Leanness in postnatally nutritionally programmed rats is associated with increased sensitivity to leptin and a melanocortin receptor agonist and decreased sensitivity to neuropeptide Y.

BACKGROUND: Pups of normally nourished dams that are cross-fostered after birth to dams fed a low-protein (8% by weight) diet (postnatal low protein (PLP)) grow slower during the suckling period and remain small and lean throughout adulthood. At weaning, they have increased expression in the arcuate nucleus (ARC) of the hypothalamus of the orexigenic neuropeptide Y (NPY) and decreased expression of pro-opiomelanocortin, the precursor of anorexigenic melanocortins. OBJECTIVES AND METHODS: We investigated, using third ventricle administration, whether 3-month-old male PLP rats display altered sensitivity to leptin with respect to food intake, NPY and the melanocortin 3/4-receptor agonist MTII, and using in situ hybridization or laser capture microdissection of the ARC followed by RT-PCR, whether the differences observed were associated with changes in the hypothalamic expression of NPY or the leptin receptor, NPY receptors and melanocortin receptors. RESULTS: PLP rats were smaller and had reduced percentage body fat content and plasma leptin concentration compared with control rats. Leptin (5 µg) reduced food intake over 0-48 h more in PLP than control rats (P<0.05). Submaximal doses of NPY increased the food intake less in PLP rats than in controls, whereas submaximal doses of MTII reduced the food intake more in PLP rats. Maximal responses did not differ between PLP and control rats. Leptin and melanocortin-3 receptor (MC3R) expression were increased in both ARC and ventromedial hypothalamic nuclei in PLP animals compared with the controls. MC4R, NPY Y1R, Y5R and NPY expression were unchanged. CONCLUSION: Postnatal undernourishment results in food intake in adult rats being more sensitive to reduction by leptin and melanocortins, and less sensitive to stimulation by NPY. We propose that this contributes to increased leptin sensitivity and resistance to obesity. Increased expression of ObRb and MC3R may partly explain these findings but other downstream mechanisms must also be involved.

A new, highly selective murine peroxisome proliferator-activated receptor δ agonist increases responsiveness to thermogenic stimuli and glucose uptake in skeletal muscle in obese mice.

AIM: We investigated how GW800644, the first pharmacologically selective murine peroxisome proliferator-activated receptor δ (PPARδ) agonist, affects energy balance, glucose homeostasis and fuel utilization by muscle in obese mice. METHODS: Potencies were determined in transactivation assays. Oral glucose tolerance was determined after 14 and 22 days' administration (10 mg/kg body weight, twice daily) to Lep(ob)/Lep(ob) mice. Food intake and energy expenditure were measured during a 26-day experiment, and plasma metabolites and 2-deoxyglucose uptake in vivo at termination. Palmitate oxidation and 2-deoxyglucose uptake by isolated soleus muscles were measured after 14 (in lean and obese mice) and 26 days. RESULTS: GW800644 activated murine PPARδ (EC(50) 2 nM), but caused little to no activation of PPARα or PPARγ up to 10 µM. It did not increase liver weight. GW800644 reduced food intake and body weight in obese mice after 8 days. It did not affect resting energy expenditure, but, compared to pair-fed mice, it increased the response to a ß(3)-adrenoceptor agonist. It improved glucose tolerance. GW800644, but not pair-feeding, reduced plasma glucose, insulin and triglyceride concentrations. It increased 2-deoxyglucose uptake in vivo in adipose tissue, soleus muscle, heart, brain and liver, and doubled 2-deoxyglucose uptake and palmitate oxidation in isolated soleus muscle from obese but not lean mice. CONCLUSIONS: PPARδ agonism reduced food intake and independently elicited metabolic effects that included increased responsiveness to ß(3)-adrenoceptor stimulation, increased glucose utilization and fat oxidation in soleus muscle of Lep(ob)/Lep(ob) but not lean mice and increased glucose utilization in vivo in Lep(ob)/Lep(ob) mice.

Metabolic responses to BRL37344 and clenbuterol in soleus muscle and C2C12 cells via different atypical pharmacologies and beta2-adrenoceptor mechanisms.

BACKGROUND AND PURPOSE: Picomolar concentrations of the beta3-adrenoceptor agonist BRL37344 stimulate 2-deoxyglucose uptake in soleus muscle via undefined receptors. Higher concentrations alter uptake, apparently via beta2-adrenoceptors. Effects of BRL37344 and beta2-adrenoceptor agonists are compared. EXPERIMENTAL APPROACH: Mouse soleus muscles were incubated with 2-deoxy[1-(14)C]-glucose, [1-(14)C]-palmitate or [2-(14)C]-pyruvate, and BRL37344, beta2-adrenoceptor agonists and selective beta-adrenoceptor antagonists. Formation of 2-deoxy[1-(14)C]-glucose-6-phosphate or (14)CO2 was measured. 2-Deoxy[1-(14)C]-glucose uptake and beta-adrenoceptor mRNA were measured in C2C12 cells. KEY RESULTS: 10 pM BRL37344, 10 pM clenbuterol and 100 pM salbutamol stimulated 2-deoxyglucose uptake in soleus muscle by 33-54%. The effect of BRL37344 was prevented by 1 microM atenolol but not by 300 nM CGP20712A or IC3118551, or 1 microM SR59230A; that of clenbuterol was prevented by ICI118551 but not atenolol. 10 nM BRL37344 stimulated 2-deoxyglucose uptake, whereas 100 nM clenbuterol and salbutamol inhibited uptake. These effects were blocked by ICI118551. Similar results were obtained in C2C12 cells, in which only beta2-adrenoceptor mRNA could be detected by RT-PCR. 10 nM BRL37344 and 10 pM clenbuterol stimulated muscle palmitate oxidation. In the presence of palmitate, BRL37344 no longer stimulated 2-deoxyglucose uptake and the effect of clenbuterol was not significant. CONCLUSIONS AND IMPLICATIONS: Stimulation of glucose uptake by 10 pM BRL37344 and clenbuterol involves different atypical pharmacologies. Nanomolar concentrations of BRL37344 and clenbuterol, probably acting via beta2-adrenoceptors, have opposite effects on glucose uptake. The agonists preferentially stimulate fat rather than carbohydrate oxidation, but stimulation of endogenous fat oxidation cannot explain why 100 nM clenbuterol inhibited 2-deoxyglucose uptake.

Hypothalamic orexin expression: modulation by blood glucose and feeding.

Orexins (hypocretins), novel peptides expressed in specific neurons of the lateral hypothalamic area (LHA), stimulate feeding when injected intracerebroventricularly. We investigated their role in feeding in the rat by measuring hypothalamic prepro-orexin mRNA levels under contrasting conditions of increased hunger. Prepro-orexin mRNA levels increased significantly after 48 h of fasting (by 90-170%; P < 0.05) and after acute (6 h) hypoglycemia when food was withheld (by 90%; P < 0.02). By contrast, levels were unchanged during chronic food restriction, streptozotocin-induced diabetes, hypoglycemia when food was available, voluntary overconsumption of palatable food, or glucoprivation induced by systemic 2-deoxy-D-glucose. Orexin expression was not obviously related to changes in body weight, insulin, or leptin, but was stimulated under conditions of low plasma glucose in the absence of food. Orexins may participate in the short-term regulation of energy homeostasis by initiating feeding in response to falls in glucose and terminating it after food ingestion. The LHA is known to contain neurons that are stimulated by falls in circulating glucose but inhibited by feeding-related signals from the viscera; orexin neurons may correspond to this neuronal population.

Hypoglycemia activates orexin neurons and selectively increases hypothalamic orexin-B levels: responses inhibited by feeding and possibly mediated by the nucleus of the solitary tract.

Orexins are novel appetite-stimulating peptides expressed in the lateral hypothalamic area (LHA), and their expression is stimulated by hypoglycemia in fasted rats. We investigated activation of orexin and other neurons during insulin-induced hypoglycemia using the immediate early gene product Fos. Insulin (50 U/kg) lowered plasma glucose by >50% after 5 h and stimulated feeding sixfold compared with saline-injected controls. Hypoglycemic rats allowed to feed and normoglycemic controls both showed sparse Fos-positive (Fos+) neurons in the LHA and the paraventricular nucleus (PVN) and arcuate nucleus (ARC) and showed none in the nucleus of the solitary tract (NTS), which relays visceral feeding signals to the LHA. In the LHA, total numbers of Fos+ neurons were comparable in fed hypoglycemic and control groups (60 +/- 6 vs. 52 +/- 4 cells/mm2, P > 0.05), as were Fos+ neurons immunoreactive for orexin (1.4 +/- 0.4 vs. 0.6 +/- 0.4 cells/mm2, P > 0.05). By contrast, hypoglycemic rats that were fasted showed significantly more Fos+ nuclei in the LHA (96 +/- 10 cells/mm2, P < 0.05, vs. both other groups) and Fos+ orexin neurons (8.4 +/- 3.3 cells/mm2, P < 0.001, vs. both other groups). They also showed two- to threefold more Fos+ nuclei (P < 0.001) in the PVN and ARC than both fed hypoglycemic rats and controls and showed strikingly abundant Fos+ neurons in the NTS and dorsal motor nucleus of the vagus. In parallel studies, whole hypothalamic orexin-A levels were not changed in hypoglycemic rats, whether fasted or freely fed, whereas orexin-B levels were 10-fold higher in hypoglycemic fasted rats than in control and hypoglycemic fed groups. These data support our hypothesis that orexin neurons are stimulated by falling glucose levels but are readily inhibited by signals related to nutrient ingestion and suggest that they may functionally link with neuronal activity in the NTS. Orexin-A and -B may play specific roles in behavioral or neuroendocrine responses to hypoglycemia.

Interactions between leptin and hypothalamic neuropeptide Y neurons in the control of food intake and energy homeostasis in the rat.

Leptin acts on the brain to inhibit feeding, increase thermogenesis, and decrease body weight. Neuropeptide Y (NPY)-ergic neurons of the hypothalamic arcuate nucleus (ARC) that project to the paraventricular nuclei (PVN) and dorsomedial nuclei (DMH) are postulated to control energy balance by stimulating feeding and inhibiting thermogenesis, especially under conditions of energy deficit. We investigated whether leptin's short-term effects on energy balance are mediated by inhibition of the NPY neurons. Recombinant murine leptin (11 microg) injected into the lateral ventricle of fasted adult Wistar rats inhibited food intake by 20-25% between 2 and 6 h after administration, compared with saline-treated controls (P < 0.05). Uncoupling protein mRNA levels in brown adipose tissue (BAT) rose by 70% (P < 0.01). Leptin treatment significantly reduced NPY concentrations by 20-50% (P < 0.05) in the ARC, PVN, and DMH and significantly decreased hypothalamic NPY mRNA levels (0.61 +/- 0.02 vs. 0.78 +/- 0.03 arbitrary units; P < 0.01). A second study examined changes in leptin during 5 days' intracerebroventricular NPY administration (10 microg/day), which induced sustained hyperphagia and excessive weight gain. In NPY-treated rats, leptin mRNA levels in epididymal fat were comparable to those in saline-treated controls (0.94 +/- 0.17 vs. 1.0 +/- 0.28 arbitrary units; P > 0.1), but plasma leptin levels were significantly higher (4.88 +/- 0.66 vs. 2.85 +/- 0.20 ng/ml; P < 0.01). Leptin therefore acts centrally to decrease NPY synthesis and NPY levels in the ARC-PVN projection; reduced NPY release in the PVN may mediate leptin's hypophagic and thermogenic actions. Conversely, NPY-induced obesity results in raised circulating leptin concentrations. Leptin and the NPY-ergic ARC-PVN neurons may interact in a homeostatic loop to regulate body fat mass and energy balance.

Anti-obesity drugs: a critical review of current therapies and future opportunities.

The last 25 years have seen a great increase in the incidence of obesity, both in the Western world and in developing third world countries. Despite the seeming inexorable progression of this disease, there have been limited advances in the pharmacotherapy of this condition. Of the newest introductions to the obesity drug portfolio, orlistat, which acts to prevent dietary fat absorption, and sibutramine, which seems to affect both arms of the energy balance equation, were the first new chemical entities to be introduced for the treatment of obesity in 30 years. In this article, we review these and other agents available in various countries for the treatment of obesity. Perhaps more importantly, we have focussed on areas of potential productivity in the future. The huge recent increase in our knowledge in this area has largely stemmed from discovery research at the genomics level. Over the last 5 or so years, this impetus in obesity research has provided us with exciting new drug targets involved in the regulation of feeding behaviour and cellular mechanisms involved in energy expenditure. Compared with the last 25 years, the future offers more hope.

Brown adipose tissue in the parametrial fat pad of the mouse.

Cold acclimation has been shown to produce a substantial increase in the number of brown adipocytes in the parametrial fat pad of female BALB/c mice-a site normally thought to consist of typical white adipocytes. The brown adipocytes have been identified not only on the basis of their morphology using light and electron microscopy, but also on the basis of the content of the mitochondrial 'uncoupling protein' (Mr = 32000) which is characteristic of the proton conductance pathway of brown adipose tissue.

Leptin treatment increases suppressors of cytokine signaling in central and peripheral tissues.

Leptin concentrations are elevated in the majority of obese individuals raising the possibility that leptin resistance contributes to their obesity. Peripheral leptin administration for 48 h caused a several-fold increase in mRNA encoding the suppressors of cytokine signaling SOCS-3 and CIS in hypothalamus and peripheral tissues. Paradoxically, CIS and SOCS-3 mRNAs are also elevated in the leptin-deficient ob/ob mouse. Forced expression of CIS in insulinoma cells prevented transactivation mediated by leptin. Thus tissues continuously exposed to leptin and/or other factors associated with obesity accumulate excessive amounts of SOCS-3 and CIS which could provide a potential mechanism for leptin resistance.

The contribution of increased thermogenesis to the effect of anorectic drugs on body composition in mice.

The study investigated whether changes in body composition of normal and genetically obese C57BL/6J (ob/ob) mice caused by the anorectic drugs phentermine, diethylpropion, fenfluramine, and mazindol are entirely due to reduced food intake. Mice were dosed daily (25 mg/kg po) for 28 days after which time carcass composition was determined. Compared to controls fed at libitum, reductions in food intake were for phentermine, 7%; fenfluramine, 17%; diethylpropion, 17%, whereas reduction in body lipid content were for phentermine, 16%; mazindol, 18%; fenfluramine, 8%; diethylpropion, 10%. Since diet restriction by 22% (in the absence of treatment with any drug) resulted in a body lipid content 12% below that of controls fed ad libitum, these results suggest that some of the lipid loss caused by phentermine and possibly mazindol is due to increased energy expenditure. In support of this conclusion, phentermine and mazindol increased energy expenditure in normal mice by 35% compared to untreated controls in the 6 h after dosing but diethylpropion and fenfluramine had little or no effect. Determination of the carcass composition of the normal mice confirmed that phentermine has a metabolic antiobesity effect. Fenfluramine had an unexpected effect on carcass composition in normal mice.

Thermogenic and antiobesity activity of a novel beta-adrenoceptor agonist (BRL 26830A) in mice and rats.

The effects of a novel compound, BRL 26830A, on energy balance in normal and obese mice have been investigated. BRL 26830A reduced body weight or weight gain in genetically (ob/ob), goldthioglucose, and cafeteria diet obese mice and genetically obese (fa/fa) Zucker rats. Weight reduction was due to reduced body lipid content. BRL 26830A caused little or no reduction in food intake in these animals but it increased metabolic rate and in genetically obese mice this thermic effect was increased by repeat dosing. BRL 26830A did not reduce body weight gain in the lean counterparts of the genetically obese animals. Its thermic effect was smaller in the lean than the genetically obese mice and it caused an increase in food intake in the lean mice. The thermic effect of BRL 26830A was inhibited by dl- but not d-propranolol. BRL 26830A largely overcame the depression in metabolic rate caused by fasting.

A simple apparatus for comparative measurements of energy expenditure in human subjects: the thermic effect of caffeine.

A simple inexpensive indirect calorimeter that is suitable for the estimation of energy expenditure in man is described. Its usefulness is demonstrated by a study of the effect of coffee on energy expenditure. Caffeinated coffee increased energy expenditure by 16% over 1 2-h period compared with decaffeinated coffee.

BRL 35135, a potent and selective atypical beta-adrenoceptor agonist.

BRL 35135, via its active deesterified metabolite BRL 37344, is a potent example of a new group of beta-adrenoceptor agonists that stimulate selectively a novel beta adrenoceptor that was originally shown to be present in brown adipose tissue in rodents. BRL 35135 produces a dose-related increase in energy expenditure in rodents and, in genetically obese (ob/ob) mice, a dose of 0.5 mg.kg-1.d-1 has significant antiobesity activity. This weight loss is entirely due to loss of fat; muscle protein is preserved. In studies in nonobese men, BRL 35135 (0.1 mg/kg) increased both resting metabolic rate and the thermic response to a glucose load. BRL 35135 is effective in improving glucose tolerance in genetically obese (ob/ob) mice and obese Zucker (fa/fa) rats at doses that have no significant antiobesity activity. The improved glucose tolerance is the result of significant improvement in insulin sensitivity. In 10-d studies in obese and diabetic patients, BRL 35135 produced improvements in glucose tolerance and insulin sensitivity.

Fatty acids inhibit leptin signalling in BRIN-BD11 insulinoma cells.

The effect of treatment with a 0.03% fatty acid (FA) cocktail on leptin-receptor-mediated STAT (signal transducers and activators of transcription) activation in the rat insulinoma cell line BRIN-BD11 was investigated. Leptin (10 nM) stimulated the tyrosine phosphorylation of STAT3 and STAT5b. Acute treatment with FAs prevented leptin-stimulated STAT3 tyrosine phosphorylation and significantly raised basal STAT5 phosphorylation. A chronic treatment (5 days) of BRIN-BD11 cells with FAs similarly attenuated leptin-stimulated STAT tyrosine phosphorylation. Chronic FA treatment also attenuated prolactin-stimulated STAT5b tyrosine phosphorylation but not interleukin-6-stimulated STAT3 tyrosine phosphorylation, suggesting that the effect is receptor/ligand specific. TaqMan analysis of gene expression following chronic FA treatment showed neither a decrease in the amount of leptin receptor (Ob-R) mRNA, nor an increase in the negative regulators of STAT signalling, SOCS3 (suppressors of cytokine signalling) or cytokine inducible sequence (CIS). These data demonstrate that FAs modulate leptin and prolactin signalling in beta-cells, implying that high levels of circulating FAs present in obese individuals affect the action of selective cytokines in beta-cell function.

Relaxant activity of 4-amido-3,4-dihydro-2H-1-benzopyran-3-ols and 4-amido-2H-1-benzopyrans on guinea pig isolated trachealis.

A series of 4-amido-3,4-dihydro-2H-1-benzopyran-3-ols and 4-amido-2H-1-benzopyrans related to the potassium channel activator cromakalim have been prepared and evaluated for their relaxant activity in guinea pig isolated tracheal spirals. Several analogues show enhanced relaxant activity relative to cromakalim in this preparation and the rank order of potency for those substituents investigated at C-6 was CF3 greater than CN greater than C2H5 greater than aza greater than or equal to CH3. One compound, trans-3,4-dihydro-2,2-dimethyl-4-(2-oxopiperidin-1-yl)-7-(trifluor omethyl)-2H- 1-benzopyran-3-ol (24), was resolved into its two enantiomers and the activity was shown to reside essentially in the (+)-isomer, adding further support to the suggestion that the smooth muscle receptor for these potassium channel activators is stereoselective.

Inhibition of cyclic nucleotide phosphodiesterase by derivatives of 1,3-bis(cyclopropylmethyl)xanthine.

Alkylation of the selective type IV phosphodiesterase inhibitor, 8-amino-1,3-bis(cyclopropylmethyl)-xanthine (1, BRL 61063), led exclusively to the N-7 substituted derivatives 2-9, which showed varying selectivities for the PDE type IV isoenzyme relative to PDE Va. The 4-methoxybenzyl derivative 6 in particular was a highly potent PDE Va inhibitor (IC50 0.14 microM) and showed a 24-fold selectivity for this isoenzyme relative to PDE IV. Sulfonation of 1 was more complex, with the product profile being highly dependent on the reaction conditions. As with alkylation, sulfonation at N-7 generally increased potency against PDE Va, especially in the aryl-containing moieties lacking strongly electron-withdrawing substituents (12, 15-17, 19). Bis-arylsulfonation at the exocyclic amino group generally reduced inhibitory potency against both PDE IV and Va. An 8-amidino compound 33, formed by the unusual reaction of 1 with N-methylpyrrolidinone in the presence of benzenesulfonyl chloride, had an IC50 value of 0.05 microM against PDE Va and is believed to be the most potent inhibitor of this isoenzyme reported. No correlation of PDE IV inhibition with displacement of [3H]rolipram from its high-affinity binding site was demonstrated. This suggests that either the catalytic site and the rolipram binding site are not the same or that PDE IV can exist in two conformations, only one of which binds to rolipram with high affinity, and that the compounds described vary in their selectivity for this isoform.

Synthesis and smooth muscle relaxant activity of a new series of potassium channel activators: 3-amido-1,1-dimethylindan-2-ols.

The synthesis of a novel series of smooth muscle relaxants which have been shown to act via the opening or activation of potassium channels is described. Compounds have been evaluated for their ability to inhibit spontaneous tone in guinea pig isolated trachealis and structure-activity relationships are discussed. One compound in particular, 1,1-dimethyl-5-nitro-3-(2-pyridon-1-yl)indan-2-ol, (16) was identified as a potent relaxant of airways smooth muscle in vitro with IC50 = 0.15 microM and was found to significantly inhibit histamine-induced dyspnoea in conscious guinea pigs when given orally 30-45 min prior to challenge.

Relaxant activity of 6-cyano-2,2-dimethyl-2H-1-benzopyran-4-carboxamides and -thiocarboxamides and their analogues in guinea pig trachealis.

Structural modifications of the potassium channel activator cromakalim (1) are described in which the amide moiety at C-4 has been replaced by carboxamide and thiocarboxamide functions. Analogues in which the hydroxyl group at C-3 has been oxidized or removed are also disclosed. Such analogues display an interesting profile of smooth muscle relaxant activity in the guinea pig isolated trachea, not all of which appears to result from the opening of potassium channels, but few compounds retain useful in vivo activity. However, one compound in particular, 6-cyano-2,2-dimethyl-N-methyl-2H-1-benzopyran-4-thiocarboxamide (13) was shown to be a potent potassium channel activator in vitro and to provide prolonged protection to guinea pigs from the respiratory effects of inhaled histamine.

Comparative effects of BRL 38227, nitrendipine and isoprenaline on carbachol- and histamine-stimulated phosphoinositide metabolism in airway smooth muscle.

1. The ability of BRL 38227 and nitrendipine to affect muscarinic agonist and histamine-stimulated [3H]-inositol phosphate accumulation in slices of bovine tracheal smooth muscle has been studied and compared with the established inhibitory effects of isoprenaline on this pathway. 2. Pre-addition of BRL 38227 (5 microM), nitrendipine (1 microM) or isoprenaline (10 microM) significantly inhibited the subsequent inositol phosphate response to histamine at all concentrations studied (10- 1000 microM). BRL 38227 and nitrendipine also significantly inhibited the [3H]-inositol phosphate response to low (1 microM), but not high (100 microM) concentrations of carbachol. Isoprenaline had no effect at any concentration of carbachol studied. 3. Nitrendipine (IC50 = 95 nM) and BRL 38227 (IC50 = 322 nM) caused concentration-related inhibitions of the inositol phosphate response to histamine (100 microM). Similar maximal inhibitions were caused by each agent (55-58%). Inhibitory effect of BRL 38227 was reduced in potency (IC50 = 5.5 microM), but not magnitude, in the presence of glibenclamide (0.5 microM). 4. Time-course studies comparing the effects of BRL 38227 addition 15 min before, and 10 min after histamine challenge showed that for pre-addition a distinct (less than 2 min) lag occurred following histamine addition before the inhibitory effect of BRL 38227 was manifest. In contrast, when BRL 38227 was added 10 min after histamine, an inhibitory effect was immediately apparent. 5. Further evidence for an initial, 'protected' phase of inositol phosphate accumulation was provided by the finding that BRL 38227 pre-addition had no effect on the early (0-300 s) time-course of inositol 1,4,5-trisphosphate mass accumulation. 6. The inhibitory effect of BRL 38227, but not that of nitrendipine or isoprenaline, on histaminestimulated [3H]-inositol phosphate accumulation was completely prevented in the presence of an elevated extracellular K+ (65 mM) concentration. 7. The results demonstrate that membrane hyperpolarization, and/or blockade of voltage-operated Ca2"-channels can regulate agonist-stimulated phosphoinositide metabolism in airway smooth muscle. The possible contribution of this regulatory mechanism to the relaxant properties of these agents is discussed.

Comparison of the airways relaxant and hypotensive potencies of the potassium channel activators BRL 55834 and levcromakalim (BRL 38227) in vivo in guinea-pigs and rats.

1. BRL 55834, a novel potassium channel activator, has been compared with levcromakalim (BRL 38227) for its relaxant effects in vivo on the airways and vasculature of the guinea-pig and rat. 2. When administered intravenously 2 min prior to challenge, BRL 55834 and levcromakalim each inhibited histamine-induced increases in airways resistance (Raw) in the anaesthetized guinea-pig, with BRL 55834 showing a 4.5 fold greater potency than levcromakalim (ED25 = 2.5 micrograms kg-1 and 11.3 micrograms kg-1 respectively). By contrast, both compounds had similar hypotensive potencies (ED18 = 8.5 micrograms kg-1 and 6.5 micrograms kg-1 respectively). 3. In the same guinea-pig model, intraduodenally administered BRL 55834 (100 and 250 micrograms kg-1) and levcromakalim (500 micrograms kg-1) each protected against histamine-induced changes in Raw and dynamic lung compliance (Cdyn), both compounds showing a rapid onset of action that persisted for more than 50 min. The lower dose of BRL 55834 had a similar bronchodilator effect to that of levcromakalim, yet both doses of BRL 55834 elicited substantially smaller effects than levcromakalim on mean arterial blood pressure. 4. In the anaesthetized rat, BRL 55834 and levcromakalim each evoked a dose-related inhibition of inhaled methacholine-induced changes in Raw and Cdyn when given i.v., with BRL 55834 showing some four fold greater potency than levcromakalim (BRL 55834: Raw ED35 = 3.7 micrograms kg-1, Cdyn ED35 = 5.9 micrograms kg-1; levcromakalim: Raw ED35 = 16 micrograms kg-1, Cdyn ED35 = 23.5 micrograms kg-1). As in the guinea-pig,BRL 55834 had a reduced propensity to lower mean arterial blood pressure (ED11 = 8 microg kg-1 for BRL55834, 11 +/- 3% being its maximum effect; ED11= 16 microg kg-1, maximum effect= 34 +/- 6% for levcromakalim.5. When administered intraduodenally to anaesthetized rats, BRL 55834 (10, 20 and 100 microg kg-1)evoked rapid and dose-related inhibitions of methacholine-induced Raw and Cdyn changes which persisted for over 30 min. At the lower and middle dose there was little effect on mean arterial blood pressure(<10% fall). Levcromakalim (500 microg kg-1) by contrast elicited transient airways responses that diminished rapidly after 5 min, while the effects on blood pressure were well maintained (>20% at 65 min). Levcromakalim (100 microg kg-1) did not affect airways responses but also evoked a marked and sustained fall in blood pressure.6. BRL 55834, administered per os, prolonged the time to histamine-induced dyspnoea in conscious guinea-pigs. The greatest effect of BRL 55834 was observed when it was administered 60 min prior to challenge, a dose of 0.20 mg kg-1 doubling the mean time to collapse. A similar level of protection was afforded by levcromakalim (1.25 mg kg-1), with maximal activity occurring between 30 and 60 min.7. The present studies in guinea-pigs and rats indicate that BRL 55834 is the first potassium channel activator to exhibit greater bronchodilator potency than levcromakalim but reduced tendency to lower arterial blood pressure. It is suggested that BRL 55834 may have greater potential than levcromakalim as a bronchodilator for therapeutic use in man.