Rabbit Iris Dilator Muscle Research Articles

Pre- and postsynaptic effects of brimonidine on isolated rabbit iris dilator muscles.

PurposeBrimonidine is an imidazoline compound used for the treatment of glaucoma, but having very little effect on pupil diameter. Like para-aminoclonidine, most imidazoline compounds interact with postsynaptic α-adrenoceptors and cause pupil dilatation. Therefore, as part of an investigation of the mechanism of action of brimonidine on pupil diameter, the present study was initiated to measure, in vitro, the relative potency of brimonidine on the pre- and postsynaptic α-adrenoceptors of rabbit iris dilator muscle.MethodsThe contractile activity of brimonidine and its effect on twitch contraction evoked by electrical field stimulation were studied in isolated rabbit iris dilator muscles by isometric tension recording.ResultsBrimonidine significantly inhibited the twitch contraction of the dilator muscle caused by field stimulation, without affecting the response to exogenously applied phenylephrine. Compared to phenylephrine, brimonidine caused only a small contractile response with % maximum contraction values of <10%.ConclusionThese results suggest that brimonidine may act on nerve endings to inhibit adrenergic neurotransmission with very little effect on postsynaptic α-adrenoceptors. This may indicate that brimonidine reduced the pupil diameter just a little, thus improving night vision.

Comparative effect of alfuzosin and tamsulosin on the contractile response of isolated rabbit prostatic and iris dilator smooth muscles: Possible model for intraoperative floppy-iris syndrome

To compare the pharmacologic properties of tamsulosin and alfuzosin in isolated prostatic and iris dilator smooth muscle from pigmented rabbits. UROsphere Laboratories, Université Paul Sabatier, Toulouse, France. Prostatic and iris dilator smooth muscle strips were placed in organ baths. A concentration-response curve to phenylephrine was compared before and after incubation with tamsulosin or alfuzosin. Both drugs were approximately 30 times less potent in iris dilator than prostatic smooth muscle. In the iris, tamsulosin acted as a competitive antagonist starting at the 0.03 microM concentration (pA(2)=7.96). This is in the same range as the maximum plasma concentration after a 0.4 mg dose of tamsulosin in humans (0.025 microM). The antagonistic effect of alfuzosin in the iris was weaker (calculated mean pA(2) value of 5.63+/-0.19). Concentrations with an equipotent antagonistic effect on rabbit iris dilator muscle (3.0 and 10.0 microM) were approximately 100 to 300 times higher than the maximum plasma concentrations after a 10.0 mg dose of alfuzosin in humans (0.032 microM). Tamsulosin was more effective than alfuzosin at blocking adrenergic contraction of the iris dilator muscle in pigmented rabbits. Both drugs were less potent in the iris than in the prostate, which suggests that an additional iris receptor could be involved. If valid in humans, our results suggest that attainable plasma concentrations of tamsulosin are able to antagonize iris dilator smooth muscle contraction, whereas those of alfuzosin are not. This could explain the higher frequency of intraoperative floppy-iris syndrome in patients treated with tamsulosin than with alfuzosin.

Effect of somatostatin and galanin on isolated rabbit iris sphincter and dilator muscles

The neuropeptides somatostatin and galanin are present in the iris and may modulate pupil diameter. We examined the effects of somatostatin and galanin on isolated rabbit iris dilator and sphincter smooth muscles that were mounted in an organ bath. An isometric transducer recorded changes in tension in response to electric field stimulation (100 Hz, 0.3 m sec in duration, 10 V in strength) delivered by a pair of platinum plate electrodes. The dilator muscle response to field stimulation was not changed by either peptide, even at the highest concentrations examined. The sphincter muscle response consisted of two components: a fast component mediated by acetylcholine and slow component mediated by substance P. Both somatostatin and galanin attenuated the cholinergic component in a dose-dependent manner (from 0.3 n m to 0.1 μ m) but had no effect on responses mediated by substance P. Galanin was more effective (attenuation of 43% at 0.1 μ m) compared with somatostatin (attenuation of 16% at 0.1 μ m) in reducing the cholinergic response. Neither peptide affected the contraction induced by acetylcholine (1 m m). Therefore both peptides inhibited cholinergic transmission in the sphincter muscle, although the degree of inhibition by each was different. We conclude that somatostatin and/or galanin may induce mydriasis by attenuating cholinergic neurotransmitter release.

Presynaptic Effects of Botulinum Toxin Type A on the Neuronally Evoked Response of Albino and Pigmented Rabbit Iris Sphincter and Dilator Muscles

Presynaptic Effects of Botulinum Toxin Type A on the Neuronally Evoked Response of Albino and Pigmented Rabbit Iris Sphincter and Dilator Muscles

Presynaptic Effects of Botulinum Toxin Type A on the Neuronally Evoked Response of Albino and Pigmented Rabbit Iris Sphincter and Dilator Muscles

Purpose: To investigate the effects of botulinum toxin type A (botulinum A toxin) on the autonomic and other nonadrenergic, noncholinergic nerve terminals. Methods: The effects of botulinum A toxin on twitch contractions evoked by electrical field stimulation (EFS) were studied in isolated albino and pigmented rabbit iris sphincter and dilator muscles using the isometric tension recording method. Results: Botulinum A toxin inhibited the fast cholinergic and slow substance P-ergic component of the contraction evoked by EFS in the rabbit iris sphincter muscle without affecting the response to carbachol and substance P. These inhibitory effects were more marked in the albino rabbit than in the pigmented rabbit. Botulinum A toxin (150 nmol/L) did not affect the twitch contraction evoked by EFS in the rabbit iris dilator muscle. Conclusions: These data indicated that botulinum A toxin may inhibit not only the acetylcholine release in the cholinergic nerve terminals, but also substance P release from the trigeminal nerve terminals of the rabbit iris sphincter muscle. However, the neurotoxin has little effect on the adrenergic nerve terminals of the rabbit iris dilator muscle. Furthermore, the botulinum A toxin binding to the pigment melanin appears to influence the response quantitatively in the two types of irides.

Calcitonin gene-related peptide relaxes rabbit iris dilator smooth muscle via cyclic AMP-dependent mechanisms: cross-talk between the sensory and sympathetic nervous systems

PURPOSE. The purpose of these studies is to determine whether or not cyclic AMP is involved in the relaxant action of calcitonin gene-related peptide (CGRP) in rabbit iris dilator muscle. METHODS. Iris dilator muscle isolated from rabbit was used. Accumulation of cAMP and cGMP in the tissue extracts was measured by radioimmunoassay (RIA), IP 3 production was measured by ion-exchange chromatography, and changes in tension were recorded isometrically. RESULTS. CGRP, vasoactive intestinal peptide, prostaglandin E2, isoproterenol and forskolin (1 µM of each) increased cAMP accumulation by 136, 256, 78, 141 and 315%, respectively. CGRP dose-dependently increased cAMP accumulation (EC 50 = 5.25 nM), inhibited IP 3 production (EC 50 = 5.4 nM) and induced relaxation (EC 50 = 10 nM) in muscle precontracted with norepinephrine (NE) (10 µM). Prostaglandin E 2, isoproterenol and forskolin also induced relaxation. CGRP stimulated cAMP formation either in the presence or absence of 3-isobutyl-1-methylxanthine (IBMX), a cAMP phosphodiesterase inhibitor, in a time – and concentration-dependent manner. The neuropeptide had no effect on cGMP accumulation. CGRP(8-37), a CGRP receptor antagonist, reversed the relaxant action of the neuropeptide and inhibited CGRP-induced cAMP accumulation in a concentration-dependent manner (IC 50 = 12.5 nM). 2',5'-dideoxyadenosine (DDA), a specific adenylate cyclase inhibitor, significantly reduced the inhibitory actions of CGRP on NE-induced contraction and IP 3 production and inhibited CGRP-induced cAMP accumulation in a concentration-dependent manner (IC 50 = 6.9 nM). CONCLUSIONS. These results strongly suggest that cAMP mediates the relaxant action of CGRP in rabbit iris dilator. The mechanism of cAMP inhibition of NE-induced IP 3 production and contraction is unclear. Modulation of a 1 -adrenergic function in the iris dilator by CGRP-induced cAMP formation is yet another example of cross-talk between the cAMP and IP 3 -Ca 2+ second messenger systems, it demonstrates a cross-talk between the sympathetic and sensory nervous systems. CGRP-containing sensory nerve fibers could play an important role in regulation of smooth muscle function in the iris-ciliary body.

Inhibitory effect of phorbol 12,13-dibutyrate on norepinephrine-induced contraction in rabbit iris dilator muscle

The hypothesis that the increase in Ca 2+ sensitivity on norepinephrine-induced contraction of smooth muscles and also the decrease of the norepinephrine-induced sustained level of intracellular Ca 2+ concentration are produced by the activation of protein kinase C was tested. Phorbol 12,13-dibutyrate (PDB; 10 −6 M) relaxed the norepinephrine-induced sustained contraction in a concentration-dependent manner. On pretreatment with PDB a transient contraction was produced by the application of norepinephrine, but the sustained contraction was significantly reduced. The sustained elevations of intracellular Ca 2+ concentration ([Ca 2+] i) and the contraction induced by norepinephrine in fura-2-loaded preparations were decreased by the application of PDB. These inhibitory effects were antagonized by potent protein kinase inhibitors, 2-(1-(3-dimethylaminopropyl)-indol-3-yl)-3-(-indol-3-yl)-maleimide (GF 109203X) (10 −6 M) and 1-(5-isoquinolinesulfonyl)-2-methylpiperazine dihydrochloride (H-7) (10 −6 M), but were not affected by a protein kinase A/G inhibitor, N-(2-cinnamylaminoethyl)-5-isoquinolinesulfonamide (H-88) (10 −6 M). The slope of the regression line for norepinephrine for [Ca 2+] i and tension was significantly steeper than those obtained with high K +. Also, on pretreatment with PDB the Ca 2+ sensitivity of the K +-induced contraction was decreased, but the Ca 2+ sensitivity of norepinephrine-induced contraction tended to be increased. These observations indicate that PDB induces a decrease of [Ca 2+] i on Ca 2+ mobility and an increase of Ca 2+ sensitivity on contraction of smooth muscle through the activation of protein kinase C.

Calcitonin gene-related peptide induced relaxation of the rabbit iris dilator muscle

Calcitonin gene-related peptide (CGRP) and Substance P (SP) -immunoreactive nerves have been found in the anterior uvea of various mammalian species. Although SP is known to play a major role in control of pupil motility in rabbits, little is known about the effect of CGRP on the iris smooth muscles. We isolated iris sphincter and dilator muscles from rabbit eyes and investigated the mechanical responses and intracellular cyclic AMP (cAMP) levels in these muscles. CGRP (up to 0.1 microM) had no effect on either the resting muscle tone or the amplitude of contraction evoked by field stimulation of the sphincter. On the other hand, CGRP (0.1 microM) relaxed dilator muscle which had been pre-contracted by phenylephrine and reduced the amplitude of contraction evoked by field stimulation. These responses were antagonized by CGRP (8-37), a CGRP antagonist. The phosphodiesterase inhibitor, 3-isobutyl-1-methylxanthine (IBMX), dose-dependently inhibited the contraction evoked by field stimulation. However, 3 microM IBMX had no effect on CGRP inhibition of twitch contraction in this preparation. CGRP had little effect on cAMP production in dilator muscle either with or without IBMX. In conclusion, the miosis which occurs during an ocular inflammatory response, when both CGRP and SP are thought to be released from terminals of sensory neurons, results from CGRP relaxation of the dilator and from the strong contractile effect of SP on the sphincter. Adenylate cyclase activation does not seem to be involved in the relaxant effect of CGRP on the dilator.

Contractile responses and calcium movements induced by α1-adrenoceptor stimulant, norepinephrine, in rabbit iris dilator muscle

1. 1. The mechanisms involved in contraction of rabbit iris dilator muscle induced by norepinephrine (NE) were studied. 2. 2. The concentration-response curve of NE was not influenced by Ca 2+ blockers in the normal physiological saline solution (PSS) and removal of Ca 2+ from PSS. 3. 3. In 0.01 mM EGTA containing Ca 2+-free PSS, the NE-induced contraction was phasic, which was suppressed by TMB-8, cyclopiazonic acid, ionomycin and A23187 but still partly remained. 4. 4. In 2 mM EGTA containing Ca 2+-free PSS, NE increased the intracellular Ca 2+ ([Ca 2+] i) and muscle tension. Ryanodine abolished the increase in [Ca 2+] i induced by NE but slightly inhibited the tension. 5. 5. These results suggest that the NE-induced contraction of rabbit iris dilator in normal PSS is mainly due to the increase in the release of intracellularly sequestered Ca 2+ and partly due to the Ca 2+-independent processes.

P-517 - α1-adrenoceptor induces contractile responses and calcium movements in rabbit iris dilator muscle.

P-517 - α1-adrenoceptor induces contractile responses and calcium movements in rabbit iris dilator muscle.

P-606 - Inhibitory effect of endothelins on field stimulation-induced contraction of the rabbit iris dilator muscle.

P-606 - Inhibitory effect of endothelins on field stimulation-induced contraction of the rabbit iris dilator muscle.

Role of neuronal and extraneuronal uptake in responses of rabbit iris dilator muscle to levarterenol and phenylephrine.

The roles of neuronal and extraneuronal uptake mechanisms in the response of iris dilator muscles of rabbit to levarterenol (norepinephrine) and phenylephrine were investigated. Chemical denervation with 6-hydroxydopamine was used to eliminate neuronal uptake. Exposure to corticosterone prior to and during exposure to levarterenol or phenylephrine was used to assess the importance of extraneuronal uptake. Dose-response curves and ED50 values for levarterenol or phenylephrine in control and 6-hydroxydopamine-denervated tissues, both in the presence and absence of corticosterone, are shown. Curves illustrating the decay of tension on washout of levarterenol or phenylephrine from treated tissues were analyzed. 6-Hydroxydopamine denervation affected the response to levarterenol more than that to phenylephrine. Washout of both agonists was slower after 6-hydroxydopamine pretreatment, and washout of phenylephrine was more rapid when corticosterone was present. These data indicate that extraneuronal uptake in iris dilator muscles was more important in determining the response to phenylephrine than to levarterenol and that neuronal uptake was more important in determining tissue responsiveness to levarterenol than to phenylephrine.

Adrenoceptors and cholinoceptors in the rabbit iris

The adrenoceptors and cholinoceptors in the isolated rabbit iris dilator and sphincter muscles were studied. It was concluded that the dilator, as well as the sphincter, contains α- and β-adrenoceptors according to Ahlquist's classification. In the dilator, the α-receptor was the dominant receptor, and stimulation of these receptors caused a contraction. There were few β-receptors in this muscle and they were inhibitory in function. The sphincter contained both kinds of adrenoceptors and stimulation caused relaxation. Cholinoceptors of the atropine sensitive type, were numerous in the constrictor. In the isolated dilator preparation, acetylcholine produced a weak contraction that was reversed by atropine into a weak relaxation. Atropine itself produced a weak contraction of the muscle.