent on prostaglandins release in the first case and dependent on GHSR-1a in the second. Regarding ocular pathology, ghrelin levels in the aqueous humor appear to be decreased in individuals with glaucoma. Moreover, ghrelin has been shown to decrease the intraocular pressure in animal models of ocular hypertension through GHSR-1a. In the posterior segment, the ghrelin-GHSR-1a system interferes with the development of oxygen-induced retinopathy, being protective in the vaso-obliterative phase and deleterious in the vaso-proliferative stage of the disease. Thus, the ghrelin-GHSR-1a system presents as a possible local regulatory mechanism in the eye, with pathophysiological implications, constituting a target for future clinical and therapeutic research and interventions.

Stimulation of adrenergic β-receptors enhances mydriasis in a porcine eye model

PURPOSE

To compare the mydriatic effect of intracamerally injected isoprenaline plus phenylephrine to phenylephrine alone and to epinephrine in a porcine eye model, aiming to eventually find the best combination of adrenergic substances for surgical mydriasis in humans.

METHODS

In this study, we used 89 intact eyes from newly slaughtered pigs, pretreated with 2.0 mg of intracameral acetylcholine. After waiting 60 seconds for miosis to develop, 0.15 ml 0.3% isoprenaline and 0.15 ml 3.0% phenylephrine were injected sequentially with a 90-second interval in 21 eyes. In another 22 eyes, the same substances were given in the reverse order. In 20 eyes, 0.15 ml of 0.025% epinephrine was injected, and as a negative control 0.15 ml of balanced salt solution was injected in 26 eyes. The pupils were filmed during the treatments, and the mean pupil diameters were measured every 15 seconds from the video recordings.

RESULTS

Phenylephrine injected after isoprenaline had a larger mydriatic effect than epinephrine (p < 0.01). Without isoprenaline pretreatment, the mydriatic effect of phenylephrine was significantly smaller than that of epinephrine (p < 0.05). Isoprenaline also exhibited a small mydriatic effect of its own.

CONCLUSIONS

The β-receptor stimulator isoprenaline enhances the mydriatic effect of intracameral phenylephrine, indicating a role for the β-receptor in the mydriatic response. Mydriasis mediated by β-receptors may explain why nonspecific adrenergic stimulators such as epinine and epinephrine can have larger mydriatic effects than the specific α(1)-receptor stimulator phenylephrine.

Mydriatic response to different concentrations of intracameral phenylephrine in humans

PURPOSE

To assess the mydriatic response to concentrations of intracamerally injected phenylephrine from 0.15 mg/mL to 30.00 mg/mL (0.015% to 3.000%) in human eyes.

SETTING

Department of Clinical Science/Ophthalmology, Umeå University Hospital, Umeå, Sweden.

DESIGN

Comparative case series.

METHODS

This prospective randomized double-masked study comprised patients scheduled for phacoemulsification cataract surgery. At the beginning of the procedure, patients received an intracameral injection of 0.15 mL of phenylephrine 0.15, 0.5, 1.5, 5.0, 15.0, or 30.0 mg/mL. To assess the mydriatic response, the pupil size was registered over 60 seconds using digital video recording. Then, the surgery was performed in the standard manner.

RESULTS

The study evaluated 42 patients. The mydriatic response was almost identical at the 4 lower phenylephrine concentrations (0.15 to 5.00 mg/mL; 0.015% to 0.500%), with final pupil sizes of approximately 4.3 mm. The 2 higher concentrations gave significantly larger pupils as follows: mean 5.80 mm ± 0.79 (SD) for 15.0 mg/mL (1.5%) and 6.65 mm ± 0.57 for 30.0 mg/mL (3.0%).

CONCLUSIONS

Results show that phenylephrine injected intracamerally does not have a linear mydriatic dose-response relationship in humans. At very high concentrations, phenylephrine may bind to and stimulate receptors other than the α(1)-receptor, explaining this phenomenon.

Shaping the pupil’s response to light in the hooded rat

The contribution of iris muscle steady state and dynamic response characteristics to the shaping of the pupil response to light in the hooded rat were studied using electrical stimulation of the parasympathetic fibers in the III nerve. The waveforms of pupillary contractions to single or brief trains of electrical impulses applied to the III nerve were virtually identical to those elicited with short duration light flashes. Individual contractions could be resolved at stimulation rates of 2 Hz and below, and the size of the contractions increased with the decrease in frequency. The pupil responded to long trains of stimuli above 2 Hz with smooth tonic contractions. Steady state contraction amplitude was linearly related to log stimulation frequency. The mean time constant of pupil constriction to stimulus trains was 1.41 s (SD +/- 0.71 s) and the shortest mean latency was 292 ms (SD +/- 30 ms). The fastest mean latency of pupil constriction to the brightest light flash used was 295 ms. In contrast, the time constant of pupillary dilation was 7 s (SD +/- 1.4 s) and the shortest latency was 485 ms (SD +/- 74 ms). Therefore, the sluggish dynamic properties of the iris musculature are responsible for the asymmetries in pupil contraction, dilation, and latencies as well as low flicker fusion frequency and constriction amplitude characteristics of pupil responses to light.