Comparison of the muscarinic cholinoceptors in the rabbit ciliary body and the guinea-pig ileum

Pharmaceutical intervention for myopia control

Myopia is the result of a mismatch between the optical power and the length of the eye, with the latter being too long. Driving the research in this field is the need to develop myopia treatments that can limit axial elongation. When axial elongation is excessive, as in high myopia, there is an increased risk of visual impairment and blindness due to ensuing pathologies such as retinal detachments. This article covers both clinical studies involving myopic children, and studies involving animal models for myopia. Atropine, a nonselective muscarinic antagonist, has been studied most extensively in both contexts. Because it remains the only drug used in a clinical setting, it is a major focus of the first part of this article, which also covers the many shortcomings of topical ophthalmic atropine. The second part of this article focuses on in vitro and animal-based drug studies, which encompass a range of drug targets including the retina, retinal pigment epithelium and sclera. While the latter studies have contributed to a better understanding of how eye growth is regulated, no new antimyopia drug treatments have reached the clinical setting. Less conservative approaches in research, and in particular, the exploration of new bioengineering approaches for drug delivery, are needed to advance this field.

Inhibitory effects of d-nicotine on the responses evoked by 1-isomer in trachea and bronchus isolated from guinea-pig and rabbit

1. The effects of d-nicotine on the responses induced by 1-isomer were studied in tracheae and bronchi isolated from guinea-pigs and rabbits. In guinea-pigs trachea 1-nicotine produced a biphasic response consisting of initial contraction and following relaxation. In other airway preparations 1-nicotine produced only contraction. 2. d-Nicotine did not produce any responses except for the case of guinea-pig trachea. d-isomer produced only relaxation and relative potency was approximately 0.44 in guinea-pig trachea. 3. Pretreatment with d-nicotine (30-300 microM) reduced concentration response curves for 1-isomer in a non-competitive manner in all preparations used in this study. 4. 1-nicotine at the concentration of 3 microns, which did not produce any response itself, reduced the concentration response curve of 1-nicotine in guinea-pig trachea. 5. Inhibition by d-nicotine or 1-nicotine (3 microM) of the concentration-response curve of 1-nicotine may be due to desensitization of nicotine receptors.

Pharmacological profiles of the subtypes of muscarinic cholinoceptors that mediate aggregation of pigment in the melanophores of two species of catfish

Using selective antagonists, including pirenzepine, adiphenine, AF-DX 116, gallamine, and 4-DAMP, we attempted to characterize the muscarinic cholinoceptors on the melanophores of the translucent glass catfish Kryptopterus bicirrhis and the mailed catfish Corydoras paleatus. The M3 receptor-selective antagonist, 4-DAMP, potently inhibited the acetylcholine-induced aggregation of pigment in both species. It appeared, therefore, that the receptors that mediated the cholinergically evoked aggregation of melanosomes in these species were of the M3 muscarinic subtype.

Synthesis and structural, biochemical, and pharmacological study of 3 beta-acyloxy-3 alpha-methoxycarbonyltropane derivatives

A series of 3 beta-acyloxy-3 alpha-methoxycarbonyltropanes were synthesized and studied by 1H and 13C NMR spectroscopy, and the crystal structure of 3 alpha-methoxycarbonyl-3 beta-pyridincarbonyloxytropane (5d) was determined by X-ray diffraction. In CDCl3 solution, compounds 5a-f display the same preferred conformation. The pyrrolidine and piperidine rings adopt an envelope conformation flattened at N-8 and a distorted chair conformation puckered at N-8 and flattened at C3, respectively, with the N-substituent in the equatorial position with respect to the piperidine ring. The pharmacological profile of one of these compounds makes it an adequate candidate for the design of novel GABAB antagonist agents.

Muscarinic receptor subtypes in human and rat colon smooth muscle

Muscarinic receptor subtypes in human and rat colon smooth muscle homogenates were characterized with [3H]N-methylscopolamine ([3H]NMS) by ligand binding studies. [3H]NMS saturation experiments show the existence of a homogeneous population of non-interacting binding sites with similar affinity (KD values of 1.38 +/- 0.20 nM in human colon smooth muscle and 1.48 +/- 0.47 nM in rat colon smooth muscle) and with Hill slopes close to unity in both samples of tissue. However, a significant (P less than 0.01) increase in muscarinic receptor density (Bmax) is found in human colon (29.9 +/- 2.9 fmol/mg protein) compared with rat colon (17.2 +/- 1.5 fmol/mg protein). Inhibition of [3H]NMS binding by non-labelled compounds shows the following order in human colon: atropine greater than AF-DX 116 greater than pirenzepine. Whereas in rat colon the rank order obtained is atropine greater than pirenzepine greater than AF-DX 116. Atropine and pirenzepine bind to a homogeneous population of binding sites, although pirenzepine shows higher affinity to bind to the sites present in rat colon (Ki = 1.08 +/- 0.08 microM) than those in human colon (Ki = 1.74 +/- 0.02 microM) (P less than 0.05). Similarly, IC50 values obtained in AF-DX 116 competition experiments were significantly different (P less than 0.01) in human colon (IC50 = 1.69 +/- 0.37 microM) than in rat colon (IC50 = 3.78 +/- 0.75 microM). Unlike atropine and pirenzepine, the inhibition of [3H]NMS binding by AF-DX 116 did not yield a simple mass-action binding curve (nH less than 1, P less than 0.01) suggesting the presence of more than one subtype of muscarinic receptor in both species. Computer analysis of these curves with a two binding site model suggests the presence of two populations of receptor. The apparent Ki1 value for the high affinity binding site is 0.49 +/- 0.07 microM for human colon smooth muscle and 0.33 +/- 0.05 microM for rat colon smooth muscle. The apparent Ki2 for the low affinity binding site is 8.01 +/- 1.0 microM for human samples and 6.07 +/- 1.1 microM for rat samples. These values are close enough to suggest that the first subtype of muscarinic receptor may be considered cardiac (M2) and the second subtype glandular (M3). The relative densities of the receptor subtypes are significantly different for both species. Human colon samples show the major densities of subtype M2, 22.62 +/- 1.11 fmol/mg protein, this represents 75.66 +/- 3.73% of the total receptors.(ABSTRACT TRUNCATED AT 400 WORDS)

Pre- and postjunctional muscarinic receptor subtypes in dog airways

To examine muscarinic receptor subtypes involved in cholinergically mediated contractions of the airway, we studied the effects of the M1-selective antagonist, pirenzepine, the M2-selective antagonist, AF-DX 116, the M3-selective antagonist, 4-diphenyl-acetoxy-N-methylpiperidine (4-DAMP) methiodide, and the non-selective antagonist, atropine, on acetylcholine (ACh)- and electrically induced contractions in dog bronchi and bronchioles. The relative potencies of the antagonists based on IC50 values of each antagonist for contractions induced by the two concentrations of ACh that produced 50% of the maximum (ED50) and the maximum (EDmax) contractions and the pA2 values were atropine greater than or equal to 4-DAMP methiodide greater than pirenzepine = AF-DX 116 in both the bronchi and bronchioles. The IC50 and pA2 values of each antagonist did not differ significantly between the bronchi and bronchioles. 4-DAMP methiodide significantly inhibited the contractile response to electrical field stimulation (EFS) at 5 Hz at concentrations that did not alter the contractile responses to exogenous ACh in both the bronchi and bronchioles, whereas pirenzepine, AF-DX 116 and atropine inhibited the EFS-induced contraction only at the concentrations that reduced the contraction induced by exogenous ACh. The present results suggest that the cholinergic contraction is mediated via the postsynaptic receptor M3, based on functional potencies of muscarinic antagonists and presynaptic receptor auto-facilitatory M3, based on the suppression of the contractile response to EFS by 4-DAMP methiodide in central and peripheral airways.

Measurements of intracellular calcium and contractility in human ciliary muscle

Electromechanical and pharmacomechanical coupling was investigated in human ciliary muscle by measuring the intracellular free calcium in single cultured ciliary muscle cells and the contractility in meridional ciliary muscle strips. The basal resting calcium concentration was 75 +/- 8.7 nmol/l, n = 23. Application of acetylcholine (0.1 mmol/l) and carbachol (0.1 mmol/l) resulted in an initial [Ca2+]i peak followed by a recovery phase and a [Ca2+]i plateau. The initial [Ca2+]i peak was still observed in the absence of extracellular calcium and in the presence of verapamil (0.1 mmol/l). During its plateau [Ca2+]i was decreased by withdrawal of extracellular calcium or application of verapamil (0.1 mmol/l). Depolarization induced by a high level of extracellular potassium yielded only a small transient [Ca2+]i peak without a [Ca2+]i plateau. In isolated ciliary muscle strips, muscarinic stimulation (carbachol 0.1 mmol/l) resulted in an initial phasic and a subsequent tonic contraction. Removal of external calcium reduced the phasic contraction to 30.6 +/- 4.4% (n = 8) and completely abolished the tonic one. Verapamil (0.1 mmol/l) had only a slight relaxing effect when applied during the tonic contraction. We conclude that human ciliary muscle contraction is mediated by calcium release from intracellular stores and calcium entry through calcium channels, which are most probably receptor-operated. Depolarization of the muscle cell membrane and calcium entry through voltage-operated calcium channels do not contribute significantly to human ciliary muscle contraction.

Characterization of muscarinic receptors in dog tracheal smooth muscle cells

1. The tritiated muscarinic antagonist N-methyl scopolamine, [3H]-NMS, was used to characterize the muscarinic receptors associated with the intact dog tracheal smooth muscle cells. Based on receptor binding assays, the intact tracheal smooth muscle cells had specific, saturable, high-affinity binding sites for [3H]-NMS. 2. Specific binding was cell concentration- and time-dependent. The specific binding of [3H]-NMS was increased linearly with increasing cell concentrations. The equilibrium for association of [3H]-NMS with the muscarinic receptors was attained within 30 min at 37 degrees C. 3. Binding was saturable with respect to [3H]-NMS concentrations. Analysis of binding isotherms yielded an apparent equilibrium dissociation constant (KD) of 320 +/- 20 pM and a maximum receptor density (Bmax) of 13.7 +/- 1.4 fmole per 5 x 10(4) cells. The Hill coefficient for [3H]-NMS binding was 1.00 +/- 0.01. The association (K1) and dissociation (K-1) rate constants were determined to be (1.19 +/- 0.23) x 10(8) M-1 min-1 and 0.034 +/- 0.09 min-1, respectively. KD, calculated from the ratio of K-1 and K1, was 286 +/- 65 pM; this value is close to the value of KD calculated from Scatchard plots of binding isotherms. 4. The non-selective muscarinic antagonist atropine and M1 selective antagonist pirenzepine did not reveal any selectivity of these muscarinic receptors. Pirenzepine competed with [3H]-NMS for a single binding site with a Ki value of (6.02 +/- 0.69) x 10(-7) M which is close to the value of M2 or M3 receptors, indicating that the M1 receptor subtype did not exist in the intact tracheal smooth muscle cells. 5. Competition with cardioselective antagonist (M2), methoctramine; smooth muscle selective antagonists (M3), hexahydrodifenidol and hexahydrosiladifenidol; as well as carbachol, were best fit by a two-binding site model. The results suggest that both M2 and M3 receptor subtypes exist at the cell surface of tracheal smooth muscle cells.

Characterization of muscarine receptors in rabbit ciliary body smooth muscle using propylbenzilylcholine mustard

1. A piece of rabbit ciliary body was mounted in organ bath and used as a test organ containing muscarine receptors. 2. A concentration response curve of carbachol was completely inhibited by a 50 min treatment of the smooth muscle preparation with propylbenzilylcholine mustard(PrBCM; 10(-6) M). This treatment was reported to block PrBCM-sensitive muscarine receptors in intestinal smooth muscle completely. 3. It is suggested that an existance of only PrBCM-sensitive muscarinic receptors, though there are two subtypes of muscarine receptors; PrBCM-sensitive and -resistant ones in intestinal smooth muscles.

Characterization of the muscarinic receptor subtype involved in phosphoinositide metabolism in bovine tracheal smooth muscle

1. The muscarinic receptor subtype involved in the methacholine-induced enhancement of phosphoinositide metabolism in bovine tracheal smooth muscle was identified by using the M2-selective antagonist AF-DX 116 and the M3-selective antagonist 4-diphenylacetoxy-N-methylpiperidine (4-DAMP) methobromide, in addition to the M1-selective antagonist pirenzepine, in a classical Schild analysis. 2. All the antagonists shifted the methacholine dose-response curve to the right in a parallel and concentration-dependent fashion, yielding Schild plots with slopes not significantly different from unity. The pA2 values (6.94, 6.32 and 8.54 for pirenzepine, AF-DX 116 and 4-DAMP methobromide respectively) indicate that it is the M3 (smooth muscle/glandular), but not the M2 (cardiac) muscarinic receptor subtype, present in this tissue, that mediates phosphoinositide turnover, in accordance with our previous contractile studies. 3. The results provide additional evidence for the involvement of phosphoinositide turnover in the pharmacomechanical coupling between muscarinic receptor stimulation and contraction in (bovine tracheal) smooth muscle.

M2 muscarinic receptors on the iris sphincter muscle differ from those on iris noradrenergic nerves

The pre- and postjunctional affinity constants of a series of muscarinic antagonists were determined in guinea pig and rabbit irises. Field stimulation-evoked [3H]noradrenaline release from superfused isolated irises was concentration dependently inhibited by (+/-)-methacholine, confirming the presence on the iris noradrenergic nerves of prejunctional inhibitory muscarinic receptors. The affinity constants of the antagonists at the pre- and postjunctional receptors are compatible with the coexistence in the iris of two different M2 receptors: the cardiac (M2 alpha) subtype on the noradrenergic nerves and the smooth muscle (M2 beta) subtype on the iris sphincter muscle. The rank order of potency of the antagonists studied at the prejunctional site was: atropine greater than himbacine greater than AF-DX 116 greater than pirenzepine greater than hexahydrosiladifenidol. The order of potency at the postjunctional receptors mediating the methacholine-induced isotonic contraction of the isolated rabbit iris sphincter was: atropine greater than hexahydrosiladifenidol greater than pirenzepine greater than himbacine greater than AF-DX 116.

Involvement of threshold level in the contractile responses for some alpha 1-andrenoceptor agonists in the rabbit iris dilators

1. The relationship between receptor occupancies and contractile responses for some alpha 1-adrenoceptor agonists were investigated in rabbit iris dilator smooth muscles. 2. Noradrenaline acted as a full agonists, while oxymetazoline and xylometazoline behaved as partial agonists with moderately higher intrinsic activity, and tizanidine and clonidine were partial agonists with lower intrinsic activity. 3. The pD2-values of oxymetazoline and xylometazoline were practically equal to the corresponding pKB-values, the negative log of dissociation constant, estimated by the partial irreversible blockade of alpha 1-adrenoceptors with phenoxybenzamine. However, the pD2-values of tizanidine and clonidine were significantly lower than the corresponding pKB-values. 4. The threshold phenomena lay between the receptor occupations and tissue responses, therefore, the pKB-values of partial agonists with lower intrinsic activity were different from their pD2-values. 5. These results suggest that the threshold phenomena in the tissue used may be an important factor in determining the agonist sensitivity.

Muscarinic inhibition of [3H]-noradrenaline release on rabbit iris in vitro: effects of stimulation conditions on intrinsic activity of methacholine and pilocarpine

1. Rabbit isolated irides were loaded with [3H]-noradrenaline and superfused with Tyrode solution. The inhibition by the muscarinic agonists (+/-)-methacholine and pilocarpine of the [3H]-noradrenaline overflow into the superfusate evoked by field stimulation (pulses of 1 ms duration, 75 mA) was measured as an index of activation of presynaptic muscarinic receptors. 2. The fractional rate of release per pulse during the first stimulation period (S1) was low with 360 pulses at 3 Hz, intermediate with 360 pulses at 10 Hz and high with 1200 pulses at 10 Hz. Upon repetitive stimulation (7 periods at 20 min intervals), the fractional rates of release per pulse during S7 no longer differed, suggesting a 'long-term' regulation of [3H]-noradrenaline release depending on the stimulation conditions. 3. The evoked [3H]-noradrenaline overflow was depressed by (+/-)-methacholine in a concentration-dependent manner. The EC50 ranged from 0.29 to 0.42 microM. Methacholine nearly abolished the transmitter release evoked at 3 Hz but reduced that induced at 10 Hz by only 50%. Under the latter condition the methacholine concentration-inhibition curve was bell-shaped and no muscarinic inhibition was observed in the presence of methacholine 30 microM. After washout of methacholine the evoked [3H]-noradrenaline release was temporarily enhanced. 4. Atropine 0.1 microM enhanced the [3H]-noradrenaline overflow (evoked by stimulation with 360 or 1200 pulses at 10 Hz), probably antagonizing a presynaptic inhibition by endogenous acetylcholine. The inhibition by methacholine was competitively antagonized by atropine 0.1 microM (apparent -log KB = 8.5-9.0). 5. Depending on the concentration, pilocarpine reduced the [3H]-noradrenaline overflow evoked by 360 pulses at 3 Hz up to 63%. However, at 10 Hz stimulation frequency the compound was inactive as an agonist but competitively antagonized the presynaptic inhibition induced by methacholine. The KB under the latter condition (0.95 microM) was very close to the EC50 value determined at 3 Hz (0.85 microM). 6. The results demonstrate a muscarinic inhibition of noradrenaline release from the rabbit isolated iris. The activation by pilocarpine of the presynaptic receptors provides an alternative explanation for the miosis induced in the rabbit in vivo, which might be the result of a decreased sympathetic tone in the iris dilator muscle.

Muscarinic receptor heterogeneity in guinea pig intestinal smooth muscle: binding studies with AF-DX 116

Muscarinic receptor subtypes in longitudinal and circular smooth muscles of the guinea pig ileum were characterized with the use of the cardioselective antagonist AF-DX 116 in binding competition experiments against 0.3 nM [3H] N-methylscopolamine [( 3H]NMS). This compound recognized a heterogeneous receptor population in both smooth muscles, revealing the existence of different percentages of the cardiac (KD = 92-110 nM) and the glandular (KD = 1150-2541 nM) muscarinic receptor subtypes. These results, together with the low potency displayed by AF-DX 116 to inhibit the agonist-stimulated smooth muscle contraction and salivary secretion allow the suggestion that the glandular muscarinic receptor subtype, showing a low affinity for AF-DX 116, is involved in smooth muscle contraction.