pA2 and receptor differentiation: a statistical analysis of competitive antagonism

N-terminal truncated analogs of men 10376 as tachykinin NK-2 receptor antagonists

Three N-terminal fragments of the selective tachykinin NK-2 receptor antagonist MEN 10376 (H-Asp-Tyr-DTrp-Val-DTrp-DTrp-Lys-NH2) have been synthesized and tested in several mammalian tissues in order to establish the minimum length of the peptide chain for maintenance of the antagonist activity. Biological activity has been determined on the rabbit pulmonary artery (RPA) and hamster trachea (HT) preparations, chosen as representative of the NK-2A and NK-2B receptor subtypes, respectively, and on the rabbit bronchus (RB), guinea-pig bronchus (GPB), human urinary bladder (HuUB), human ileum (HuI) and human colon (HuC) preparations to verify the previously described NK-2A character of these tissues. The N-terminal tetrapeptide was inactive in the RPA and HT, while the N-terminal hexa- and penta- peptides maintained antagonist activity in all preparation investigated. The selectivity of the latter two peptides confirms that the receptor expressed in RB, GPB, HuUB, HuC and HuI tissues is of the NK-2A type.

Pharmacological profile of a potent, efficacious fentanyl derivative in rhesus monkeys

The recent synthesis of fentanyl derivatives, some of which appear to have novel profiles of pharmacological effects, has provided compelling evidence that mu opioid efficacy might be altered systematically by modifications in the parent compound fentanyl. In the present study a new 4-(heteroanilido)-piperidine, compound 28, was studied for its effects in rhesus monkeys. In self-administration studies compound 28 maintained rates of lever pressing similar to those maintained by alfentanil; the reinforcing effects of compound 28 were attenuated by the opioid antagonist quadazocine. In drug discrimination studies compound 28 did not substitute for the kappa agonist ethylketocyclazocine and did substitute for the mu agonist alfentanil. In morphine-treated subjects discriminating between saline and naltrexone, compound 28 did not substitute for naltrexone; however, in morphine-abstinent subjects compound 28 reversed naltrexone lever responding. Moreover, this discriminative stimulus effect in morphine-abstinent subjects was antagonized by naltrexone and by quadazocine in a manner consistent with mu receptor mediation. Compound 28 also was an effective analgesic in a warm-water, tail-withdrawal procedure and it decreased markedly respiratory function. The analgesic effects as well as the respiratory depressant effects of compound 28 were antagonized by quadazocine. Together, these results show compound 28 to be a potent, efficacious mu agonist of similar potency to alfentanil. Large differences in apparent efficacy at mu receptors between compound 28 and another compound in this series (mirfentanil), clearly demonstrate that, within this chemical family, small chemical changes can confer significant differences in pharmacologic effect.

Stereoselectivity at ?-adrenoreceptor subtypes: observations with the enantiomers of WB 4101 separated through their amides ofN-Tosyl-(S)-proline

We present a chromatographic method for the separation and determination of the optical purity of the enantiomers of WB 4101 [(+/-)-1], one of the most potent and selective alpha 1-adrenoreceptor antagonists. (+/-)-1 was converted into the amide of N-tosyl-(S)-proline. The two diastereoisomers were separated on silica gel and analysed by HPLC reversed phase. The analytical method described is both accurate and sensitive and allows the optical purity to be determined at very low concentrations and to obtain WB 4101 enantiomers with a purity of more than 99.95%.

Pharmacological characterization of muscarinic receptor subtypes in rabbit isolated tissue preparations

1 The affinity of some muscarinic antagonists for muscarinic receptors was determined in functional isolated tissue studies in order to compare the muscarinic receptor subtypes in the rabbit. 2 Our attention was specially focused on the question of whether the muscarinic receptors mediating vasodilatation in the aorta resemble or not the ones present on the jejunum of the gastrointestinal tract. 3 Isolated aorta, jejunum, stimulated left atrium and vas deferens preparations of rabbit were investigated with the following muscarinic antagonists: atropine, pirenzepine, methoctramine (N,N'-bis[6-92-methoxybenzyl)amino hexyl]-1,8-octane-diamine tetrahydrochloride) and 4-DAMP (4-diphenylacetoxy-N-methylpiperidine methiodide). 4 The results demonstrate that the receptors on aorta are unlike those on the other rabbit tissues: pirenzepine pA2 was 6.4 on aorta but 8.1 on vas deferens; methoctramine pA2 was 5.9 on aorta but 7.1 on heart; 4-DAMP pA2 was 8.7 on aorta and 8.0 on jejunum. This raises the question: what subtype might be involved?

Vasodilatation elicited by 5-HT1A receptor agonists in constant-pressure-perfused rat kidney is mediated by blockade of alpha 1A-adrenoceptors

The vasodilator mechanism of the putative serotonin1A (5-HT) receptor agonists, urapidil, 5-methyl-urapidil, ipsapirone, flesinoxan and 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) was investigated in constant-pressure perfused rat kidneys. The compounds (10(-12)-10(-7) mol bolus injection) neither enhanced basal flow nor evoked vasodilatation in kidneys preconstricted by 27 mM KCl, 1.5 mM BaCl2 or 10(-6) M prostaglandin (PG)F2 alpha, but evoked a dose-dependent, reversible and spiroxatrine-resistant increase in vasodilatation of organs preconstricted by 6 x 10(-7) M noradrenaline. 5-Carboxamidotryptamine and sumatriptan did not reverse the vasoconstriction induced by all stimuli or that induced by noradrenaline in the presence of 5-HT2 plus 5-HT3 receptor blockade. No correlation for the vasorelaxant drugs was found between their -log ED50 in rat kidney and pKi values at 5-HT1A binding sites in pig cortex as determined in radioligand experiments. The relaxation in rat kidney induced by 5-HT1A receptor agonists and alpha 1A-adrenoceptor-selective antagonists (WB 4101 and (+)-niguldipine) was significantly correlated with pKi values at alpha 1A binding sites in rat cortex and the pA2 values derived from contraction studies for competitive antagonism at alpha 1-adrenoceptors in prostatic portions of the rat vas deferens, but differed from pKi values for alpha 1B binding sites in rat cortex. Thus, the vasodilator effect of the 5-HT1A receptor agonists urapidil, 5-methyl-urapidil, ipsapirone, flesinoxan and 8-OH-DPAT in the noradrenaline-perfused rat kidney appears to be mediated by their concomitant alpha 1A-adrenoceptor blockade. No evidence for a vasodilator effect mediated through 5-HT1A receptors was found under our experimental conditions.

Further evidence for the existence of NK2 tachykinin receptor subtypes

1. We have evaluated the biological activity of a number of neurokinin A (4-10), (NKA (4-10)) analogues in the endothelium-deprived rabbit isolated pulmonary artery (RPA) and hamster isolated trachea (HT), two tissues rich in different NK2 receptor subtypes. 2. MDL 28,564, a pseudopeptide selective for NK2 receptor sites, behaved as a full agonist in the RPA, while in the HT it competitively antagonized NKA or [beta Ala8]-NKA (4-10) contractile effects. 3. The peculiar behaviour of MDL 28,564 in the RPA and HT may be explained neither by a difference in receptor reserve between the two organs (the reserve being three times greater in RPA than in the HT) nor by a different affinity for the two receptor subtypes (identical dissociation constants, pKA or pKB, calculated in the RPA and in the HT). On the other hand, MDL 28,564 displayed a very different intrinsic efficacy for the two receptor subtypes. 4. The novel peptides MEN 10,295 ([Trp7, beta Ala8]-NKA-(4-10)) and MEN 10,296 ([Tyr5, Trp7, beta Ala8]-NKA-(4-10] behaved as weaker agonists than MDL 28,564 in the RPA, but retained appreciable agonist activity also in the HT. 5. The novel peptides: MEN 10,282 ([Tyr5, D-Trp6,8, Trp9, Arg10]-NKA-(4-10], MEN 10,449 ([diI-Try5, D-Trp6,8,9, Arg10]-NKA-(4-10] and the cyclic hexapeptide L 659,877 (cyclo [Leu-Met-Gln-Trp-Phe-Gly]) behaved as competitive antagonists against NKA contractile effects both in the RPA and HT. MEN 10,282 and MEN 10,449 were unable to distinguish between the NK2 receptor subtypes, having almost the same affinity in the two organs. On the other hand L 659,877 was about 15 times more potent in the HT than in the RPA. 6. These results provide further evidence for NK2 receptors heterogeneity and are useful in outlining pharmacological features of the two subtypes present in the RPA and HT.

Kinin receptors of the central nervous system of spontaneously hypertensive rats related to the pressor response to bradykinin

1. Kinin analogues bradykinin (BK), T-kinin, Met-Lys-BK, Lys-Lys-BK, Des-Arg9-BK with agonist activity and D-Arg0-Hyp3-Thi5,8-D-Phe7-BK (DAHTDBK) and Arg9-Leu8-BK with antagonist activity were injected into the posterior portion of the fourth cerebral ventricle of unanaesthetized rats implanted with permanent cannulae and arterial pressure was measured directly from the abdominal aorta. 2. The spontaneously hypertensive rats (SHR) were more sensitive than normotensive Wistar rats (NWR) to the pressor effect of BK and other kinin analogues. The SHR did not differ in sensitivity of the pressor response to centrally administered angiotensin II or endothelin-1. 3. Experiments with selective kinin agonists and antagonists revealed that in the SHR, as in the NWR, the receptors which mediated the central pressor response are of the BK2 subtype. 4. Measurements of the pressor activity of kinins with different degrees of susceptibility to degradation, as well as experiments with kininase inhibitors, enalaprilat and CPP-Ala-Ala-Phe-pAB, suggest that the kininase activity in the central nervous system of SHR is reduced in comparison to that of NWR. 5. The SHR also showed increased sensitivity to BK and Lys-Lys-BK, compared with the NWR, when the kinins were injected following the administration of a mixture of the kininase inhibitors, suggesting that mechanisms other than kininase activity may play a role in the increased sensitivity of the SHR to the central pressor action of kinins. 6. An in vivo characterization of the kinin receptors which mediate the central pressor response showed that the interaction with DAHTDBK was reversible and of competitive nature. The pA2 in vivo estimated for the kinin receptors of the SHR was 0.7 logarithm units larger than that obtained in the NWR. 7. The kinin receptors which mediate the central BK pressor effect in the SHR are of the BK2 subtype and are similar to receptors in the NWR. The increased sensitivity to kinins in the SHR may be explained, at least in part, by their decreased kininase activity. At present it is impossible to conclude whether the difference observed in the pA2 represents an increased affinity of the kinin receptors or can be attributed to differences amongst strains in the enzymatic degradation of the antagonist.

Characterization of muscarinic receptors mediating endothelium-dependent relaxation of bovine coronary artery

In order to identify the receptor subtype responsible for acetylcholine (ACh)-induced relaxation of bovine coronary artery, we determined the affinity of six subtype-selective muscarinic antagonists and compared them with affinity estimates obtained for bovine left atria. At low concentrations, ACh potently relaxed circular strips of coronary artery with endothelium (EC50 0.15 microM), but contracted them at higher agonist concentrations with potencies that depended on the presence or absence of endothelium: EC50 1.8 microM (without endothelium); 4.6 microM (with endothelium). The pA2 values obtained for antagonism of relaxant responses to ACh were: pirenzepine (M1-selective) 7.38 +/- 0.12; AF-DX 116 (11-[2-(diethylamino-methyl)-1-piperidinyl-acetyl]-5,11- dihydro-6H-pyrido(2,3-b)1,4-benzodiazepine-6-one; M2-selective) 5.79 +/- 0.09; and 4-diphenylacetoxy-N-methyl-piperidine-methobromide (4-DAMP; M3/M1-selective) 9.07 +/- 0.12. The corresponding Schild slopes were 0.98 +/- 0.07 for pirenzepine, 1.17 +/- 0.09 for AF-DX 116 and 1.01 +/- 0.04 for 4-DAMP. For the following three antagonists, pKB values were determined at two different antagonist concentrations: dicyclomine (M1-selective) 7.49 +/- 0.10, cyclohexylphenyl-(2-piperidinoethyl)-silanol (CPPS; M3-selective) 8.0 +/- 0.10, and parafluoro-hexahydrosila-difenidol (pFHHSiD; M3-selective) 7.87 +/- 0.10. For comparison, the antagonism of methacholine-induced negative inotropy in left atria was determined for three antagonists, yielding the following pA2 values: pirenzepine 5.98 +/- 0.14; AF-DX 116 6.81 +/- 0.14 and 4-DAMP 7.99 +/- 0.14. The slopes of the corresponding Schild plots were 1.05 +/- 0.10, 1.14 +/- 0.12 and 1.08 +/- 0.08, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)

Muscarinic inhibition of excitatory neurotransmission in guinea-pig olfactory cortex slices: weak antagonism by M3-muscarinic receptor antagonists

Dose-dependent depression of the electrically evoked surface-negative field potential (N-wave) produced by bath-superfusion of carbachol was measured in guinea-pig olfactory cortex slices maintained in vitro. The possible involvement of M3 (smooth muscle/glandular) type muscarinic receptors in partly mediating this response was investigated by testing the effectiveness of the muscarinic M3 receptor antagonists hexahydro-sila-difenidol (HHSiD) and p-fluoro-hexahydro-sila-difenidol (p-F-HHSiD). Low doses of HHSiD (10-100 nM) or p-F-HHSiD (up to 1 microM), pre-applied for 30 min, produced no obvious antagonism of carbachol responses. However, a clear competitive-type inhibition of carbachol effects was observed in 250 nM-1 microM HHSiD or 10-50 microM p-F-HHSiD respectively. Schild plot analysis (regression slope constrained to unity) of pooled data yielded pA2 values of 6.6 for HHSiD (n = 6 slices) and 5.5 for p-F-HHSiD (n = 6 slices) respectively, suggesting a weak competitive antagonism by both compounds. In addition, combination experiments using either HHSiD or p-F-HHSiD with atropine, produced dose-ratio shifts close to those predicted for two antagonists competing for a common receptor site. By comparison, another suggested M3-receptor antagonist, 4-diphenyl-acetoxy-N-methyl-piperidine methiodide (4-DAMP) was a potent competitive blocker of carbachol responses. Schild analysis for 4-DAMP versus carbachol gave a pA2 of 7.9 (n = 6 slices). It is concluded that the muscarinic receptors involved in the suppression of the olfactory cortical N-wave possess a low affinity for HHSiD and particularly for p-F-HHSiD, but not 4-DAMP.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of potent bombesin antagonist on exocrine pancreatic secretion in rats

Recent synthesis of specific, potent bombesin receptor antagonists allows examination of the role of bombesin-like peptides in physiological processes in vivo. We characterized effects of [D-Phe6]bombesin(6-13)-methyl-ester (BME) on pancreatic enzyme secretion stimulated by the C-terminal decapeptide of gastrin releasing peptide (GRP-10), food intake, and diversion of bile-pancreatic juice in rats. In isolated pancreatic acini, BME had no agonistic effects on amylase secretion but competitively inhibited responses to GRP-10, yielding a pA2 value of 8.89 +/- 0.19. In conscious rats with gastric, jugular vein, bile-pancreatic, and duodenal cannulas, basal enzyme secretion (bile-pancreatic juice recirculated) was not affected by the antagonist. Maximal amylase response to GRP-10 (0.5 nmol/kg/h) was inhibited dose dependently by BME, reaching 97% inhibition at a dose of 400 nmol/kg/h. The dose response curve of amylase secretion stimulated by GRP-10 was shifted to the right by 40 nmol/kg/h BME, but maximal amylase response was unaltered, suggesting competitive inhibition in vivo. Liquid food intake and bile-pancreatic juice diversion caused substantial increases in amylase secretion; neither response was altered during administration of 400 pmol/kg/h BME. These results demonstrate that BME is a potent, competitive antagonist of pancreatic responses to bombesin-like peptides in vitro and in vivo. Lack of effect of BME on basal pancreatic secretion or responses to liquid food intake or diversion of bile-pancreatic juice in rats suggests that endogenous bombesin-like peptides do not act either directly or indirectly to mediate these responses.

Characterization of muscarinic receptors of bovine coronary artery by functional and radioligand binding studies

The nature of the muscarinic receptor subtype mediating contraction of the endothelium-denuded bovine coronary artery was investigated in vitro by functional measurements and radioligand binding studies. The acetylcholine (ACh)-induced isotonic contraction of circularly cut muscle strips was recorded and expressed as a percentage of the maximum contraction obtained with 80 mM K+. In order to distinguish between M1, M2 and M3 receptors, the potency of the five subtype-selective antagonists, 4-diphenylacetoxy-N-methyl-piperidine methobromide (4-DAMP), parafluor-hexahydro-siladifenidol (pFHHSiD), pirenzepine, AF-DX 116 and methoctramine, to block the ACh-induced contraction was estimated. All the antagonists competitively inhibited the responses induced by ACh, with one exception, namely, 4-DAMP, whose Schild plot had a slope greater than one. The low affinity of pirenzepine (pA2 7.14 +/- 0.14) excluded an action at the M1 subtype. The low affinity of AF-DX 116 (pA2 6.49 +/- 0.18) and methoctramine (pA2 5.88 +/- 0.07) suggest that the bovine coronary artery smooth muscle receptor is not of the M2 (cardiac) subtype. In contrast, 4-DAMP (pA2 9.04 +/- 0.03) and pFHHSiD (pA2 7.64 +/- 0.04) potently inhibited the ACh-induced contraction with affinities similar to those reported for the M3 (glandular) receptor. In addition, the muscarinic receptors mediating coronary artery contraction were characterized in antagonist/[3H]N-methyl-scopolamine ([3H]NMS) competition binding studies. With the exception of AF-DX 116, all antagonists bound to a homogeneous population of receptors with pseudo-Hill slopes not different from unity. The pKi values, albeit somewhat lower, essentially substantiated the functional affinity estimates.(ABSTRACT TRUNCATED AT 250 WORDS)

D-Arg[Hyp3-Thi5-D-Tic7-Tic8]-bradykinin, a potent antagonist of smooth muscle BK2 receptors and BK3 receptors

D-Arg[Hyp3-Thi5-D-Tic7-Tic8]-bradykinin (NPC 16731) inhibited bradykinin (BK) binding and BK-induced contraction in guinea-pig ileum, being markedly more potent than D-Phe7-BK analogues as a BK2 receptor antagonist. In isolated trachea NPC 16731, unlike other BK2 antagonists, inhibited BK binding and BK-induced contraction, and 45Ca2+ efflux in tracheal smooth muscle cells. That NPC 16731 potently inhibits BK effects in trachea provides further evidence for the existence of the airway BK3 receptor.

CGS 15943, a nonxanthine adenosine receptor antagonist: effects on locomotor activity of nontolerant and caffeine-tolerant rats

CGS 15943 (0.1-10 mg/kg, IP) dose-dependently increased the locomotor activity of rats to the same extent as caffeine (1.0-100 mg/kg, IP) did and was approximately 26 times more potent than caffeine. N-Ethylcarboxamidoadenosine (0.001-0.01 mg/kg, SC), an analog of adenosine, dose-dependently decreased locomotor activity; this effect was antagonized surmountably by concurrent administration of CGS 15943. The apparent pA2 value for this interaction, 6.57, was approximately 1.5 log-units (28-fold) higher than the pA2 for caffeine-NECA reported previously. Rats consuming 70 mg/kg/day of caffeine via their drinking water were tolerant to the stimulation of locomotor activity induced by both caffeine and CGS 15943. These results suggest that caffeine and CGS 15943 increase locomotor activity by a common mechanism of action possibly involving adenosine receptors or a cellular element conformationally similar to adenosine receptors.

Organic Ca(2+)-antagonist-resistant response to FMRF-NH2 on the molluscan smooth muscle

1. FMRF-amide (10(-7)-10(-5) M) contracted molluscan anterior byssus retractor muscle in a concentration-dependent fashion. 2. The concentration-response curve of FMRF-amide was shifted rightward by an analogue of FMRF-amide, FMRf-amide ([D-Phe4]FMRF-amide, putative FMRF-amide receptor antagonist) in a parallel manner (pA2 = 4.87 +/- 0.04). 3. Although a contractile response to KCl was reduced by the organic Ca2+ antagonists (verapamil, diltiazem and high concentration of nifedipine and nicardipine). FMRF-amide-induced contraction was not markedly reduced by them. 4. In the Ca(2+)-free medium, FMRF-amide-induced contraction was diminished. The response was also reduced by TMB-8 (10(-4) M), suggesting that FMRF-amide-induced contraction might be partly dependent on intracellular Ca2+. 5. An inorganic Ca(2+)-antagonist, MnCl2, markedly reduced the FMRF-amide- and KCl-induced contraction. The results show that FMRF-amide-induced contraction might be dependent on extracellular Ca2+. 6. These findings suggest that FMRF-amide-induced contraction might be mediated through an action on FMRF-amide receptors and not through the activation of organic Ca2+ antagonist-sensitive Ca2+ channels.

Neurokinin receptors antagonists: old and new

Four neurokinin antagonists of different size have been used to counteract the myotropic effects of substance P, neurokinin A and neurokinin B in isolated organs containing a single receptor type (monoreceptor systems). These are: the dog carotid artery, the rabbit jugular and cava veins and the guinea pig ileum (NK-1), the rabbit pulmonary artery (NK-2) and the rat portal vein (NK-3). Undeca and octapeptides containing 2 D-Trp residues in their sequences were slightly more active on the NK-1, than on the NK-2 and NK-3 receptors and showed little selectivity. In contrast, compound AcThr-D.Trp(For)-Phe.NMe Bz was found to be as good an antagonist as the larger compounds and showed some selectivity for the NK-1 receptors. When tested against kinins or angiotensin, all compounds were found to be inactive, suggesting that they are specific for neurokinins. The present results show that NK-1 receptor antagonism can be obtained with compounds of different size, including tripeptides and nonpeptides.

Contractile responses of rat aorta to phenylephrine and serotonin, and aging

1. The potency (pD2 value) of phenylephrine increased with age from 3 to 10 weeks, but decreased thereafter from 10 to 80 weeks, while the affinity (pKA value) of phenylephrine to alpha 1-adrenoceptors did not alter with aging. 2. The potency (pD2 value) of serotonin did not alter with aging. 3. There is no significant difference between slopes of regression lines between a cytosolic free Ca2+ level ([Ca2+]i) and tension in the presence of phenylephrine in aorta strips from 10- and 60-week-old rats, suggesting that the sensitivity of contractile system to Ca2+ did not alter with aging. 4. Effect of ryanodine on the transient increase of [Ca2+]i and the followed sustained contraction induced by phenylephrine or serotonin in Ca2+ free solution did not alter with aging.

Receptor classification using antagonists without determining pA2 values

Albeit time-consuming, the most rigorous method of classification of receptor subtypes is the determination of pA2 values. However, this methodology requires the knowledge of absolute concentrations of both agonists and antagonists--something that may not necessarily be calculable under certain experimental conditions. Wolfgang Kromer offers a different method for classification using antagonists which is not dependent on absolute affinities, and which has the additional advantage of speed.

Effects of aging on postsynaptic alpha 1-adrenoceptor mechanisms in rat aorta

1. Effects of aging on alpha 1-adrenoceptor and S2-serotonin receptor mechanisms in rat aorta were studied. 2. In rat aorta, the potency (pD2 value) of norepinephrine or phenylephrine increased with age from 3 to 10 weeks, but decreased thereafter with age from 10 to 80 weeks. The affinity (pKA value) of norepinephrine or phenylephrine and of prazosin (pA2 value) did not alter with aging. 3. In rat vas deferens, the efficacy of norepinephrine and the maximum binding sites of [3H]prazosin increased with age from 3 to 18 weeks, but decreased thereafter with age from 18 to 60 weeks. The affinity (pKA value) of norepinephrine and the dissociation constant (KD value) of prazosin did not alter with aging. 4. In rat aorta, the potency (pD2 value) and affinity (pKA value) of serotonin, and affinity (pA2 value) of ketanserin did not alter with aging. 5. There is no significant difference between slopes of regression lines between a cytosolic free Ca2+ level [( Ca2+]i) and tension in the presence of phenylephrine in aorta strips from 10- and 60-week-old rats. 6. These results suggest that changes in alpha 1-adrenoceptor mechanisms with aging are due to changes in receptor density or receptor reserve, but not to changes in affinity of drugs to alpha 1-adrenoceptor or sensitivity of contractile system to Ca2+ mediated through alpha 1-adrenoceptor, and that S2-serotonin receptor mechanisms in rat aorta do not alter with aging.

Antagonism of kinin-induced contraction of isolated rat uterus by the crude hydroalcoholic extract from Mandevilla illustris

1. The crude aqueous/alcoholic extract (CE) of Mandevilla illustris (Apocynaceae) rhizomes was analysed against contractile response elicited by bradykinin (BK), lysyl-bradykinin (L-BK), oxytocin(Ot), acetylcholine (Ach), angiotensin II (AII) and barium chloride (BaCl2) in the isolated uterus of the rat. 2. The CE of this plant (0.5-2.0 mg/ml) caused a parallel and concentration-related rightward displacement of BK and L-BK contractile responses. Schild plot revealed a linear relationship (r close to one) and yielded nominal PA2 values of 3.6 and 3.2 respectively, but the slopes were significantly different from unity. 3. However, the anti-BK action of the CE of M. illustris was not selective to kinin action, since in the same range concentration the CE also affected uterine contractile responses induced by Ot, Ach, AII and BaCl2.

Pharmacological interaction of opiates with various classes of centrally acting dopaminergic drugs

The comparative analgesic and sedative (narcosis potentiating) efficacy of mu and kappa opioids was studied as a function of time in rats and mice. The mu agonists, morphine and fentanyl, produced antinociceptive actions against both heat and chemical noxious agents, but the half-lives of their ED50s were longer in the writhing than in the hot plate test. The kappa agonist drugs, bremazocine, ethylketocyclazocine and pentazocine, proved to be inactive against heat nociception, and produced a potent, long-lasting analgesia in the acetic acid writhing test, similar to mu agonists. The combination of two mu agonists resulted in a synergistic interaction and a remarkable prolongation of antinociceptive action. When the kappa-drug bremazocine was coadministered with morphine, there was a significant prolongation of the duration of analgesic action, without any influence on the potency. The interactions of mu and kappa opioids with agonists and antagonists at dopamine receptors were also studied in narcosis. The time course of the naloxone-morphine antagonism in analgesiometric assays revealed similarities, when apparent pA2 values were estimated at the peak of agonist and antagonist activity, but it was different in the writhing test when the pA2 was determined 84 minutes after morphine administration (EDt1/2, half-life belonging to the ED50) while naloxone was given at its peak effect.

Selectivity profile of the novel muscarinic antagonist UH-AH 37 determined by the use of cloned receptors and isolated tissue preparations

1. Functional in vitro experiments were carried out to determine the antimuscarinic potencies of the pirenzepine derivative UH-AH 37 (6-chloro-5,10-dihydro-5-[(1-methyl-4-piperidinyl)acetyl]-11H-dibenzo- [b,e] [1,4] diazepine-11-one hydrochloride) at M1 muscarinic receptors of rabbit vas deferens, M2 receptors of rat left atria and M3 receptors of rat ileum. Furthermore, N-[3H]-methylscopolamine competition binding experiments were performed to obtain its affinities for the five cloned human muscarinic receptors (m1-m5) stably expressed in CHO-K1 cells. Pirenzepine served as a reference drug throughout all experiments. 2. In all preparations used, UH-AH 37 interacted with muscarinic receptors in a fashion characteristic of a simple competitive antagonist. 3. In the functional studies, UH-AH 37, like pirenzepine, showed high affinity for M1 (pA2 8.49) and low affinity for M2 muscarinic receptors (pA2 6.63). In contrast to pirenzepine, UH-AH 37 also displayed high affinity for M3 receptors (pA2 8.04). 4. In agreement with its functional profile, UH-AH 37 bound with highest affinity to m1 (pKi 8.74) and with lowest affinity to m2 receptors (pKi 7.35). Moreover, it showed a 7 fold higher affinity for m3 (pKi 8.19) than for m2 receptors, whereas pirenzepine bound to both receptors with low affinities. 5. The binding affinity of UH-AH 37 for m4 and m5 receptors (pKi 8.32 for both receptors) was only ca. 2.5 fold lower than that for m1 receptors, while the corresponding affinity differences were 6 and 13 fold in case of pirenzepine. 6. In conclusion, the receptor selectivity profile of UH-AH 37 differs clearly from that of its parent compound, pirenzepine, in both functional and radioligand binding studies, the major characteristics being its pronounced M2 (m2)/M3 (m3) selectivity. UH-AH 37 thus represents a useful tool for the further pharmacological characterization of muscarinic receptor subtypes.

Effect of ageing on response to nicotine in rabbit bronchial preparation

1. Effect of ageing on the response to nicotine was tested in the bronchial muscle preparations from 5, 13, 100 and 125 week-old rabbits. The pD2 value (potency) of nicotine significantly increased in the preparation from the 125 week-old rabbits. No age-related change was found in the pD2 value of carbamylcholine or pA2 value of atropine. 2. No age-related change in characteristics of nicotine receptors. Choline acetyltransferase activity, the amount of acetylcholine released by nicotine and acetylcholineesterase activity decreased in the preparations from the 125 week-old rabbits. 3. Decrease in the pD2 value of nicotine in the preparation from the older rabbit is due to a decline in choline acetyltransferase activity followed by a reduction in the acetylcholine released, and not to a change in characteristics of nicotine receptors. 4. These results also suggest that enzymes may be influenced more easily with age than drug receptors.

Dual antagonism by L-636,499 of serotonin and thromboxane A2 induced aggregation of canine platelets

We evaluated the ability of L-636,499 (3-carboxyl-dibenzo-[b,f] thiepen-5,5-dioxide), a compound structurally similar to cyproheptadine, to antagonize U46619 (a TXA2/PGH2 mimetic) and serotonin (5-hydroxytryptamine; 5HT)-induced aggregation of canine platelets in vitro. L-636,499 antagonized competitively and dose-dependently aggregation induced by both 5HT and U46619, with pA2 values of 5.8 +/- 0.6 and 4.8 +/- 0.2, respectively. L-670,596, a potent TXA2/PGH2 receptor antagonist, and ketanser in, a potent 5HT2 receptor antagonist, yielded pA2 values of 7.0 +/- 0.3 and 9.0 +/- 0.2 vs. their respective agonists. These results show that despite its low potency vs. TXA2- and 5HT2-induced aggregation, L-636,499 antagonizes both physiologic mediators comparably.

Spantide II, a novel tachykinin antagonist having high potency and low histamine-releasing effect

Two undecapeptide substance P (SP) analogues, Spantide I and Spantide II, were tested for their capacity to block the contractile effect of SP on the guinea pig isolated taenia coli and the contractile effect of electrical stimulation of the rabbit isolated (and atropinized) iris sphincter, and for their capacity to mobilize histamine from rat isolated peritoneal mast cells. Spantide I and Spantide II have one feature in common, namely D-tryptophan in positions 7 and 9. Spantide I: D-Arg, Pro2, Lys3, Pro4, Gln5, Gln6, D-Trp7, Phe8, D-Trp9, Leu10, Leu11-NH2. Spantide II: D-NicLys1, Pro2, 3-Pal3, Pro4, D-Cl2Phe5, Asn6, D-Trp7, Phe8, D-Trp9, Leu10, Nle11-NH2. Both Spantide I and II were found to be competitive antagonists to SP on the taenia coli and to be capable of blocking the electrically induced non-cholinergic contraction of the iris sphincter. Spantide II had higher pA2 value (taenia coli) than Spantide I, 7.7 versus 7.0, and higher pIC50 value (blockade of tachykinin-mediated neurotransmission in iris sphincter), 6.0 versus 5.1. Both Spantide I and II mobilized histamine from rat peritoneal mast cells but Spantide II was less effective. Spantide I and II were tested for antagonistic specificity. Both blocked contractions of the taenia induced by SP and neurokinin A. In the concentration used, Spantide II in addition blocked the response to neurokinin B. The contractions induced by carbachol, 5-hydroxytryptamine, histamine and prostaglandins (F2 alpha and E1) were not affected; the contractile response to bombesin was inhibited by Spantide I but not by Spantide II.

Photoaffinity receptor antagonist for human platelet thromboxane A2/prostaglandin H2 receptors

9,11-Dimethylmethano -11,12-methano-16-(3-azido-4-iodophenoxy)-13,14- dihydro-13-aza-15 alpha beta-omega-tetranor TXA2 (I-PTA-PON3) was synthesized and evaluated as a potential photoaffinity probe of the human platelet thromboxane A2/prostaglandin H2 (TXA2/PGH2) receptor. I-PTA-PON3 inhibited the aggregation of washed human platelets induced by the TXA2 mimetic U46619 [(15S)-hydroxy-11 alpha, 9 alpha-(epoxymethano)prosta-5Z,13E-dienoic acid]. Schild analysis of the data revealed a Kd of 9.5 nM and a slope not significantly different from -1. Equilibrium binding studies using [125I]PTA-OH, a TXA2/PGH2 receptor antagonist, showed that I-PTA-PON3 plus photolysis resulted in a 52% reduction in the number of binding sites (1252 +/- 202/platelet) compared to the nonphotolyzed group (2557 +/- 293/platelet) (N = 5, P less than 0.05) with no significant change in the Kd. Repetition of the incubation with I-PTA-PON3 and photolysis a second time resulted in a further 77% (578 +/- 163 binding sites/platelet) reduction in the number of binding sites. Incubation of washed human platelets with I-PTA-PON3 (163 nM) followed by photolysis and removal of the non-covalently bound I-PTA-PON3 resulted in no change in the EC50 value for the TXA2 mimetic, U46619, when compared to controls that were either exposed to I-PTA-PON3 and not photolyzed or exposed only to photolysis. The second photolysis of I-PTA-PON3 resulted in a significant 42% increase in the EC50 value of U46619-induced aggregation compared to the non-photolyzed group (N = 4, P less than 0.05). These results suggest that I-PTA-PON3 is a useful probe for the study of TXA2/PGH2 receptors and that spare TXA2/PGH2 receptors may exist in the platelet.

Competitive antagonists discriminate between NK2 tachykinin receptor subtypes

1. We have compared the ability of various tachykinins and selective tachykinin receptor agonists to induce contraction of the endothelium-denuded rabbit pulmonary artery (RPA) and hamster trachea (HT) and have estimated the affinity of some newly developed NK2 selective antagonists in the same tissues. 2. In confirmation of previous findings, experiments with the agonists indicated that NK2 receptors are the main if not the sole mediators of the response to tachykinins in both RPA and HT. No evidence for significant degradation of neurokinin A (NKA) was found in either tissue when experiments were repeated in the presence of a mixture of peptidase inhibitors (thiorphan, captopril and bestatin, 1 microM each). 3. The peptide antagonists tested were: Peptide I = [Tyr5, D-Trp6,8,9, Arg10]-NKA(4-10); Peptide II = [Tyr5, D-Trp6,8,9, Arg10]-NKA(3-10); Peptide III = Ac-Leu-Asp-Gln-Trp-Phe-Gly-NH2. The three peptides produced a concentration-dependent rightward shift of the concentration-response curve to NKA in both RPA and HT with no significant depression of the maximal response attainable. The slopes of the Schild plots were not significantly different from unity, indicating a competitive antagonism. Peptides I and II were about 100 times more potent in the RPA than in the HT, while Peptide III was about 100 times more potent in the HT than RPA. 4. The pA2 values obtained in these two tissues with the three antagonists were not significantly different when tested in the absence or presence of peptidase inhibitors, or when a selective NK2 receptor agonist, [beta Ala8]-NKA(4-10) was used instead of NKA. Similar pA2 values were obtained after 15 or 90min of incubation with the antagonists. Peptides I, II and III had no inhibitory effect on contractions produced by noradrenaline in the RPA or by carbachol in the HT. 5. Peptides I, II and III showed weak or no antagonistic activity toward the vasodilatator effect of substance P in the dog carotid artery (NK, receptor-mediated) or toward the contractile effect of neurokinin B in the rat portal vein (NK3 receptor-mediated). 6. These results provide pharmacological evidence for heterogeneity of NK2 receptors in the RPA and HT. The NK2 receptors present in these tissues are not discriminated by natural tachykinins or selective agonists, but are recognized with very different affinity by NK2 receptor antagonists.

Effect of aging on the response of guinea pig trachea to isoprenaline

To study the effect of aging on the beta-adrenergic receptor potency of isoprenaline, tracheas from 3-, 6-, 10-, 40- and 100-week-old guinea pigs were used as test tissues. The pD2 values (potency) of isoprenaline increased with age in tracheas treated with corticosterone, but decreased in untreated tracheas. The pA2 value of propranolol against isoprenaline estimated in the treated tracheas did not change with age. Specific binding of [3H]-dihydroalprenolol to the microsomal fractions from the tracheal muscles from 6-, 10- and 40-week-old guinea pigs was analyzed with Scatchard plots. The capacity of maximum binding sites of [3H]-dihydroalprenolol increased with increasing age, while its dissociation constant did not change. Age-related increase in the potency of isoprenaline in the tracheas treated with corticosterone is considered to be due to the increase in the total amount (density) of beta-adrenoceptors. However, the potency of the drug in untreated tracheas decreased with age. These results suggest that extraneuronal uptake plays an important role in the beta-adrenergic action of isoprenaline in older (40 and 100 weeks) guinea pigs.

Telenzepine enantiomers block muscarinic M1-receptors with opposite kinetics

Stimulation of muscarinic M1-receptors in isolated rabbit vas deferens by McN-A-343 inhibited electrically induced twitch contractions susceptible to competitive blockade by (+)-, (+/-), (-)-telenzepine and pirenzepine (pA2 = 9.12, 8.86, 6.98 and 7.79, respectively). The inhibition of twitch contractions by 10(-6) M McN-A-343 (EC70-80) was reversed by the antimuscarinic agents (at concentrations 10-fold higher than pA2) in a time-dependent manner. The antagonists were then displaced by 3 X 10(-5) M McN-A-343 (30-fold EC70-80), which again led to inhibition of twitch contractions. Assuming first-order kinetics for M1-receptor blockade by the antagonists, t1/2 values for the start and end of blockade were calculated. For (+)-telenzepine, the t1/2 values for the rates for the start and end of blockade were 23 and 174 min, respectively, whereas (-)-telenzepine exhibited an inverse kinetic pattern of 3.0 and 0.38 min, respectively. The extremely slow dissociation of (+)-telenzepine from muscarinic M1-receptors may explain the long-lasting pharmacological effect of this compound in vivo.

Adenosine modulation of resting vascular tone in rabbit skeletal muscle

The effects of adenosine and adenosine antagonists on arteriolar vessel diameters were studied in the rabbit tenuissimus muscle in situ by means of intravital microscopy. Topically applied adenosine dose-dependently dilated transverse and terminal arterioles, an effect which was competitively antagonized by alkylxanthines. Topically applied alkylxanthines per se evoked dose-dependent vasoconstriction with the same potency order as for their adenosine antagonism. Furthermore, the concentrations of alkylxanthines eliciting vasoconstriction correlated with the observed pA2 values for their adenosine antagonism. The data indicate a physiological role of adenosine, or other purines, as regulator of resting vascular tone in mammalian skeletal muscle.

Opioid agonist affinity in the guinea-pig ileum and mouse vas deferens

The affinity of morphine, normorphine, methadone, Tyr-D-Ala-Gly-MePhe-NH(CH2)2(N-O)(CH3)2 (RX 783030), [D-Ala2,D-Leu5]enkephalin (DADLE), ketazocine and ethylketocyclazocine (EKC) were determined for their pharmacological receptors in two bioassay tissues, the guinea-pig ileum and the mouse vas deferens (MVD). The method involved the use of the irreversible antagonist, beta-chlornaltrexamine (beta-CNA), and the method of partial receptor blockade. The agonist concentration-effect curves were displaced to the right with decreasing maximum effect, a pattern typical of partial, irreversible blockade of receptors. The concentrations of beta-CNA required to produce a rightward displacement in the concentration-effect curves for different agonists, ranged between 2 and 3000 nM. No similarity was found between the IC50 and the dissociation constant (KA), values predicted to be equivalent only if a linear relationship exists between receptor occupation and observed effect; the dissociation constant for the agonists were between 3 and 218 times larger than the IC50 values. When methadone was used as the agonist in the guinea-pig ileum, beta-CNA produced parallel displacement of the concentration-effect curve, regardless of the blocking concentration chosen, preventing the determination of KA for this agonist, in this tissue; this problem was not encountered in the mouse vas deferens. The KA of morphine, RX 783030 and ketazocine were found not to differ in the guinea-pig ileum and mouse vas deferens. As expected, DADLE had significantly different affinity in the two tissues, showing 117-fold lower affinity in the guinea-pig ileum. Surprisingly, the normorphine affinity was found to be 7-fold higher in the guinea-pig ileum. While the difference in affinity of DADLE may be due to the suggested lack of functional delta receptors in the guinea-pig ileum, the difference in affinity seen with normorphine, but not morphine, in the two tissues is difficult to explain. Taken together with the insensitivity of methadone to beta-CNA blockade in the guinea-pig ileum, but not mouse vas deferens, the difference in the affinity of normorphine in these tissues may suggest the possibility of differences in local milieu of mu receptors or of mu receptor subtypes in the two tissues. The results provide fundamental information regarding opioid agonist affinity in two standard bioassays in vitro, and support the view of (1) a difference in receptors activated by DADLE in the guinea-pig ileum (mu) and mouse vas deferens (delta), as well as (2) possible differences in mu-receptors in these tissues.

Muscarinic suppression of the evoked N-wave by oxotremorine-M recorded in the guinea-pig olfactory cortex slice

The effect of the muscarinic agonist oxotremorine-M has been studied on the surface-negative field potential (N-wave) evoked by orthodromic stimulation of the lateral olfactory tract in slices of guinea-pig olfactory cortex. Bath-application of oxotremorine-M (5-80 microM) or carbachol (10-300 microM) produced a reversible depression of the N-wave amplitude without affecting the lateral olfactory tract compound action potential. Oxotremorine-M was approximately 5 times more potent than carbachol in this respect, and the effects of both agonists were competitively blocked by telenzepine (5-100 nM), a selective M1-receptor antagonist. In contrast, methoctramine or AF-DX 116, two 'cardioselective' M2-receptor antagonists, had little or no blocking effect on the agonist responses. It is suggested that oxotremorine-M (like carbachol) inhibits the evoked field potential by activating presynaptic M1-type muscarinic receptors in the olfactory cortex slice.

Stereoselective inhibition of muscarinic receptor subtypes by the enantiomers of hexahydro-difenidol and acetylenic analogues

1. The affinities of the (R)- and (S)-enantiomers of hexahydro-difenidol (1) and its acetylenic analogues hexbutinol (2), hexbutinol methiodide (3) and p-fluoro-hexbutinol (4) (stereochemical purity greater than 99.8%) for muscarinic receptors in rabbit vas deferens (M1), guinea-pig atria (M2) and guinea-pig ileum (M3) were measured by dose-ratio experiments. 2. The (R)-enantiomers consistently showed higher affinities than the (S)-isomers. The stereoselectivity ratios [(R)/(S)] were greatest with the enantiomers of 1 (vas deferens: 550; ileum: 191; atria: 17) and least with those of the p-Fluoro-analogue 4 (vas deferens: 34; ileum: 8.5; atria: 1.7). 3. The enantiomeric potency ratios for compounds 1-4 were highest in rabbit vas deferens, intermediate in guinea-pig ileum and much less in guinea-pig atria. Thus, these ratios may serve as a predictor of muscarinic receptor subtype identity. 4. (S)-p-Fluoro-hexbutinol [(S)-4] showed a novel receptor selectivity profile with preference for M3 receptors: M3 greater than M2 greater than or equal to M1. 5. These results do not conform to Pfeiffer's rule that activity differences between enantiomers are greater with more potent compounds.

Stereoselectivity in beta-adrenomimetic and beta-adrenolytic actions of carteolol, a beta-adrenoceptor blocker with intrinsic sympathomimetic action in guinea-pig taenia caecum

1. The beta-adrenomimetic and beta-adrenolytic activities of S(-) and R(+) isomers of carteolol, a beta-adrenergic partial agonist (a beta-adrenoceptor blocker with intrinsic sympathomimetic action) were tested in the guinea-pig taenia caecum. 2. The beta-adrenoceptor blocking activities (pA2 values) of S(-) and R(+) isomers of carteolol were significantly larger than the corresponding beta-adrenomimetic activities (pD2 values), supporting our views that beta-adrenoceptors contain two different binding sites; high and low affinity sites. 3. In beta-adrenoceptor blocking action S(-) carteolol was about 10 times as potent as R(+) carteolol while beta-adrenomimetic action of S(-) carteolol was about 2 times as potent as that of R(+) carteolol. Further, intrinsic activity for S(-) carteolol was slightly but significantly larger than that for R(+) carteolol. 4. These results suggest that the binding site for competitive antagonism between S(-) isoprenaline and S(-), R(+) and RS(+/-) carteolol is more stereoselective than the binding site to induce beta-adrenomimetic action.

Blockade of kinin-induced responses of the guinea-pig isolated urinary bladder by the extract of Mandevilla velutina

1. The crude extract (CE) from M. velutina (0.2-0.8 mg/ml), antagonized in a dose-dependent and reversible manner bradykinin (BK)- and lysyl-BK-induced contractions of the guinea-pig urinary bladder in vitro. 2. Schild plots yielded nominal pA2 values (as g/ml) of 3.5 and 3.7, but the slopes were significantly greater than unity. 3. At concentrations 2-10-fold higher, the CE failed to affect responses to acetylcholine or histamine. 4. In electrically-stimulated tissue, BK, L-BK and M-L-BK potentiated nerve-mediated twitch responses and increased muscular tone. The B1 agonist [des-Arg9]-BK, was inactive up to 10 microM. 5. The CE from M. velutina (0.4 mg/ml) inhibited both actions of the kinins.

Cyproheptadine displays high affinity for muscarinic receptors but does not discriminate between receptor subtypes

The affinity of cyproheptadine for different muscarinic receptor subtypes was investigated in vitro by functional experiments in field-stimulated vas deferens of the rabbit (ganglionic M1- and cardiac M2-receptors) and in guinea-pig ileum (smooth muscle M3-receptors). Cyproheptadine displayed high but similar affinity for all muscarinic receptor subtypes studied (pA2 = 7.99-8.02). In contrast, (+)-telenzepine (M1 over M2 and M3), and for mefurtramine (M2 over M3 and M1) were found to be selective.

Chronic opioid antagonist treatment: assessment of receptor upregulation

Changes in specific brain opioid binding and opioid pharmacodynamics were determined in mice treated with the opioid antagonist naltrexone (subcutaneously implanted pellets) for 8 days. Chronic opioid antagonist treatment increased the number of binding sites (upregulation) for [3H]naloxone (+55%) and [3H][D-Ala2, D-Leu5]enkephalin (+41%) but did not alter the affinity of the ligands, as determined in saturation studies. Displacement studies of [3H]naloxone by morphine also indicated that there was no change in morphine's affinity. In vivo estimation of naloxone affinity (pA2), agreed with the in vitro results indicating that chronic naltrexone treatment did not alter naloxone affinity. Chronic naltrexone treatment (0.5, 1.0, 15.0 mg pellets) increased the analgesic potency of morphine (supersensitivity) in a dose-dependent manner, up to a maximal increase in relative potency of 1.8. However, in mice tested with the naltrexone pellets still implanted, the 15 mg naltrexone pellet was able to shift the dose-response function for morphine analgesia more than 300-fold. The lowest dose naltrexone pellet (0.5 mg), produced significant antagonism of morphine analgesia, but did not produce significant supersensitivity. Thus, supersensitivity and upregulation are not proportional to the degree of antagonism of opioid effects; and supersensitivity in the mouse is related to increased binding sites and not to changes in receptor affinity as determined by in vivo and in vitro methods.

Affinity profiles of BTM-1086 and BTM-1041 at muscarinic receptor subtypes and at H1- and alpha 1-receptors

The affinities of the (+) and (-) enantiomers of the antimuscarinic benzothiazepinone derivative, cis-2,3-dihydro-3-(4-methyl-1-piperazinylmethyl)-2-phenyl-1,5-benzoth iazepin-4 (5H)-one (BTM-1041 and BTM-1086), for muscarinic receptor subtypes, histamine H1-receptors and alpha 1-adrenoceptors were determined in vitro using isolated organs: field-stimulated rabbit vas deferens (M1-receptors), guinea-pig left atrium (M2-receptors), guinea-pig ileum (M3- and histamine H1-receptors) and rat vas deferens (alpha 1-adrenoceptors). We also assessed the binding profile of BTM-1041 and BTM-1086 at muscarinic receptor subtypes in guinea-pig cortex (M1), heart (M2) and salivary glands (M3) as well as at alpha 1-adrenoceptors in rat cerebral cortex. Functional and binding experiments showed that the (-) enantiomer (BTM-1086) had a high affinity (pA2 = 7.98-8.81; pKi = 8.31-9.15) for the three muscarinic receptor subtypes, whereas the (+) enantiomer (BTM-1041) showed a low antimuscarinic potency (pA2 = 4.87-5.31; pKi = 4.85-5.55). This results in an extremely high stereoselectivity for these optical isomers [-)/(+) ratios = 1023 to 6918). The affinity of the (-) enantiomer BTM-1086 was lower for both histamine H1- and alpha 1-receptors than for muscarinic receptors, whereas the reverse was true for the (+) enantiomer, BTM-1041. Thus, the stereochemical demands for the two optical isomers were most stringent at muscarinic receptors but were inverse and less pronounced at histamine H1- and alpha 1-receptors (stereoselectivity ratios = 0.16-0.22).

Muscarinic suppression of the M-current in the rat sympathetic ganglion is mediated by receptors of the M1-subtype

1. Under voltage-clamp dissociated adult and foetal rat superior cervical ganglion (s.c.g.) cells exhibited a non-inactivating voltage- and time-dependent component of K+ current termed the M-current (IM). IM was detected and measured from the current decay during hyperpolarizing voltage steps applied from potentials where IM was pre-activated. 2. Neither the resting membrane current nor the amplitude of these current decay relaxations were reduced by omitting Ca from the bathing fluid, showing that the M-current was not a 'Ca-activated' K-current dependent on a primary Ca-influx. Concentrations of (+)-tubocurarine sufficient to block the slow Ca-activated K-current IAHP did not inhibit IM or antagonize the effect of muscarinic agonists on IM, showing that IM was not contaminated by IAHP. Tetraethylammonium (1 mM), which blocks the fast Ca-activated K-current IC, produced a small inhibition of IM. This was not due to contamination of IM by IC since muscarinic agonists did not consistently block IC. 3. The muscarinic agonists muscarine, oxotremorine, McN-A-343 and methacholine reversibly suppressed IM, resulting in an inward (depolarizing) current. The rank order of potency was: oxotremorine greater than or equal to muscarine greater than McN-A-343 greater than methacholine. 4. The suppression of IM by muscarine was similar in cultured cells derived from adult and foetal tissue to that seen in the intact ganglia. 5. IM-suppression by muscarine was inhibited by pirenzepine (Pz) and AF-DX 116 with mean pKB values of 7.53 +/- 0.13 (n = 3) and 6.02 +/- 0.13 (n = 4) respectively. 6. The suppression of IM by muscarinic agonists was not affected by gallamine (10-30 microM). 4-Diphenylacetoxy-N-methylpiperidine methiodide inhibited the response at 300 nM. 7. Pirenzepine inhibited the contractions of the guinea-pig isolated ileum produced by muscarine with a mean pKB of 6.37 +/- 0.03 (n = 8). 8. These results suggest that the receptors mediating suppression of the M-current accord with those designated pharmacologically as M1 and that these receptors reach maturity at a very early stage in the development of the rat s.c.g.

Effects of ONO-3708, an antagonist of the thromboxane A2/prostaglandin endoperoxide receptor, on blood vessels

The pharmacological properties of a novel thromboxane A2/prostaglandin endoperoxide receptor antagonist, ONO-3708, on blood vessels were examined in vitro and in vivo. ONO-3708, 10 microM, inhibited the rabbit aorta contractions induced by thromboxane A2, prostaglandin H2, 11,9-epoxymethano-prostaglandin H2 (U-46619) or prostaglandin F2 alpha without affecting the contractions induced by angiotensin II, serotonin or norepinephrine. ONO-3708, at a concentration of 1 to 100 nM, appeared to be a competitive inhibitor of the contractile responses of the canine basilar artery to 9,11-epithio-11,12-methano-thromboxane A2 (STA2), U-46619 and PGF2 alpha, and a non-competitive inhibitor of the contractile responses to 15-hydroperoxy-eicosatetraenoic acid (15-HPETE). In in vivo studies, ONO-3708 (10 and 100 micrograms/kg per min i.v.) ameliorated the decrease in diameter of the basilar artery induced by the i.v. infusion of STA2 (0.1 microgram/kg per min) in cats. Furthermore, infusion of ONO-3708 (10 and 30 micrograms/kg per min i.v.) prevented the cerebral vasospasm in an experimental subarachnoid hemorrhage model in dogs. These results indicate that ONO-3708 is a potent antagonist of the thromboxane A2/prostaglandin endoperoxide receptor in vitro and in vivo and may be of therapeutic use in preventing cerebral vasospasm.

Affinity profiles of hexahydro-sila-difenidol analogues at muscarinic receptor subtypes

In an attempt to assess the structural requirements of hexahydro-sila-difenidol for potency and selectivity, a series of analogues modified in the amino group and the phenyl ring were investigated for their affinity to muscarinic M1-(rabbit vas deferens), M2- (guinea-pig atria) and M3- (guinea-pig ileum) receptors. All compounds were competitive antagonists in the three tissues. Their affinities to the three muscarinic receptor subtypes differed by more than two orders of magnitude and the observed receptor selectivities were not associated with high affinity. The pyrrolidino and hexamethyleneimino analogues, compounds substituted in the phenyl ring with a methoxy group or a chlorine atom as well as p-fluoro-hexahydro-difenidol displayed the same affinity profile as the parent compound, hexahydro-sila-difenidol: M1 approximately M3 greater than M2. A different selectivity pattern was observed for p-fluoro-hexahydro-sila-difenidol: M3 greater than M1 greater than M2. This compound exhibited its highest affinity for M3-receptors in guinea-pig ileum (pA2 = 7.84), intermediate affinity for M1-receptors in rabbit vas deferens (pA2 = 6.68) and lowest affinity for the M2-receptors in guinea-pig atria (pA2 = 6.01). This receptor selectivity profile of p-fluoro-hexahydro-sila-difenidol was confirmed in ganglia (M1), atria (M2) and ileum (M3) of the rat. Furthermore, dose ratios obtained with either pirenzepine (M1) or hexahydrosila-difenidol (M2 and M3) and the p-fluoro analogue used in combination suggested that the antagonism was additive, implying mutual competition with a single population of muscarinic receptor subtypes. These results indicate that p-fluoro-hexahydro-sila-difenidol represents a valuable tool for characterization of muscarinic receptor subtypes.

Binding and functional properties of antimuscarinics of the hexocyclium/sila-hexocyclium and hexahydro-diphenidol/hexahydro-sila-diphenidol type to muscarinic receptor subtypes

1. In an attempt to assess the structural requirements for the muscarinic receptor selectivity of hexahydro-diphenidol (hexahydro-difenidol) and hexahydro-sila-diphenidol (hexahydro-sila-difenidol), a series of structurally related C/Si pairs were investigated, along with atropine, pirenzepine and methoctramine, for their binding affinities in NB-OK 1 cells as well as in rat heart and pancreas. 2. The action of these antagonists at muscarinic receptors mediating negative inotropic responses in guinea-pig atria and ileal contractions has also been assessed. 3. Antagonist binding data indicated that NB-OK 1 cells (M1 type) as well as rat heart (cardiac type) and pancreas (glandular/smooth muscle type) possess different muscarinic receptor subtypes. 4. A highly significant correlation was found between the binding affinities of the antagonists to muscarinic receptors in rat heart and pancreas, respectively, and the affinities to muscarinic receptors in guinea-pig atria and ileum. This implies that the muscarinic binding sites in rat heart and the receptors in guinea-pig atria are essentially similar, but different from those in pancreas and ileum. 5. The antimuscarinic potency of hexahydro-diphenidol and hexahydro-sila-diphenidol at the three subtypes was influenced differently by structural modifications (e.g. quaternization). Different selectivity profiles for the antagonists were obtained, which makes these compounds useful tools to investigate further muscarinic receptor heterogeneity. Indeed, the tertiary analogues hexahydro-diphenidol (HHD) and hexahydro-sila-diphenidol (HHSiD) had an M1 = glandular/smooth muscle greater than cardiac selectivity profile, whereas the quaternary analogues HHD methiodide and HHSiD methiodide were M1 preferring (M1 greater than glandular/smooth muscle, cardiac).

Photoaffinity labelling of the human platelet thromboxane A2/prostaglandin H2 receptor

The synthesis, binding and photoincorporation of a thromboxane A2/prostaglandin H2 (TXA2/PGH2) analog (9,11-dimethylmethano-11,12-methano-16-(3-[125I]iodo-4-azidophenyl )-13,14- dihydro-13-aza-15 alpha beta-omega-tetranor-TXA2) [( 125I]PTA-Azido) to washed human platelets was characterized. Kinetic analysis of the binding of [125I]PTA-Azido at 30 degrees C yielded a k1 of 1.83.10(7) M-1.min-1 and k -1 of 0.195 min-1, Kd = k -1/k1 = 11 nM. Incubation of washed human platelets with [125I]PTA-Azido followed by photolysis resulted in the radiolabelling of a number of platelet proteins as assessed by SDS-PAGE autoradiography. The radiolabelling of three of these protein bands could be either uniformly blocked or reduced with a series of structurally dissimilar TXA2/PGH2 receptor antagonists or agonists and corresponded to proteins with a molecular mass of 43, 39 and 27 kDa. In addition, the incorporation of [125I]PTA-Azido into the three proteins was stereoselectively blocked by a pair of optically active stereoisomers that are TXA2/PGH2 receptor antagonists. Two-dimensional gel electrophoresis indicated that the 43 kDa protein possessed a pI value of 5.6 and that the 27 kDa protein exists in at least three isoforms with pI values of 4.9, 5.1 and 5.3. The labelling pattern was not altered by a mixture of proteinase inhibitors. The data suggest that one or more of these specifically radiolabelled proteins may represent the human platelet TXA2/PGH2 receptor.

The affinity, selectivity and biological activity of telenzepine enantiomers

The binding of the enantiomers of telenzepine to muscarinic receptor subtypes present in guinea-pig cerebral cortex, myocardium and salivary glands has been examined. The (+) enantiomer is more potent in all assays and exhibits a greater selectivity than the (-) enantiomer for the different receptor subtypes. As a consequence, the enantiomeric potency ratio varies from ca. 400 (cortical 'M1' receptors) to ca. 50 (cardiac receptors). In functional assays in vitro in the rabbit vas deferens and rat atria, the affinity constants and enantiomeric potency ratios for the two isomers agree with those found for the appropriate muscarinic receptor subtype in binding assays. A high enantiomeric potency ratio, 180, is found in vivo for the ability of the telenzepine enantiomers to inhibit the production of lesions in the modified Shay rat preparation. The data are compatible with the blockade of M1 receptors by (+)-telenzepine being responsible for this action of telenzepine and would tend to exclude the possibility that the anti-ulcer action of telenzepine is mediated via a muscarinic or non-muscarinic action of the (-) enantiomer.

Can equal pA2 values be compatible with receptor differences?

Situations exist in which pA2 analysis leads to conclusions about receptor differentiation that conflict with those drawn from other convincing lines of evidence. Robert Raffa and colleagues address this issue in its broadest context, particularly in relationship to the concept of accessory binding sites, using the controversy of delta-opioid receptor-mediated antinociception as an example. When the possibility of accessory binding sites is considered, it becomes clear that the finding of equal pA2 values does not necessarily mean action at the same receptor, and that the finding of unequal pA2 values does not necessarily confirm action at different receptors.

Effects of ONO-3708, an antagonist of the thromboxane A2/prostaglandin endoperoxide receptor, on platelet aggregation and thrombosis

The beneficial effects of an antagonist of the thromboxane A2/prostaglandin endoperoxide receptor, 7-[2 alpha,4 alpha-(dimethylmethano)-6 beta-(2-cyclopentyl-2 beta- hydroxyacetamido)-1 alpha-cyclohexyl]-5(Z)-heptenoic acid (ONO-3708) on thrombosis were examined. ONO-3708 at 0.1-3 microM inhibited the human platelet aggregation induced by thromboxane A2, prostaglandin H2, collagen, ADP (secondary phase) and epinephrine (secondary phase) without affecting prostanoid synthesis and the content of cyclic AMP in platelets. The in vivo effects, on coronary thrombosis in this case, were examined in two canine models. ONO-3708, 3 to 300 micrograms/kg i.v., prevented dose dependently the coronary thrombosis induced by partial obstruction of the coronary artery. ONO-3708, 3 micrograms/kg per min i.v., significantly prevented electrically stimulated coronary thrombosis without affecting systemic blood pressure and heart rate. These results indicate that the thromboxane A2/prostaglandin endoperoxide receptor could play an important role in the pathogenesis of thrombosis and that ONO-3708 may have therapeutic advantages in preventing thrombosis.

Evidence for alpha 1-adrenoceptor subtype predominance in the rat seminal vesicle

Noradrenaline (0.6-19 microM) and phenylephrine (2-130 microM) induced contractions in the rat seminal vesicle that were competitively antagonized by the alpha 1-adrenoceptor-selective antagonist corynanthine (120-920 nM). Yohimbine (60-450 nM), an alpha 2-adrenoceptor-selective antagonist, produced a non-competitive antagonism of noradrenaline responses, suggesting that the responses were not alpha 2-adrenoceptor mediated. It is concluded that the rat seminal vesicle has a predominance of alpha 1-adrenoceptors.

The effect of strontium on the drug-receptor interaction on cholinergic drugs

The effect of replacing calcium with strontium in the perfusion fluid was qualitatively and quantitatively studied in the isolated longitudinal muscle of the guinea-pig ileum. In the presence of strontium hyoscine could be considered a competitive antagonist of acetylcholine for the acetylcholine receptor of the longitudinal muscle of the guinea-pig ileum; dibenamine still blocked this receptor in an irreversible way. The equilibrium constants for acetylcholine (KA) and hyoscine (KI) were obtained in the presence of calcium (KA = 3.16 +/- 0.63 microM; KI = 0.38 +/- 0.07 nM), and strontium (KA = 7.00 +/- 0.89 microM; KI = 0.93 +/- 0.16 nM). The results show a decrease in the affinity of both drugs for the muscarinic receptor in the presence of strontium.