In vivo electrical stimulation of the sympathetic nerve of the eye increases inositol phosphate production and prostaglandin release in the rabbit iris muscle

α1-Adrenergic Receptor Antagonists and the Iris: New Mechanistic Insights into Floppy Iris Syndrome

Understanding the role of adrenergic receptors in iris biology has gained widespread interest due to the recently described intraoperative floppy iris syndrome sometimes encountered during cataract surgery. alpha(1)AR-mediated iris dilator smooth muscle contraction occurs via alpha(1a)ARs whereas alpha(1b)ARs mediate iris arteriolar contraction. Because alpha(1)AR antagonists are first-line therapy for benign prostatic hyperplasia and lower urinary tract symptoms, more elderly patients requiring cataract surgery now receive these drugs. After reviewing intraoperative floppy iris syndrome, strengths/weaknesses of supporting data, and reviewing iris biology, a case is made that rather than being drug specific (alpha(1)AR antagonists), intraoperative floppy iris syndrome may represent the "tip of the iceberg." Relaxed iris dilator muscle resistant to adrenergic agonists should be expected with clinical drugs shown to relax the iris dilator (e.g., antagonists at alpha(1)AR, endothelin-A, angiotensin receptors, nitric oxide donors such as nitrates), and/or diseases associated with endothelial dysregulation (e.g., congestive heart failure, diabetes, hypertension). Rather than a rare, unexpected, unpredictable syndrome due to one drug, a careful medical history should elucidate intraoperative floppy iris syndrome predisposition. Just as anticoagulants are discontinued prior to elective surgery, conservative management of elderly patients suggests discontinuation of drugs that relax iris dilator muscle, in consultation with the patient's primary physician, should be considered prior to cataract surgery.

In vivo studies on the effects of α1-adrenoceptor antagonists on pupil diameter and urethral tone in rabbits

Alpha1-adrenoceptors mediate contraction of iris dilator smooth muscle and hence pupil dilatation. We compared the ability of i.v. bolus injections of alfuzosin, doxazosin, naftopidil, prazosin, tamsulosin and terazosin to antagonise phenylephrine-induced mydriasis relative to their potency for inhibiting phenylephrine-induced elevations of intraurethral pressure (IUP) in rabbits. Moreover, we compared the ability of these drugs to induce miosis in conscious rabbits in the absence of phenylephrine. All antagonists inhibited the effects of phenylephrine on pupil size and IUP, and the ratio of the respective ED50 values was close to unity in all cases. The doses required to induce statistically significant miosis in the absence of phenylephrine were 30- to 100-fold higher than those inhibiting phenylephrine-induced mydriasis for all antagonists, except for naftopidil. Moreover, the miotic effects of all alpha1-adrenoceptor antagonists were fully reversible within 8 h. We conclude that alfuzosin, doxazosin, naftopidil, prazosin, tamsulosin and terazosin inhibit phenylephrine-induced mydriasis in the same dose range as they inhibit elevations in IUP. Higher doses of all antagonists are required to induce miosis in the absence of an exogenous agonist, and such miosis is always reversible within hours.

Prejunctional prostaglandin receptors in the human iris-ciliary body

Prostaglandins (PGs) of the E series have been shown to modulate sympathetic neurotransmitter release in a variety of peripheral tissues and organs, including the eye. In this study, we evaluated the inhibitory effects of a series of naturally-occurring and synthetic PGs on field stimulation-evoked release of 3H-norepinephrine (3H-NE) from isolated, superfused segments of human iris-ciliary body. Field-stimulated 3H-NE secretion was calcium-dependent, blocked by selective inhibitors of voltage-sensitive calcium and sodium channels, and originated from a desipramine-sensitive transmitter pool. Evoked 3H-NE release was inhibited in a concentration-dependent manner by PGE2 (EC50 = 45 nM) and several closely related compounds with the following rank order of potency: sulprostone greater than 16,16-dimethyl-PGE2 greater than PGE2 greater than 11-deoxy-PGE1. By contrast, PGF2 alpha was relatively inactive (EC50 greater than 10 microM) in this system. None of the above compounds significantly modified spontaneous 3H-NE efflux. PGE2-mediated inhibition was not antagonized by the selective prostanoid EP1-receptor antagonists AH 6809 (10 microM) or SC-19220 (30 microM), nor did these agents alone affect basal or field-stimulated 3H-NE release. The results suggest that human ocular sympathetic nerves possess inhibitory PG receptors which have the pharmacological properties of the EP3 subtype. These receptors may play a role in local feedback regulation of sympathetic transmission in the iris-ciliary body, and may contribute to symptoms of acute ocular inflammation, including vasodilation, miosis and hypotony.

Muscarinic-agonist and guanine nucleotide stimulation of myo-inositol trisphosphate formation in membranes isolated from bovine iris sphincter smooth muscle: effects of short-term cholinergic desensitization

The effect of short-term cholinergic desensitization on muscarinic acetylcholine receptor (mAChR)-mediated activation of phospholipase C was investigated in membranes isolated from the bovine iris sphincter smooth muscle. Membranes prepared from normal or desensitized muscles, prelabeled with either [3H]myo-inositol or 32P from [gamma-32P]ATP, were incubated with a hydrolysis-resistant analogue of GTP, GTP gamma S, or GTP gamma S plus carbachol (CCh), and the production of [3H]myo-inositol 1,4,5-trisphosphate (IP3) and the breakdown of polyphosphoinositides were assessed. In normal membranes, GTP (greater than or equal to 1 mM), GTP gamma S (greater than 10 microM) and GTP gamma S (1 microM) plus CCh (10 microM), but not GDP or GDP beta S, increased phosphatidylinositol 4,5-bisphosphate (PIP2) hydrolysis and IP3 production. GTP gamma S increased IP3 accumulation in a time- and dose-dependent manner, and CCh, which had no effect on phospholipase C activity in the absence of GTP gamma S, potentiated the effects of GTP gamma S. The effect of CCh plus GTP gamma S on IP3 production was inhibited by atropine, had an absolute requirement for nM amounts of Ca2+ and was not affected by pertussis toxin. At higher concentrations (greater than 1 microM), Ca2+ alone induced PIP2 hydrolysis. Short-term exposure (less than 60 min) of the muscle to CCh (100 microM) did not affect the total number (Bmax) of mAChRs nor their affinity (KD) for [3H]-N-methylscopolamine. Desensitization did, however, result in: (1) a loss of the CCh-high affinity binding state of the sphincter mAChRs in a manner analogous to that produced by GTP gamma S; (2) a loss of the ability of GTP gamma S to affect CCh binding to the receptors; and (3) an attenuation of the GTP gamma S plus CCh-stimulated PIP2 hydrolysis. In conclusion, the data presented suggest that, in the iris smooth muscle, G-proteins are involved in the coupling of mAChRs to phospholipase C and that short-term cholinergic desensitization results in (1) the uncoupling of the receptor-G-protein complex and (2) the attenuation of mAChR-activation of phospholipase C.

Calcium-mobilizing receptors, polyphosphoinositides, generation of second messengers and contraction in the mammalian iris smooth muscle: historical perspectives and current status

It is well established now that activation of Ca2+ -mobilizing receptors results in the phosphodiesteratic breakdown of phosphatidylinositol 4,5-bisphosphate (PIP2), instead of phosphatidylinositol (PI), into myoinositol 1,4,5-trisphosphate (IP3) and 1,2-diacylglycerol (DG). There is also accumulating experimental evidence which indicates that IP3 and DG may function as second messengers, the former to mobilize Ca2+ from intracellular sites and the latter to activate protein kinase C (PKC). In this review, I have recounted our early studies, which began in 1975 with the original observation that activation of muscarinic cholinergic and adrenergic receptors in the rabbit iris smooth muscle leads to the breakdown of PIP2, instead of PI, and culminated in 1979 in the discovery that the stimulated hydrolysis of PIP2 results in the release of IP3 and DG and that this PIP2 breakdown is involved in the mechanism of smooth muscle contraction. In addition, I have summarized more recent work on the effects of carbachol, norepinephrine, substance P, the platelet-activating factor, prostaglandins, and isoproterenol on PIP2 hydrolysis, IP3 accumulation, DG formation, myosin light chain (MLC) phosphorylation, cyclic AMP production, arachidonic acid release (AA) and muscle contraction in the iris sphincter muscle. These studies suggest: (a) that the IP3-Ca2+ signalling system, through the Ca2+ -dependent MLC phosphorylation pathway, is probably the primary determinant of the phasic component of the contractile response; (b) that the DG-PKC pathway may not be directly involved in the tonic component of muscle contraction, but may play a role in the regulation of IP3 generation; (c) that there are biochemical and functional interactions between the IP3-Ca2+ and the cAMP second messenger systems, cAMP may act as regulator of muscle responses to agonists that exert their action through the IP3-Ca2+ system; and (d) that enhanced PIP2 turnover is involved in desensitization and sensitization of alpha 1-adrenergic- and muscarinic cholinergic-mediated contractions of the dilator and sphincter muscles of the iris, respectively. The contractile response is a typical Ca2+ -dependent process, which makes smooth muscle an ideal tissue to investigate the second messenger functions of IP3 and DG and their interactions with the cAMP system.