Arthur H. Neufeld

Cyclic-AMP in the aqueous humor: The effects of adrenergic agents

Adrenergic agents, which reduced intraocular pressure when applied topically to the rabbit eye, increased the cyclic-AMP concentration in the aqueous humor. The order of potency for both reducing intraocular pressure and increasing cyclic-AMP was: epinephrine > norepinephrine > isoproterenol. Intravenous phenoxybenzamine partially blocked the decrease in intraocular pressure and the increase in cyclic-AMP. Intravenous propranolol was ineffective. Topically applied aminophylline, theophylline, and dibutyryl cyclic-AMP were also ineffective. n nThe time course for the epinephrine induced mydriasis was different from the time course for the decrease in intraocular pressure and the increase in cyclic-AMP. The pupil dilated rapidly and then slowly returned to baseline values over the next 4 hr. The intraocular pressure and cyclic-AMP responses peaked at approximately the same time (1·5 hr) and were sustained for at least 5 hr. n nSubstantial tissue levels of cyclic-AMP were found in the cornea, iris-ciliary body and retina-choroid. However, no differences in tissue level could be demonstrated in the control versus epinephrine-treated eye. Thus the site of production of the increased cyclic-AMP in the aqueous humor is not yet resolved. n nIntracameral injection of high concentrations of cyclic-AMP (estimated final concentration in anterior chamber: 4 × 10−4m) caused a marked decrease in intraocular pressure. Similar injections of 5′AMP into the other eye of the same animal were ineffective. n nThe experiments indicate that cyclic-AMP plays a central role in mediating the action of catecholamines on aqueous humor dynamics. The role of the increase in the aqueous humor and possible sites of production and action of the cyclic-AMP are discussed.

The site of action of prostaglandin E2 on the disruption of the blood-aqueous barrier in the rabbit eye

Abstract The protein content of the anterior chamber and posterior chamber was determined soon after trauma to the eye. The trauma was topical administration of prostaglandin E 2 or paracentesis of the anterior chamber in untreated rabbits or rabbits treated with aspirin. The results are consistent with the hypothesis that the site of action of prostaglandin E 2 is on the tight junctions of the non-pigmented epithelial cells of the ciliary body. The possible routes of flow of plasmoid aqueous humor into the anterior chamber after trauma is discussed.

Characterization of α- and β-adrenergic receptors in membranes prepared from the rabbit iris before and after development of supersensitivity

Abstract α-Adrenergic and β-adrenergic receptors were studied by measuring the binding of [ 3 H]dihydroergocryptine and [ 3 H]dihydroalprenolol, respectively, to membranes prepared from homo-genized rabbit irides. The binding of [ 3 H]dihydroergocryptine appears to be specific for α-adrenergic receptors as adrenergic agents displace this radioligand with the following order of potency: phentolamine > epinephrine ⩾ norepinephrine ⪢ isoproterenol = propranolol. The binding of [ 3 H]dihydroalprenolol appears to be specific for β-adrenergic receptors as adrenergic agents displace this radioligand with the following order of potency: propranolol ⪢ isoproterenol ⩾ epinephrine > norepinephrine ⪢ phentolamine. Several weeks after removal of the superior cervical ganglion, when all the adrenergic nerves to the tissue have degenerated, membranes prepared from denervated irides have an increased density of β-adrenergic receptors with no increase in the density of α-adrenergic receptors. A small decrease in the total number of α-adrenergic receptors probably occurs, which is due to the loss of pre-junctional receptors. The affinities of the receptors do not change. These findings suggest that unlike skeletal muscle, the supersensitivity that occurs in smooth muscle is not due to an increase in the population of receptors governing contraction. However, the change in population of β-adrenergic receptors is consistent with the hypothesis that, as in other tissues, the level of cyclic AMP modulates the density of the β-adrenergic receptor.

Cyclic nucleotides and mitosis in the rabbit cornea following superior cervical ganglionectomy

Abstract Twenty hours after removal of the superior cervical ganglion, norepinephrine, released from degenerating adrenergic nerve terminals, causes a decrease in mitotic activity of the corneal epithelium of the rabbit. This is mediated through cyclic AMP which is elevated between 20 and 24 hr after surgery in ipsilateral corneas. Intravenous propranolol, administered 19 hr after surgery, blocks the increase in cyclic AMP and the decrease in mitotic activity. These findings suggest that adrenergically stimulated increases in cyclic AMP in the corneal epithelium may regulate mitotic activity during wound healing.

Ion pumps in the cornea and their stimulation by epinephrine and cyclic-AMP.

Abstract The type and location of the ionic pumps in the cornea of the amphibian and the mammalian is described, as well as the action of epinephrine and derivatives that increase the cellular level of cyclic-AMP. The cornea of the frog transports chloride ions from aqueous to tear side by a pump located in the epithelium which can dehydrate partially opaque corneas. This mechanism is activated by epinephrine and cyclic-AMP and aminophylline. These substances increase the net flux of chloride measured with 36Cl and they do not have effect on the short circuit current in chloride free solutions. The cornea of the rabbit pumps normally sodium ions inwards by a mechanism located in the epithelium. However, at rest and especially during the action of epinephrine and Db cyclic-AMP a transport of chloride from aqueous to tear side was well described in the epithelium of the rabbit cornea. The implications of the presence of these pumps and a revision of the information leading to the characterization and localization of these pumps is included.

Regulation of adrenergic neuromuscular transmission in the rabbit iris.

Abstract The regulation of the release of norepinephrine from adrenergic nerve terminals has been studied in the isolated, rabbit iris. When field stimulated, irides, previously incubated in [ 3 H]norepinephrine, release tracer from a cocaine-sensitive pool. This calcium-dependent release can be observed as an overflow of [ 3 H]norepinephrine that is enhanced by phenoxybenzamine, an inhibitor of extraneuronal reuptake. Prostaglandins E 1 and E 2 decrease the overflow of norepinephrine; prostaglandin F 2α is not effective. Prostaglandins decrease overflow by affecting electrically stimulated, calcium-dependent release of norepinephrine and not by stimulating reuptake. During field stimulation of the rabbit iris, endogenous synthesis of prostaglandins occurs, as demonstrated with 5,8,11,14 eicosatetraynoic acid, an inhibitor of the synthesis of prostaglandins, which further increases the overflow of norepinephrine. In addition, a direct action of extraneuronal neurotransmitter decreases the release of norepinephrine and this is not blocked by phenoxybenzamine or phentolamine. Thus, in the rabbit iris, as in other adrenergically innervated tissues, regulation of the release mechanism for norepinephrine from stimulated nerves occurs by negative feedback through prostaglandin-dependent and prostaglandin-independent pathways.

Cyclic-AMP in the aqueous humor: the effects of repeated topical epinephrine administration and sympathetic denervation

Abstract Cyclic-AMP (c-AMP) may mediate the effects of topical epinephrine on intraocular pressure in the rabbit eye. Experiments which further correlate the decrease in intraocular pressure with the increase in c-AMP in the aqueous humor in response to adrenergic agonists are presented. Repeated, daily topical administration of epinephrine led to diminished responsiveness to this drug on the fifth day. The treated eye demonstrated a smaller decrease in intraocular pressure and a smaller increase in c-AMP in the aqueous humor on the fifth day when compared to the response of a previously untreated eye. Unilateral superior cervical ganglionectomy, after a 2–3-week recovery period, causes a denervation supersensitivity to alpha-adrenergic agonists. Topical norepinephrine and epinephrine caused a greater fall in intraocular pressure and greater increase in c-AMP in the aqueous humor of the denervated eye than of the control eye. The shift to the left of the dose-response curve for epinephrine indicates a greater affinity for agonists in the denervated eye. Intravitreal injections of a high concentration of epinephrine produced a greater maximal response of c-AMP in the aqueous humor of the denervated eye. This is consistent with an increased number of sites for c-AMP production following sympathetic denervation. Speculations on the location of the adenyl cyclase system are discussed.

Current Topics in Eye Research

The latest annual volume of a series entitled Current Topics in Eye Research appeared recently. Jose A. Zadunaisky and Hugh Davson are the editors of the series, and each volume contains four to six chapters about selected topics written by experts in the field. The chapters are reviews that primarily, but not exclusively, concentrate on the authors work and are aimed at the basic scientist in the same or a related field. This volume contains chapters about glutamate metabolism in the retina (Liane Reif-Lehrer, PhD), aqueous humor outflow (Paul Kaufman, MD), noninvasive corneal redox fluorophotometry (Barry Masters, PhD), and fluid and electrolyte transports across the corneal endothelium (Jorge Fischbarg, PhD, and Jong Jim Lim, PhD). The chapters are consistently informative, detailed, and well written. The chapter on glutamate metabolism in the retina is encyclopedic, with over 500 references. Dr Reif-Lehrer examines the molecule through many facets, including its metabolism and