Effects of cyproheptadine on insulin secretion by isolated perifused rat islets.

The effects of cyproheptadine on basal and glucose-induced insulin release by isolated rat islets was studied by use of a perifusion system. A forty-five minute preincubation of islets with a medium containing both 34.3 mug./ml. (10(-4) M) cyproheptadine and 1.0 mg./ml. glucose completely abolished the biphasic pattern of increased insulin secretion normally obtained after islets are stimulated with a medium containing 3.0 mg./ml. glucose. In another series of experiments, similar results were obtained when the cyproheptadine and 3.0 mg./ml. glucose were presented together. Here, however, the inhibition of the first phase of insulin secretion did not achieve statistical significance and some recovery of the islets' secretory capacity was observed late during the second phase. In studies designed to investigate the influence of cyproheptadine on basal insulin secretion, no obvious effect was observed. These results are discussed in relation to the species-specific alterations in pancreatic beta-cell morphology that have been reported in rats after the oral administration of cyproheptadine.

Proposals for the classification and nomenclature of functional receptors for 5-hydroxytryptamine.

As a result of controversy in the literature regarding the classification and nomenclature of functional receptors for 5-hydroxytryptamine (5-HT), a framework for classification is proposed. The formulation of these proposals has only been made possible by the recent advent of new drug tools. It is considered that there are three main types of 5-HT receptor, two of which have been well characterised pharmacologically, using selective antagonists, and which it is proposed to name 5-HT2 and 5-HT3. These two groups broadly encompass the "D" and "M" receptors, respectively, which Gaddum identified in the guinea-pig ileum (Gaddum and Picarelli, 1957). The 5-HT2 receptor, which mediates a variety of actions of 5-HT, has been definitively shown to correlate with the 5-HT2 binding site in the brain. No binding studies in brain tissue have yet been published with radiolabelled ligands specific for 5-HT3 receptors. A number of other actions of 5-HT appear to be mediated via receptors distinct from 5-HT2 or 5-HT3 receptors. Since selective antagonists are not yet available, these receptors cannot be definitively characterised, although in many cases they do have some similarities with 5-HT1 binding sites, which are a heterogeneous entity. Criteria are proposed for tentatively classifying these receptors as "5-HT1-like" (Table 1). Definitive characterisation of these receptors will await the identification of specific antagonists. This classification of 5-HT receptors into three main groups (Table 1) is based largely, but not exclusively, on data from studies in isolated peripheral tissues where definitive classification is possible. However, it is believed that this working classification will be relevant to functional responses to 5-HT in the central nervous system.

The calcium antagonistic effects of cyproheptadine on contraction, membrane electrical events and calcium influx in the guinea-pig taenia coli.

1 The ability of cyproheptadine (Cph) to inhibit membrane translocation of calcium in smooth muscle was investigated by studying the drug's action on contraction, electrical activity and calcium influx in the guinea-pig taenia coli.2 Cph >/= 10(-6)M reduced the amplitude of normal spontaneous contractions and concurrently decreased the number of action potentials occurring with each slow-wave of depolarization (sucrose-gap recordings). These inhibitory effects of Cph were antagonized by increasing the medium [Ca] three fold to 7.68 mM.3 Intracellular recordings showed that Cph >/= 2 x 10(-6)M decreased the amplitude and extended the duration of the action potential. These effects were only partially reversible in normal medium whereas large overshooting action potentials were again seen in 7.68 mM Ca medium.4 High frequency mechanical activity was produced by inclusion of veratridine 5 x 10(-6)M in the perfusate. Low concentrations of Cph (>/= 10(-7)M) reduced the amplitude of such contractions at a faster rate than they did normal spontaneous contractions.5 At concentrations between 10(-7) and 10(-6)M, Cph fully reduced the tonic component of contractions elicited in 112 mM isotonic KCl whilst having little or no effect on either (i) the initial phasic KCl contraction or (ii) the ;repolarization contracture' normally produced on wash-out of the KCl or (iii) the spontaneous contractions before and after KCl treatment. In contrast, at Cph 2 x 10(-6)M, the repolarization contracture, as well as the isotonic KCl contraction, was totally blocked whereas spontaneous contractions were still unaffected. Progressively higher Cph concentrations inhibited all components of this contractile cycle.6 Dose-response curves for the rate of drug-induced relaxation of tonic contractures produced in hypertonic 42.7 mM high-potassium medium, showed the calcium antagonistic potency of Cph to be intermediate between that of chlorpromazine and D600. The minimum Cph concentration for effect lay between 1 and 5 x 10(-7)M, and the effects of Cph 2 x 10(-6)M (approximately the ID(50)) were totally antagonized by 12.8 mM Ca.7 By means of a lanthanum wash procedure, Cph >/= 2 x 10(-6)M was found to decrease the (45)Ca uptake occurring into strips of taenia coli in normal medium, although the maximum effect (at Cph 10(-5)M) amounted to only 25% inhibition of the uptake occurring into control strips (also found with D600). The increased uptake occurring in hypertonic 44.7 mM high-potassium medium was inhibited in a dose-dependent manner by Cph 1 x 10(-7)M.8 The results are consistent with an action of Cph in reducing the flow of Ca(2+) through voltage-dependent Ca channels in the smooth muscle cell membrane. It is suggested that the interaction of Cph molecules with such sites is dependent upon membrane potential as well as drug concentration.

Mechanism by which cyproheptadine inhibits insulin secretion.

1 Isolated islets of Langerhans from the rat have been used in studies designed to elucidate the mechanism by which cyproheptadine inhibits insulin secretion. 2 D-Glucose and tolbutamide, both of which require extracellular Ca2+ to produce insulin release, failed to evoke a secretory response from islets pretreated with cyproheptadine. Conversely veratridine, the calcium ionophore A23187 and theophylline, all of which are capable of mobilizing sufficient intracellular Ca2+ to evoke insulin secretion in the absence of extracellular Ca2+, produced similar responses from cyproheptadine pretreated and control islets. 3 Cyproheptadine completely inhibited Ca2+ uptake induced by D-glucose and high Ko+, two agents which depolarize the islet beta-cell membrane, whilst Ca2+ uptake elicited by removal of extracellular Na+ (i.e. Na+-Ca2+ counter transport) was only slightly reduced. 4 A significant increase in Na+ uptake produced by veratridine was sensitive to tetrodoxin but only partially reduced by cyproheptadine. 5 These results suggest that cyproheptadine inhibits depolarization-dependent calcium entry into pancreatic beta-cells.

beta-Adrenoceptor subtypes in sections of rat and guinea-pig kidney.

The present autoradiographical study examines the distribution of the two beta-adrenoceptor subtypes in sections of rat and guinea-pig kidney. The radioligand [125Iodo]-(--)-cyanopindolol was used for the labelling of beta-adrenoceptors and the selective beta-adrenoceptor blocking agents ICI 89-406 (beta 1-antagonist) and ICI 118-551 (beta 2-antagonist) were utilized to differentiate both subclasses unequivocally. beta-Adrenoceptors in rat kidney were found to be almost exclusively beta 1. They were located mainly on glomeruli and to a lesser extent on the straight part of the distal tubules and on the cortical portion of the collecting ducts. Some beta 2-adrenoceptors were localized around the corticomedullary junction. Grain localization in the auto-radiograms was absent in the inner medulla and papilla. Glomeruli and distal tubules of the guinea-pig kidney also possess only beta 1-adrenoceptors, but, in contrast to the rat, extremely high concentrations of beta 2-adrenoceptors were associated with the straight part of the proximal tubules in the cortex and possibly with the cortical portion of the collecting duct. Labelling was not detected on the proximal convoluted tubule in either species.

5-Hydroxytryptamine receptor antagonism by metoclopramide and ICS 205-930 in the guinea-pig leads to enhancement of contractions of stomach muscle strips induced by electrical field stimulation and facilitation of gastric emptying in-vivo.

Contractions induced by electrical field stimulation of isolated circular muscle strips, taken from the guinea-pig stomach, were enhanced by metoclopramide, ICS 205-930 and MDL 72222 at concentrations similar to those shown to antagonize at neuronal 5-hydroxytryptamine receptor sites in a variety of preparations. Metoclopramide, MDL 72222 and ICS 205-930 also facilitated gastric emptying in-vivo. The abilities of metoclopramide, MDL 72222 and ICS 205-930 to enhance stomach muscle contraction processes and to facilitate gastric emptying may be the consequence of 5-hydroxytryptamine receptor antagonism.

Gastric ulcer and cancer.

The incidence of gastric carcinoma was studied in a series of 210 patients presenting with apparently benign gastric ulcer. In eight cases (3.9 per cent), carcinoma was diagnosed within 18 months and was almost certainly present from the outset; in 2.4 per cent the diagnosis was delayed for over three months, and the five-year death-rate due to gastric carcinoma was 3.2 per cent. In a mean follow-up period of 5.7 years after the first diagnosis of an ulcer (8.2 years after first symptoms), three fresh cases of gastric carcinoma were found, and in two of these the cancer was at a different site from the ulcer. The five-year incidence of fresh gastric carcinoma was 0.6 per cent. Unsuspected superficial spreading carcinoma was detected by histology in 5.4 per cent of gastrectomy specimens. The significance of these findings is discussed in relation to the management of gastric ulcer and the early diagnosis of cancer.

Effect of leukotriene C4 on electromechanical activity and Ca2+ uptake in taenia coli.

The actions of leukotriene C4 (LTC4) on electromechanical activity and 45Ca2+ uptake in guinea pig taenia coli were investigated. The contractile action of LTC4 was abolished by the removal of extracellular Ca2+. LTC4 concentrations eliciting a maximal contraction in normal medium produced no response in preparations depolarized with KCl. In single sucrose gap studies, LTC4 increased both the frequency of electrical spiking and tension. These effects were blocked by the dihydropyridine Ca2+-channel antagonist PY 108-068 and by the leukotriene receptor antagonist FPL 55712. In double sucrose gap experiments, LTC4 caused a small depolarization without measurable change in membrane conductivity; increased spontaneous electrical activity was again accompanied by an increase in tension. LTC4 caused a detectable increase in 45Ca2+ uptake only at extracellular Ca2+ concentrations less than 1 mM, and this was again inhibited by PY 108-068 or FPL 55712. It is concluded that the contractile effects of LTC4 in guinea pig taenia coli occur as a consequence of its ability to open voltage-sensitive Ca2+ channels, an effect that may occur independently of membrane depolarization.

Pancreatic beta cell changes induced by cyproheptadine in vitro.

The effect of cyproheptadine on the structure and function of rat pancreatic beta cells was studied in vitro by culturing isolated islets in media containing the drug (0.5 mM). After 8 days in culture the ultrastructure of islet cells maintained in control media appeared well preserved, being similar to the previously reported for islets kept in long term organ culture. In contrast beta cells from islets incubated in media containing cyproheptadine appeared degranulated and the rough endoplasmic reticulum showed cisternal dilation and vacuole formation. These vacuoles were filled with an electron-dense granular material and their surface was usually studded with ribosomes. These lesions are identical with those produced by the administration of cyproheptadine to rats in vivo. In addition to these findings, increased numbers of lysosomes and myeloid bodies were observed in both alpha and beta cells. Compared with that of the controls, the 24-hour basal insulin secretion of islets cultured in the presence of cyproheptadine was significantly reduced from the 4th day of the study onwards. It is thus concluded that cyproheptadine has direct effects on the morphology and function of the rat pancreatic beta cell.

Anomalous bronchospasm following inhalation of (+) isoprenaline by asthmatics.

Sympathomimetics (beta-adrenergic agonists) presently used in asthma therapy comprise racemic mixtures of bronchodilator and non-bronchodilator enantiomers. In a randomized, double-blind placebo-controlled study, asthmatic subjects inhaled a nebulized solution of the non-bronchodilator (+) enantiomer of isoprenaline. Substantial decreased forced expiratory volume (FEV1) was detected in 2 patients and of the remaining 8, a single subject had increased reactivity to histamine 7 h after inhalation of (+) isoprenaline. These effects of (+) isoprenaline may contribute to anomalous effects of (+/-) isoprenaline in asthma.

The effects of bronchodilators on spontaneous ventilation and oxygen consumption in rhesus monkeys.

The effects of breathing normal saline, salmeterol, fenoterol, ipratropium bromide, or formoterol, and of i.v. infusion of theophylline on oxygen consumption (VO2), carbon dioxide production (VCO2), minute ventilation (VE), heart and respiratory rates, and end-tidal carbon dioxide tension (P(ET)CO2) have been defined in 10 anesthetized, intubated rhesus monkeys (mean age 7.0 y, weight 10.2 kg). VO2 increased over control by + 17.1% after salmeterol (p < 0.001), +33.3% after fenoterol (p < 0.001), +23.7% after formoterol (p < 0.001), +3.9% after theophylline (p < 0.01), but did not change after ipratropium bromide and normal saline. VE increased by 63.0% after fenoterol (p < 0.001), 49.8% after formoterol (p < 0.001), 31.7% after salmeterol (p < 0.01), and 29.7% after theophylline (p < 0.001), but not after ipratropium bromide or normal saline. Heart rate response was greatest after fenoterol, formoterol, and salmeterol, respectively. P(ET)CO2 dropped dramatically after theophylline (-15.7%, p < 0.001), but not at all with any of the inhaled beta2-adrenoceptor agonists. In seven animals, salbutamol (albuterol) caused an increase in V(E) and VO2 of 50.1% and 45.9%, respectively, whereas in the presence of a beta2-adrenoceptor antagonist [racemic or (+/-)-propranolol (0.1 mg/kg i.v.)], inhaled salbutamol (2.5 mg/mL for 10 min) could not increase V(E) (+6.2%, p > 0.05) and VO2 (+1.6%, p > 0.05). The increase in VO2 and V(E) after administration of beta2-agonists may be partly the result of direct stimulation of the respiratory center and partly a response to increased metabolic rate. The dramatic increase in VO2 and V(E) after salbutamol was suppressed in the presence of propranolol, which is consistent with a beta-receptor-mediated mechanism.

Na+ channels in pancreatic islet cells.

Experiments are reviewed which show that Na+ channels which are pharmacologically identical with those found in spiking neurones are present in the outer membrane of pancreatic beta-cells. In both nerve and beta-cells the neurotoxins veratridine, scorpion toxin, and tetrodotoxin bind to three distinctly separate sites on the sodium channel, the affinity of each of these sites for its respective toxin being similar in the two tissues. The binding sites for veratridine and scorpion toxin exhibit reciprocal, positive, heterotropic cooperativity. The role of these Na+ channels in the physiological regulation of insulin release is discussed.

Identification of serotonin M-receptor subtypes and their specific blockade by a new class of drugs.

We describe a new class of drugs that selectively block serotonin M-receptors on peripheral neurones. Because of their high affinity, some of these drugs are the most potent of any pharmacological class yet reported. They have allowed the identification of three M-receptor subtypes, one of which is responsible for mediating the painful effects of serotonin in humans.

The functional significance of sodium channels in pancreatic beta-cell membranes.

1. The existence and functional significance of Na channels in pancreatic beta-cell membranes were investigated by studying the effects of the plant alkaloid veratridine on the temporal release of insulin from perfused isolated rat islets of Langerhans.2. 100 muM veratridine evoked a sustained threefold increase in insulin release which was almost completely inhibited by 3 muM tetrodotoxin (TTX). This action of TTX was rapidly reversible.3. The simultaneous presence of 100 muM propranolol, 100 muM phenoxy-benzamine and 10 muM atropine did not alter the magnitude of the response to 100 muM veratridine, indicating that the action of veratridine on the beta-cells was direct and was not mediated via the release of neurotrans-mitters from nerve endings within the islets.4. (45)Ca uptake by isolated islets in static incubation was increased almost threefold by 100 muM veratridine. This increase was completely inhibited by the simultaneous presence of 3 muM TTX.5. Replacement of Na(o) by choline caused a transient fourfold increase in insulin release which was associated with an increase in the uptake of (45)Ca from the extracellular space of similar magnitude. Subsequent exposure of islets to 100 muM veratridine still evoked some insulin release but this only achieved 32% of that secreted by islets exposed to veratridine in medium of normal [Na](o).6. The addition of 2.5 mM CoCl(2) to the medium caused a 62.5% inhibition of veratridine-mediated insulin release.7. In Ca-free medium supplemented with 1 mM EGTA, 100 muM veratridine evoked insulin release of equal magnitude and of similar temporal relationship to that obtained in the presence of normal [Ca](o).8. A twofold increase in insulin release that occurred in the 15 min period immediately following exposure to 1 mM ouabain was completely independent of [Ca](o). Subsequent ouabain-evoked release became increasingly dependent on [Ca](o).9. Tetrodotoxin (3 muM) inhibited the first phase of insulin release evoked by 16.7 mMd-glucose by 37% and the second phase by 20%.10. Both Na and Ca appear capable of entering through Na channels opened in the beta-cell membrane by veratridine. The increase in [Na](i), resulting from the veratridine mediated increase in P(Na+), causes depolarization of the beta-cell membrane with a consequent opening of voltage-sensitive, Co(2+)-blockable channels for additional Ca entry. An increase in [Na](i) also increases [Ca](i) by altering the equilibria of intracellular Ca-sequestering mechanisms. The small but significant reduction of glucose-mediated insulin release by TTX indicates that glucose has a rather weak action on the Na channel and a more pronounced effect on the voltage-dependent Co(2+)-blockable Ca channel.