Pharmaco-mechanical coupling in the response to acetylcholine and substance P in the smooth muscle of the rat iris sphincter

Molecular analysis of non-specific supersensitivity induced by AF64A in rat iris smooth muscle

Characteristics of supersensitivity induced by the pretreatment with AF64A, an inhibitor of choline uptake at parasympathetic nerve endings, were examined in rat iris sphincter. In preparations isolated and skinned by beta-escin after the micro injection of AF64A to eyes in vivo, the amplitude of maximum contraction in pCa 4.5 solution was increased by 180% of the control from the contralateral eyes. The Ca2+ sensitivity of the contractile system was slightly but significantly increased by AF64A injection; the half maximum contraction was obtained at pCa 5.87 and 6.05 in the control and AF64A-injected eyes, respectively. The increase in maximum contraction in AF64A injected ones was neither affected by the addition of calmodulin, GTPgammaS nor H-7. The increase in Ca2+ sensitivity by AF64A injection was not affected by calmodulin, enhanced by GTPgammaS and abolished by H-7. AF64A injection increased the total protein content only by 30% of the control. The contents of contractile proteins per iris were quantified using Western blotting with monoclonal antibodies. The contents of actin and calponin were increased by AF64A, whereas those of myosin, calmodulin and caldesmon were not affected. The results indicate that AF64A-induced enhancement of the maximum contraction is not mainly due to the increase in the contents of major contractile proteins and that the increase in Ca2+ sensitivity could be due to the mechanism in which changes in protein kinase C and/or GTP binding protein activity are involved.

Sensory nerves play an efferent role in the function of the arterioles, but not the dilator muscle, of the rat iris

We have studied the expression, distribution and function of receptors for the sensory neurotransmitters, substance P (SP) and calcitonin gene-related peptide (CGRP) in the dilator muscle and arterioles of the rat iris. Immunohistochemical studies showed that the sensory fibres containing these peptides are distributed throughout the connective tissue stroma of the iris and in association with the larger arterioles, but do not come into close association with the dilator muscle cells. Using reverse-transcription polymerase chain reaction, we have shown that both NK1 and NK3 receptor message is expressed by iris tissue, comprising both dilator muscle and stromal tissue. Binding sites for the NK1 agonist, [Sar9, Met(O2)11]-substance P (SarSP), and for CGRP are confined to the stromal layer and to the larger arterioles within that layer and do not appear to be associated with the dilator muscle itself. Application of either SarSP or CGRP produced both a vasodilatation and an inhibition of sympathetic nerve-induced vasoconstriction of the larger arterioles. Neither SarSP nor CGRP altered the resting tone of the dilator nor were they capable of modulating the contractions due to sympathetic nervous activity. These results suggest that the sensory fibres perform an efferent role in the larger irideal arterioles while their presence in the irideal stroma appears not to modulate the activity of the dilator muscle.

Pertussis toxin-sensitive muscarinic relaxation in the rat iris dilator muscle

1. The effects of pertussis toxin (PTX) on contraction and/or relaxation induced by agonists or transmural nerve stimulation (TNS) were examined in the rat iris dilator and sphincter muscles. 2. TNS in the presence of phentolamine induced an atropine-sensitive biphasic response: initial contraction followed by relaxation in dilator muscles. Exogenously applied acetylcholine (ACh) elicited a large relaxation at low doses (3 microM or less) and a concentration at high doses. 3. Only the ACh-induced relaxation was affected by injection of PTX (10 ng) into the anterior eye chamber. Relaxation was decreased 12 h after injection and had completely disappeared after 24 h. Relaxation recovered in part 3 weeks and almost completely 8 weeks after PTX treatment. A gradual decrease in muscarinic relaxation in a dilator muscle was also observed in vitro after addition of PTX to the bathing solution. 4. The pA2 values of muscarinic blockers, pirenzepine, AF-DX 116, 4-DAMP, and himbacine for competitive antagonism to ACh-induced contraction were 7.14, 6.53, 9.03, and 6.80, respectively, in PTX-pretreated dilator muscles. These values are comparable to those obtained in parasympathectomized dilator muscles and may indicate involvement of M3 or M3-like receptors in muscle contraction. 5. Pretreatment with PTX did not significantly affect contraction induced by noradrenaline or 5-hydroxytryptamine or the relaxation induced by isoprenaline in dilator muscles. 6. In conclusion, among several agonist-induced responses in the rat iris dilator and sphincter muscles, only muscarinic relaxation in dilator muscle occurs via activation of PTX-sensitive GTP binding proteins.

Pre- and postjunctional actions of endothelin in the rat iris sphincter preparation

Effects of endothelins (ETs) were studied in the rat iris sphincter preparation. Three peptides (ET-1, ET-2 and ET-3) caused contractile responses, and the rank order of agonist potency was: ET-1 = ET-2 > ET-3. The concentration-response curve to ET-1 was shifted to the right by the ETA receptor antagonist cyclo [D-Asp-L-Pro-D-Val-L-Leu-D-Trp] (BQ-123: 10(-7) M), the pA2 value of which was 7.41 +/- 0.09 (n = 4). ET-1 and ET-3, at the concentration of 10(-9) M, potentiated cholinergic contractions evoked by electrical field stimulation (5 and 20 Hz) without affecting the postjunctional sensitivity to carbachol. This potentiating effect was not influenced by BQ-123 (10(-6) M). The ET-evoked percentage increase in the stimulation-induced contraction observed at 5 Hz was significantly greater than that at 20 Hz. A release of immunoreactive ET was detected when the preparation was stimulated at 20 Hz (1.81 +/- 0.36 pg/sphincter n = 6). ET release evoked by 20 Hz stimulation was completely abolished by tetrodotoxin (10(-7) M). In conclusion, ET interacts with two different receptor types, ETA and non-ETA receptors (probably ETB) which exist post- and presynaptically at cholinergic neuroeffector junctions of the rat iris preparation. Stimulation of ETA receptor results in a direct muscle contraction and non-ETA receptor activation facilitates the acetylcholine output from cholinergic nerve endings. It is suggested that ET released from a tetrodotoxin-sensitive site is involved in the modulation of acetylcholine release in the rat iris sphincter preparation.

Functional consequences of prejunctional receptor activation or blockade in the iris

The iris is innervated by nerves of the sympathetic, parasympathetic, and sensory nervous systems. The terminal nerve fibres are endowed with prejunctional receptors which modulate neurotransmitter release. Activation or blockade of prejunctional receptors by drugs may have an influence on iris smooth muscle tone. Several findings are in favour of the hypothesis that prejunctional receptors may be involved in regulation of iris smooth muscle tone and/or pathophysiological events. (i). Release of acetylcholine from parasympathetic nerves of guinea-pig iris sphincter evoked by electrical stimulation is subject to autoinhibition via prejunctional M2 muscarinic receptors, and the release can be enhanced by M2 selective antagonists such as methoctramine or gallamine. Concomitantly with the increased neurotransmitter release, the sphincter contraction is enhanced in the presence of M2 antagonists, since the postjunctional muscarinic receptors (presumably M3, or at least not M2) are not simultaneously blocked. Unlike the non-selective blocker atropine, M2 antagonists are not expected to cause mydriasis but rather miosis. (ii). Sensory nerves are involved in pathophysiological events following ocular irritation. Release of substance P and/or neurokinin A from sensory nerves of rabbit iris is followed by a non-adrenergic-non-cholinergic iris sphincter contraction (mediated by NK1 and NK3 receptors) which can be used to estimate sensory neurotransmitter release. Exocytotic release of the sensory neurotransmitters is inhibited by activation of alpha 2B-adrenoceptors and probably also via putative prejunctional imidazoline receptors. Alpha-adrenoceptors are stimulated by oxymetazoline and other imidazoline derivatives (which are agonists at imidazoline receptors) leading to a reduction of sensory neurotransmitter release, as evident from a decrease in evoked sphincter contraction. Imidazolines in eye drops may not only cause relief in ocular inflammation due to postjunctional vasoconstriction but also possibly due to a prejunctional effect, a reduction of sensory neurotransmitter release. Reinforcement of inflammation due to release of sensory neurotransmitters may thus be prevented.

Species differences in the effects of substance P on inositol trisphosphate accumulation and cyclic AMP formation, and on contraction in isolated iris sphincter of the mammalian eye: differences in receptor density

The effects of substance P (SP) on inositol trisphosphate (IP3) accumulation, myosin light chain (MLC) phosphorylation, cAMP formation and contraction were studied in iris sphincter smooth muscle of different mammalian species. SP receptor density was also examined in membrane fractions from this tissue. The data obtained can be summarized as follows. (1) In the iris sphincters of rabbit, bovine and pig, SP receptors are coupled to the phospholipase C system, whereas in dog, cat and human these receptors are coupled to the adenylate cyclase system. (2) In those species which employ the phospholipase C system, SP induced IP3 accumulation, MLC phosphorylation and contraction in a dose-dependent manner; in contrast, in those species in which SP induced the formation of cAMP we found the neuropeptide to cause muscle relaxation. The findings on cAMP formation in intact tissue were confirmed in iris sphincter membranes. Both the effect of SP on IP3 accumulation in rabbit and bovine sphincters and its effect on cAMP formation in the dog were blocked by the SP antagonist, (D-Pro2, D-Trp7, 9)-SP. (3) The density of SP receptors in rabbit, bovine and dog were found to be 227, 110.9 and 13.6 fmol mg-1 protein, respectively, and the Kd values were 1.9, 1.8 and 1.3 nM, respectively. (4) Of the neuropeptides investigated SP, neurokinin A and neurokinin B had significant stimulatory effects on IP3 accumulation and on contraction in the rabbit iris sphincter; however, neither neurokinin Y nor the calcitonin gene-related peptide (CGRP) had any effect on these responses. In addition, none of the neuropeptides studied had any effect on IP3 or on contraction in the dog iris sphincter. While it is possible that SP may have dual actions, with the predominant action dependent on the species, the data presented could suggest the presence of two SP receptor subtypes, one coupled to phospholipase C and the other to adenylate cyclase. The results of this investigation indicate major species differences in biochemical and functional responsiveness to SP and in SP receptor density in the iris sphincter of the mammalian eye, and support a modulatory role for the neuropeptide in muscle response in this tissue.

Development of a functional innervation of the iris dilator muscle by sympathetic nerve fibres in the rat

These studies using both in situ and in vitro preparations were designed to determine the time of onset of a functional sympathetic innervation to the dilator muscle of the rat iris during postnatal development. The preganglionic nerve trunk to the superior cervical ganglion was stimulated and measurements made of pupil diameter. Alternatively, intracellular recordings were made from the dilator muscle cells and the response to stimulation of the intramural nerves was measured. During the second postnatal week, a decrease in the resting pupil diameter under low lighting conditions was observed, reflecting the normal developmental increase in size of the iris. On account of the small size of the iris in rats up to 10 days postnatal, the in vivo experiments were a reliable test of a functional innervation only in older animals. In all cases, stimulation of the preganglionic nerve trunk in situ caused dilatation of the pupil. Stimulation of the sympathetic nerves in vitro, in preparations of iris taken from rats of various ages, evoked a slow depolarisation mediated via alpha adrenoceptors. Such responses could be recorded in irides from rats as young as 3 days postnatal. Spontaneous activity was recorded in irides towards the end of the second postnatal week just prior to the time of normal eye opening, and may reflect the onset of photomechanical coupling. We conclude that the sympathetic nerves within the iris are capable of evoking membrane potential changes in the dilator muscle cells in the early postnatal period, well before the nerve plexus in the iris is fully differentiated.

Modulation of neurotransmission in the guinea-pig vas deferens by capsaicin: involvement of calcitonin gene-related peptide and substance P

1. The effects of capsaicin, calcitonin gene-related peptide and substance P were studied via three parameters in the guinea-pig vas deferens: the overflow of ATP and of tritiated noradrenaline, the mechanical responses to field stimulation and the mechanical responses to exogenous noradrenaline and alpha, beta-methylene ATP. 2. At 2 Hz, capsaicin inhibited the stimulus-evoked release of ATP, whereas it was without effect on the release of noradrenaline. At 20 Hz capsaicin did not affect the release of either of the cotransmitters. Capsaicin enhanced responses to alpha, beta-methylene ATP, but not to exogenous noradrenaline. 3. Calcitonin gene-related peptide, like capsaicin, inhibited the release of ATP, but not noradrenaline at 2 Hz and was without effect on release at 20 Hz. However, calcitonin gene related peptide inhibited responses to alpha, beta-methylene ATP and was without effect on responses to exogenous noradrenaline. 4. Substance P had no effect on the release of either noradrenaline or ATP at either frequency. However, like capsaicin it enhanced responses to alpha, beta-methylene ATP and was without effect on exogenous noradrenaline. 5. These results suggest that the actions of capsaicin on the guinea-pig isolated vas deferens are mediated via the release of both calcitonin gene-related peptide and substance P. Furthermore, as capsaicin and calcitonin gene-related peptide prejunctionally modulate purinergic, but not noradrenergic transmission, this suggests that the mechanisms for the storage and release of the sympathetic co-transmitters noradrenaline and ATP may not be the same.

The sensory-efferent function of capsaicin-sensitive sensory neurons

Capsaicin-sensitive sensory neurons convey to the central nervous system signals (chemical and physical) arising from viscera and the skin which activate a variety of visceromotor and neuroendocrine reflexes integrated at various levels (intramurally in peripheral organs, at level of prevertebral ganglia, spinal and supraspinal level). Much evidence is now available that peripheral terminals of certain sensory neurons, widely distributed in skin and viscera have the ability to release, upon adequate stimulation, their transmitter content. In addition to the well-known "axon reflex" arrangement, the capsaicin-sensitive sensory neurons have the ability to release the stored transmitter also from the same terminal which is excited by the environmental stimulus. The efferent function of these sensory neurons is realized through the direct and indirect (i.e. mediated by activation of other cells) effects of released mediators. The action of released transmitters on postjunctional elements covers a wide range of effects which may have a physiological or pathological relevance. Development of drugs capable of controlling the sensory-efferent functions of the capsaicin-sensitive sensory neurons represent a new and very promising area of research for pharmacological treatment of various human diseases.

Responses to antidromic trigeminal nerve stimulation, substance P, NKA, CGRP and capsaicin in the rat eye

In the rat eye, intracameral injections of substance P in doses of 10-30 pmol caused a maximal long-lasting miosis and a leakage of plasma proteins into the aqueous humor, indicating a breakdown of the blood-aqueous barrier. Neurokinin A seemed equipotent to SP, but calcitonin-gene-related peptide (CGRP) (17 pmol) caused neither miosis nor protein leakage into the aqueous humor. The same result was obtained when CGRP was administered intravenously. Intracameral injection of capsaicin caused only a transient miosis which could not be repeated with further injections, even though the pupillary sphincter was still able to react to exogenous SP. Antidromic electrical stimulation of the trigeminal nerve caused plasma extravasation in the skin and a breakdown of the blood-aqueous barrier with an increased protein content in the aqueous humor. The stimulation did not affect the pupil size. The results indicate that in rat eyes SP and NKA are miotics, but the amounts that can be released from sensory nerve endings are too small to cause persistent miosis. These peptides are more likely to play a role in the neurogenic breakdown of the blood-aqueous barrier. CGRP at the same dose affects neither the pupillary sphincter muscle nor the barrier.

Co-existence of neuropeptides in sympathetic, cranial autonomic and sensory neurons innervating the iris of the guinea-pig

We have used double-labelling immunofluorescence to identify the peptide content of autonomic and sensory neurons innervating the iris of albino guinea-pigs. Four major classes of neurons were identified on the basis of their distributions, origins and immunohistochemical characteristics. A dense plexus of noradrenergic axons in the constrictor and dilator muscles of the iris originated from the superior cervical ganglion, and contained immunoreactivity (IR) to both neuropeptide Y (NPY) and dynorphin (DYN). The constrictor and dilator muscles were also supplied with a dense plexus of axons with IR to substance P (SP). These axons probably originated from SP-IR nerve cell bodies located along the ciliary nerves, and are almost certainly the same axons as those producing cholinergic pupilloconstriction. The iris was also innervated by unmyelinated, capsaicin-sensitive axons with IR to both SP and calcitonin gene-related peptide. Most of these axons also contained IR to DYN and some were also IR for cholecystokinin. These axons are almost certainly sensory. Axons containing IR to both NPY and vasoactive intestinal peptide (VIP) were common in the ciliary processes, and also formed a sparse plexus near the ciliary margin of the dilator muscle. Following surgical sympathetic denervation these axons showed IR for dopamine-beta-hydroxylase; they seemed to originate from the sphenopalatine ganglion. These results demonstrate that there are well-defined patterns of coexistence of neuropeptides in the autonomic and sensory neurons supplying the iris of guinea-pigs. To understand the physiological roles of these peptides, it will be necessary to consider the possibility of complex interactions between them.

Capsaicin-induced inhibition of motility of the rat isolated vas deferens: do multiple neuropeptides mediate the visceromotor effects of capsaicin?

1 The effects of capsaicin have been investigated in the field-stimulated (0.1 Hz) or unstimulated rat isolated vas deferens and compared with those of some neuropeptides (neurokinins, calcitonin gene-related peptide (CGRP) putatively stored in capsaicin-sensitive sensory fibres. 2 Capsaicin (0.01-3 microM) produced a concentration-related transient inhibition of the field-stimulation-induced twitches. This effect was characterized by marked desensitization and could not be elicited in preparations excised from capsaicin-pretreated rats (50 mg/kg s.c., 4 days before). The amplitude of the nerve-mediated twitches was unaffected by capsaicin-desensitization. 3 Neurokinins (substance P, Kassinin) produced a potentiation of the nerve-mediated contractions while CGRP had a potent inhibitory effect. In the presence of Kassinin, CGRP still inhibited twitches although the time course of this inhibitory effect was delayed as compared to controls. 4 In the unstimulated rat vas deferens neither capsaicin (3 microM) nor CGRP (0.1 microM) had any significant motor effect. However, when phasic contractions were initiated by previous exposure to Kassinin (0.2 microM), both capsaicin (3 microM) or CGRP (10-100 nM) had a prompt inhibitory effect. Capsaicin inhibition exhibited a marked desensitization while the effect of CGRP was still evident after capsaicin-desensitization. 5 The inhibitory effect of capsaicin or CGRP on the Kassinin-stimulated rhythmic contractions of the rat isolated vas deferens was unaffected by a previous exposure to tetrodotoxin (0.5 microM). 6 Storage at 4 degrees C for 24 h produced a 65% reduction of the response of the rat isolated vas deferens to nerve-stimulation. The residual response was tetrodotoxin-sensitive and could be potentiated by Kassinin (0.2 microM) or inhibited by CGRP (10-100 nM) as observed in controls. In these preparations the inhibitory effect of capsaicin (3 microM) was significantly reduced (approximately equal to 50%) and in some preparations abolished, as compared to controls. 7 These findings indicate the existence, in the rat isolated vas deferens, of capsaicin-sensitive sensory innervation which, upon chemical stimulation, releases, through a tetrodotoxin-insensitive mechanism, a substance(s) which inhibits motility at postjunctional level. CGRP is a possible candidate for the role of inhibitory sensory transmitter released from capsaicin-sensitive nerve endings in this preparation.

Effects of substance P on inositol triphosphate accumulation, on contractile responses and on arachidonic acid release and prostaglandin biosynthesis in rabbit iris sphincter muscle

Addition of substance P (10(-7) to 10(-6) M) to rabbit iris sphincter muscle induced: (a) a rapid phosphodiesteratic breakdown of phosphatidylinositol 4,5-bisphosphate (PIP2) into 1,2-diacylglycerol (DG), measured as phosphatidic acid, and inositol triphosphate (IP3), measured by anion-exchange chromatography; (b) a rapid and strong contractile response, and (c) a rapid release of prostaglandin E2 (PGE2), measured by radioimmunoassay, and rapid release of 14C-labeled arachidonic acid, measured by radiochromatography. These substance P actions are concentration and time-dependent, and are blocked by substance P antagonist, [D-Pro2, D-Trp7,9]SP. The effects of substance P on arachidonic acid release and PG synthesis are not mediated through the cyclo-oxygenase and lipoxygenase pathways. Substance P exerted little effect on PGE2 release by the iris dilator muscle. We conclude that substance P, which is liberated from the sensory nerves that innervate the sphincter region of the iris and plays a role in miosis, may function as a Ca2+-mobilizing agonist in this tissue. Thus, a substance P-induced release of IP3 and formation of DG, a source for arachidonic acid in PG synthesis, followed by Ca2+ mobilization could underlie the mechanism for the biological actions, such as muscle contraction, of the neuropeptide reported in the eye. However, the precise relationship remains to be established.