Siv F.E. Nilsson

Increased uveoscleral outflow as a possible mechanism of ocular hypotension caused by prostaglandin F2α-1-isopropylester in the cynomolgus monkey

The effects of topical application of a single dose of prostaglandin F2 alpha, administered as the isopropylester, on the intraocular pressure (IOP), aqueous humor flow (AHF), conventional, and uveoscleral outflow were studied in cynomolgus monkeys under pentobarbital anesthesia. 1 microgram PGF2 alpha decreased the IOP by 2.9 +/- 0.6 mmHg (3 hr after the application) as compared with the vehicle-treated control eye. The mean AHF during the whole experiment was slightly higher in the experimental than in the control eye, 1.34 +/- 0.11 microliters min-1 compared with 1.16 +/- 0.09 microliters min-1. The uveoscleral outflow was significantly increased in the PGF2 alpha-treated eye, 0.98 +/- 0.12 microliters min-1 compared with 0.61 +/- 0.10 microliters min-1 for the control eye. The conventional outflow was lower in the experimental eye throughout the experiment. Topical application of 10 micrograms pilocarpine at the time when the fall in IOP was expected prevented the drop in the IOP. Simultaneously the increase in the uveoscleral outflow was abolished. After systemic pretreatment with atropine, 1 mg (kg body weight)-1 i.v., there was no significant difference in IOP, AHF, conventional or uveoscleral outflow between the PGF2 alpha-treated, and the control eye. The results of the present investigation suggest that PGF2 alpha decreases the intraocular pressure by increasing the uveoscleral outflow. The mechanism behind the increase in the uveoscleral outflow remains to be established. Relaxation of the ciliary muscle as well as enlarged intramuscular spaces and loss of extracellular material may contribute to the effect.

Characteristics of uveal vasodilation produced by facial nerve stimulation in monkeys, cats and rabbits

The effect of electrical stimulation of the facial nerve on ocular blood flow and intraocular pressure (IOP) was studied in monkeys, cats and rabbits. Ocular blood flow was determined with radioactive microspheres or by direct measurement of uveal blood flow from a cannulated vortex vein in rabbits. Frequency-response relationships were determined in monkeys (intraocular pressure) and rabbits (uveal blood flow). Stimulation of the facial nerve produced a marked increase (greater than 100%) in choroidal blood flow in all three species. The effect in the anterior uvea appeared the same, but less pronounced. Retinal blood flow was not affected by the stimulation in any of the species. In cats, local blood flow in the optic nerve was significantly increased by the stimulation. As there was no significant change in mean arterial blood pressure, the increase in blood flow must have been due to decreased vascular resistance. The uveal vasodilation was resistant to muscarinic blockade in all three species, excluding acetylcholine as the principal peripheral transmitter. Stimulation of the facial nerve also caused a moderate increase in IOP (range 1-11 cmH2O), only investigated in monkeys. This increase in IOP seems to be secondary to the intraocular vasodilation. The maximal increase in intraocular pressure, in monkeys, and vasodilation, in rabbits, was obtained at 15-20 Hz. At these frequencies, the vasodilation was always abolished by ganglionic blockade. In rabbits, stimulation at high frequencies (greater than 40 Hz) sometimes produced uveal vasodilation even after ganglionic blockade. This vasodilation was always less pronounced than before the ganglionic blockade and could be abolished by muscarinic blockade. The increase in uveal blood flow, in rabbits, was not affected by administration of indomethacin, indicating that prostaglandins are not critically involved in the vasodilation produced by facial nerve stimulation. The vasodilatory nerve fibers in the facial nerve are likely to be involved in regulation of choroidal blood flow to control the environmental temperature for the retina. The present study establishes the existence of efferent vasodilatory nerve fibers of facial nerve origin to the uvea. The peripheral transmitter causing the vasodilation is suggested to be vasoactive intestinal polypeptide (VIP).

PACAP-27 and PACAP-38: vascular effects in the eye and some other tissues in the rabbit

The effects of pituitary adenylate cyclase activating polypeptide (PACAP) on regional blood flow in the eye and other tissues were investigated in albino rabbits. Direct determination of the flow from a cannulated vortex vein, in animals pretreated with a vasopressin receptor antagonist, showed that i.v. infusion of either PACAP-27 or PACAP-38 caused a dose-dependent (0.08-0.64 pmol/kg per min) decrease in the uveal vascular resistance. Regional blood flow was determined, with radioactive microspheres, during i.v. infusion of PACAP-27 or PACAP-38 (0.64 pmol/kg per min) in rabbits pretreated with hexamethonium and a vasopressin receptor antagonist. In these experiments, both PACAP-27 and PACAP-38 increased choroidal blood flow by about 50%, whereas there was no effect in the anterior uvea. Nor was there any major effect on blood flow in the anterior uvea after intracameral injection of PACAP-27 or PACAP-38 (3 pmol). The largest blood flow increases, caused by i.v. infusion of PACAP-27 or PACAP-38, were observed in the parotid gland, submandibular gland, eyelids and nictitating membrane. Local blood flow in the choroid plexus, pineal gland, posterior pituitary gland, stomach, kidney and adrenal gland was also significantly increased during the i.v. infusion of PACAP-27. The results of the present investigation indicate that PACAP-27 and PACAP-38 are about 100 times more potent than vasoactive intestinal polypeptide and peptide histidine isoleucine as vasodilators in the rabbit choroid and, possibly, also in many other tissues of the rabbit.

Effects of atrial natriuretic factor (ANF) on intraocular pressure and aqueous humor flow in the cynomolgus monkey

Atrial natriuretic factor (ANF: human sequence) was examined for its effects on basal and terbutaline-stimulated aqueous humor flow, intraocular pressure (IOP) and uveoscleral outflow in cynomolgus monkeys under pentobarbital anesthesia. A dilution method with radioactively labeled albumin was used for the determination of aqueous humor flow. ANF was given by i.v. infusion or intracamerally. Intracameral administration of terbutaline increased the aqueous humor flow significantly; 1.10 +/- 0.05 microliter min-1 in the control eye and 1.69 +/- 0.06 microliter min-1 in the treated eye. I.v. infusion of ANF, 97 fmol kg-1 min-1, increased the aqueous humor flow by about 44% from basal values in the control eye. There was a small but not statistically significant increase on the terbutaline-treated side. The IOP was not changed by ANF at this dose. An ANF dose of 97 pmol kg-1 min-1 increased the aqueous humor flow by 51% in the control eye and by 19% in the terbutaline-treated eye. A further rise of about 8% in aqueous humor flow was registered on the control side when the infused ANF-dose was doubled. Doubling the dose also resulted in a decrease of the IOP by 1.3 +/- 0.3 mmHg on the control side and 2.2 +/- 0.4 mmHg on the terbutaline-stimulated side. Intracameral administration of ANF (81-162 pmol ml-1 perfusion fluid) increased the aqueous humor flow transiently by approximately 50% with a maximum after about 2 hr. The uveoscleral outflow tended to increase and IOP tended to decrease in the ANF-treated eye compared with the control. However, these effects were not statistically significant. These results suggest that ANF may be involved in the control of aqueous humor formation.

Effects of vasoactive intestinal polypeptide (VIP) on intraocular pressure, facility of outflow and formation of aqueous humor in the monkey

Stimulation of the facial nerve causes a non-cholinergic vasodilation in the uvea and a rise in the intraocular pressure in rabbits, cats and monkeys. Vasoactive intestinal polypeptide (VIP) has been suggested as the neurotransmitter mediating these effects. In the present investigation, the effects of VIP on aqueous humor dynamics were studied in cynomolgus monkeys. After intracameral injection of 1 microgram VIP, the outflow facility was higher in the experimental eye than in the control; 0.42 +/- 0.46 compared with 0.33 +/- 0.03 microliter cm H20-1 min-1, difference 0.09 +/- 0.04 microliter cm H2O-1 min-1. Intravenous infusion of VIP, 160 ng min-1, increased aqueous humor flow from 1.12 +/- 0.07 to 1.65 +/- 0.09 microliter min-1. Almost the same effect, a 50% increase in aqueous humor flow, was found after intracameral administration of 90 micrograms VIP. This dose of VIP caused a significant increase in intraocular pressure (IOP) in the experimental eye. The maximal difference in IOP between the experimental eye and the control eye was 7.5 +/- 0.4 cm H2O. A lower dose of VIP, 30 micrograms intracamerally, increased aqueous humor flow by about 20%, but had no consistent effect on IOP. The effect of VIP on aqueous humor flow was not affected by pretreatment with indomethacin. The results suggest that most of the rise in IOP caused by intracameral VIP administration is due to a rise in the pressure in the veins into which the aqueous humor is drained. Enhanced formation of aqueous humor plays a smaller role. The effects of VIP on aqueous humor formation and outflow facility suggest that the facial nerve may be involved in nervous control of aqueous humor dynamics, as VIP is most probably released in the eye by stimulation of the facial nerve.

Non-adrenergic sympathetic vasoconstriction in the eye and some other facial tissues in the rabbit

The effects of unilateral sympathetic nerve stimulation (SNS) on regional blood flow in the rabbit were studied with radioactive microspheres. SNS at 10 or 4 Hz caused an approximately 60% reduction in choroidal blood flow, which was partly resistant to alpha-adrenoceptor blockade with phenoxybenzamine. The vasoconstriction evoked by SNS at 2 Hz was completely abolished by alpha-adrenoceptor blockade. A similar response was seen in the iris, ciliary body, masseter muscle and lacrimal gland. In the harderian gland, however, SNS (2 Hz) after alpha-adrenoceptor blockade caused a significant reduction in blood flow. In the salivary glands, combined beta- and alpha-adrenoceptor blockade with propranolol and phenoxybenzamine revealed a slight non-adrenergic vasoconstriction during SNS at 10 Hz; however, the blood flow was significantly increased during SNS at 4 and 2 Hz following alpha-adrenoceptor blockade. These results indicate that there is a frequency-dependent, non-adrenergic component in the sympathetic vasoconstriction of the eye and several facial tissues. In the salivary glands, beta-adrenoceptor-mediated vasodilatation tends to mask a non-adrenergic vasoconstriction.

Effects of timolol on terbutaline- and VIP-stimulated aqueous humor flow in the cynomolgus monkey

The effects of timolol on terbutaline- and VIP-stimulated aqueous humor flow were investigated in cynomolgus monkeys, with a labeled albumin dilution method. The maximal increase in aqueous humor flow caused by intracameral (100 micrograms/ml) or intravenous (0.4 micrograms/kg/min) administration of terbutaline was about 100%. The effect of intravenously infused terbutaline was completely abolished by intracameral administration of timolol, 0.1 mg/ml. The same dose of timolol also abolished the effect of intravenously infused VIP, 50 ng/kg/min. Intravenous administration of timolol, 0.2 mg/kg, had no effect on VIP-stimulated aqueous humor flow, when VIP (90 micrograms) was given intracamerally, but abolished completely the effect of intracameral terbutaline, 100 micrograms/ml. The results suggest that the effect of intravenously infused VIP on aqueous humor flow is secondary to activation of the sympathetic nervous system, while the effect of intracameral administration of VIP is a direct effect on the ciliary epithelium. The maximal aqueous humor flow achieved with terbutaline is comparable to that in conscious cynomolgus monkeys.

Adenylate Cyclase Activation by VIP and PACAP in the Retina and Choroid: Effects of Antagonistsa

Adenylate cyclase activation by VIP and PACAP in the retina and choroid: effects of antagonists

Suppression of VIP-and terbutaline stimulated aqueous humor flow by increased intraocular pressure in the cynomolgus monkey

The effect of increased intraocular pressure (IOP) on stimulated aqueous humor flow (AHF) was studied in cynomolgus monkeys. Two experimental series were performed, one with unilateral VIP-treatment (60 micrograms intracamerally) and one with unilateral terbutaline-treatment (10 micrograms.ml-1 perfusion fluid). The AHF was determined with a labelled albumin dilution method, and an artificial increase in IOP was produced by clamping the outlet of the perfusion system, thus causing a net inflow of perfusion fluid. The initial AHF was significantly higher in the VIP-treated eye than in the control eye - 1.568 +/- 0.095 as compared to 1.112 +/- 0.103 microliters.min-1 (P less than or equal to 0.01). The spontaneous IOP was 5.8 +/- 0.4 mmHg (P less than or equal to 0.001) higher in the VIP-treated eye. There was no difference in pseudofacility between the VIP-treated eye (0.063 +/- 0.016 microliter.min-1.mmHg-1) and the control eye (0.065 +/- 0.022 microliter.min-1.mmHg-1), but the total and true outflow facilities were higher in the VIP-treated eye. In the experiments with terbutaline, the initial AHF was 1.729 +/- 0.114 for the experimental eye and 1.262 +/- 0.104 microliters.min-1 for the control eye (P less than or equal to 0.01). The pseudofacility tended to be higher in the terbutaline-treated eye (0.072 +/- 0.026 microliters.min-1.mmHg-1) than in the control eye (0.048 +/- 0.012 microliters.min-1.mmHg-1), but the difference was not statistically significant. There was no difference in total and true outflow facility between the experimental and control eye. The results indicate that the pressure sensitivity of the AHF is independent of the initial level of the AHF. VIP increases true outflow facility, possibly via a direct effect on the trabecular meshwork. VIP also appears to rise the IOP due to an increase in episcleral venous pressure, which could be secondary to vasodilatation in the anterior segment.

Neuropeptide Y Causes Nonadrenergic Vasoconstriction in the Eye, Brain, and Some Other Tissues in Rabbits

The eye has a rich innervation of sympathetic nerve fibers from the superior cervical ganglion and like sympathetic nerve fibers to many other tissues, they are immunoreactive to neuropeptide Y (NPY). Although the density of the NPY-fibers differs between species, the distribution pattern seems to be about the same in most species. The iris dilatator muscle and the blood vessels of the uvea (choroid, iris and ciliary body) have a rich innervation, but there are NPY-fibers present also in the ciliary processes and outflow apparatus. ’ The innervation pattern suggests that NPY may have effects on the pupil size, ocular blood flow and aqueous humor dynamics. In the present study, the effects of NPY on regional blood flows in the eye and other tissues were studied in the rabbit. The experiments were made on albino rabbits under urethane anesthesia (7 ml/kg b.w. of a 25% solution). All animals were tracheotomized and artificially ventilated. In eight rabbits, weal blood flow (UBF) was determined by direct measurement of the flow from a cannulated vortex vein simultaneously with registration of the mean arterial blood pressure (MABP) and the intraocular pressure (IOP). The uveal vascular resistance (UVR) was calculated as (MABP-IOP)IUBF.’ Regional blood flows were measured with radioactively labelled microspheres3 ( 15 pm) in twenty-five rabbits. Two different experimental series were made; one with and one without a-adrenweptor blockade (phenoxybenzamine 50 mglkg b.w.). Blood flow determinations were made before and at 2 and 10 min after the start of i.v. infusion of NPY, 120 pmol/kg x min. The cervical sympathetic nerve was sectioned on one side in all experiments. Blood flows during the NPY-infusion were compared with blood flows under control conditions by Student t test. Mean values and SE are given. Intravenous infusion of increasing doses of NPY (7.5-120 pmollkg x min) caused a dose-dependent increase in the UVR. Near maximal effect (70% increase) was achieved with 120 pmolkg x min. In the microsphere experiments without a-adrenoceptor blockade, the NPY-infusion had no significant effect on the MABP, but cardiac output was significantly decreased at 10 min. After 2 min of NPY-infusion, local blood flows were markedly decreased in the kidneys, spleen and adrenal glands (FIG. 1A). There were also significant blood flow reductions (20-60%) in different parts of the gastro-intestinal tract. In these tissues there was no further decrease in blood flow at 10 min (FIG. 1A) and the effects of NPY were not affected by a-adrenoceptor blockade (FIG. IB). In the eye the effect of NPY was moderate at 2 min, but at 10 min there were marked blood flow reductions in all parts of the uvea. The blood flow reductions on intact and sectioned side respectively were for the choroid 55 2 5 and 56 +4% (p 50.001), ciliary body 53 k 5 and 52 2 6% (p ~ 0 . 0 0 1 ) and iris 33 2 7 and 28 2 8% (p 50.01). The salivary glands, tongue and masseter muscle responded in a similar way as the eye; moderate effect at 2