Thiazolidinedione insulin sensitizers and the heart: a tale of two organs?

Thiazolidinediones (TZDs) are relatively new agents for the treatment of type 2 diabetes. They act as agonists at the PPAR-gamma nuclear receptor and their therapeutic effects include decreased insulin resistance and hyperglycaemia, an improved plasma lipid, inflammation and pro-coagulant profile, and amelioration of hypertension, microalbuminuria and hepatic steatosis. The most common side effects of TZDs include weight gain and oedema, with occasional reports of congestive heart failure (CHF). This review discusses the benefit-risk profile of TZDs in treating patients with type 2 diabetes, with particular reference to the heart. To provide context, we explore briefly the epidemiology and pathophysiology of heart failure in patients with type 2 diabetes, touch on the association of heart disease and cardiovascular mortality with antihyperglycaemic treatment modalities other than TZDs, and then focus on the effects of TZDs on the heart, cardiovascular risk factors and outcomes. We describe the cluster of host factors, which seems to predispose patients with type 2 diabetes to TZD-induced or TZD-exacerbated oedema and CHF and then provide an overview of the putative mechanisms of these TZD-related side effects. We also propose that certain diuretics (amiloride and spironolactone), by targeting the distal nephron that expresses PPARgamma in collecting duct cells, might be of benefit in ameliorating the fluid retention and oedema associated with TZDs.

Energy restriction enhances therapeutic efficacy of the PPARgamma agonist, rosiglitazone, through regulation of visceral fat gene expression

AIM

Consumption of a palatable diet can induce hyperphagia, leading to weight gain (dietary obesity) and insulin resistance in rats. Thiazolidinediones (TZDs) can also induce hyperphagia in rats but conversely have an insulin-sensitizing effect. The aim of this study was to investigate whether preventing TZD-induced hyperphagia (i.e. energy restriction) in dietary obese (DIO) rats would enhance the insulin-sensitizing effects of treatment at a therapeutic dose; and, within this paradigm, to produce an original survey of candidate TZD-gene targets in the clinically relevant visceral white adipose tissue (WAT) depot.

METHODS

DIO rats that were either freely fed or energy restricted (i.e. pair-fed to the level of untreated controls) were treated with rosiglitazone maleate (RSG; 3 mg/kg/day) for 2 weeks, the restricted group controlling for treatment-induced hyperphagia and weight gain. The outcome measures were circulating concentrations of various biochemical markers of insulin resistance, and gene expression was measured in epididymal WAT.

RESULTS

In both freely fed and pair-fed groups, compared to untreated DIO controls, RSG reduced plasma levels of insulin (-29% and -43%; p < 0.05 and p < 0.001, respectively), free fatty acids (FFAs; -45% and -48%; p < 0.01 and p < 0.001, respectively) and triglycerides (TGs; -63% and -72%; both p < 0.001), reflected in improved insulin sensitivity, as measured by homeostasis model assessment (-29% and -43%; p < 0.01 and p < 0.0001). RSG also increased the expression of the fatty acid transport/synthesis genes, fatty acid transport protein (2.4-3.2-fold), epidermal fatty acid-binding protein (FABP; 1.7-2.0-fold), heart FABP (25-29-fold) and fatty acid synthase (2.3-2.9-fold; all p < 0.05) in both groups. Adipocyte FABP was also increased by RSG treatment, but only in combination with energy restriction (1.52-fold; p < 0.05) as was hexokinase II expression (p < 0.001). In contrast, the drug had no effect on expression of several genes associated with lipolysis. Although obesity-induced hyperleptinaemia was normalized only in the energy-restricted group, leptin messenger RNA (mRNA) expression was reduced in both treated groups (all p < 0.01). Resistin and tumour necrosis factor-alpha expression was also reduced, though in the latter case, only with energy restriction (p < 0.05). Other adipokines were unaffected by RSG treatment.

CONCLUSION

Our results clearly show that energy restriction enhances the therapeutic efficacy of TZDs and suggest that this occurs, at least in part, through a modulatory effect on gene expression in visceral WAT. These findings improve our understanding of the underlying mechanistic basis for the clinical usefulness of dietary restriction as an adjunct to TZD therapy in type 2 diabetes.

Thiazolidinediones and their Fluid-Related Adverse Effects

Thiazolidinediones (TZDs) or glitazones are agents that are widely used for the treatment of type 2 diabetes mellitus. These drugs have a multitude of therapeutic effects including reduction in insulin resistance and hyperglycaemia, anti-inflammatory effects and amelioration of hypertension, microalbuminuria and hepatic steatosis. The TZD molecular target, peroxisome proliferator-activated receptor gamma (PPARgamma), a nuclear transcription factor, is expressed diffusely in humans, including many tissues comprising the cardiovascular and renal systems. This suggests a potential for TZDs to elicit perturbing effects on these systems, which are independent of their effects on glucose and lipid metabolism. One of the most common adverse effects of TZDs is fluid retention, which can result in, or exacerbate, oedema and congestive heart failure (CHF). The frequency of peripheral oedema is approximately 5% when TZDs are used in mono- or combination oral therapy, and about 15% when used with insulin. Patients with type 2 diabetes are at high risk of myriad morbid complications, including CHF. The development of CHF, particularly in the elderly, is a harbinger of premature mortality. TZD-induced oedema is largely peripheral, may have its origins in changes in haemodynamics, with some contribution from molecules, which regulate cell and tissue permeability (e.g. vascular endothelial growth factor and protein kinase Cbeta), and remains the preponderant manifestation of TZD-induced fluid retention even in those with existing heart failure. Preclinical and pilot clinical data attest to the fact that at least part of the fluid retention derives from a direct effect of TZDs on sodium reabsorption via the renal medullary collecting duct, a mechanism that is sensitive to diuretic agents that have this nephron segment as their site of action, in whole or in part (spironolactone, amiloride and hydrochlorothiazide). Our review suggests various potential clinical strategies by which TZD-induced fluid retention might be effectively monitored and addressed.

Thiazolidinediones: Pleiotropic drugs with potent anti-inflammatory properties for tissue protection

Peroxisome proliferator-activated receptor-gamma (PPARgamma) is a nuclear transcription factor that comprises the primary molecular target for thiazolidinedione (TZD) insulin-sensitizing drugs. Whilst expressed in many tissues in humans, its abundant expression in adipose tissue is believed to be the focal point through which TZDs regulate genes involved in glucose and lipid metabolism and via which these agents ultimately improve the hyperglycemia of type 2 diabetes. However, TZDs exhibit many additional properties, not least an array of effects which suggest a broad attack on the inflammatory process. Thus, TZDs have been shown to reduce plasma levels of the chemokine, monocyte chemotactic protein-1 (MCP-1), the anti-fibrinolytic protein, plasminogen activator inhibitor-1 (PAI-1), the endothelial cell adhesion molecules, e-selectin and inter-cellular adhesion molecule-1 (ICAM-1), the leucocyte-activating molecule, CD40L, and the tissue-remodeling enzyme, matrix metalloproteinase-9 (MMP-9). Further tangible evidence of a reduction by TZDs of systemic inflammation in patients with the classical metabolic syndrome stems from falls in the white blood cell count, P-selectin-positive platelets and in the acute-phase inflammatory proteins, C-reactive protein, serum amyloid A and fibrinogen. At the tissue level, TZDs improve vascular endothelial function, and reduce the rate of progression of intimal-medial thickening of the carotid artery and the microalbuminuria of type 2 diabetes. Further, TZDs have been shown to be efficacious in inflammatory diseases as wide-ranging as psoriasis, ulcerative colitis and non-alcoholic steatohepatitis (NASH). In the case of the latter, a broad spectrum of TZD-related properties is visible. Here, these drugs improve insulin sensitivity for glucose metabolism, reduce hyperinsulinemia, hepatic steatosis, inflammation and fibrosis, and lower the circulating levels of liver transaminases (ALT, AST), alkaline phosphatase and gamma glutamyl transferase. These effects in humans are also well-supported by investigative animal and in vitro studies. The ameliorative effects on liver fibrosis are of particular interest since they suggest that TZDs are able to activate a program of corrective tissue-remodeling. The basis for this action may be partly an ability to inhibit matrix protein secretion by hepatic stellate cells. An analogous action has also been seen in kidney mesangial cells. In conclusion, TZDs are important new drugs, presently indicated for the treatment of type 2 diabetes but with a spectrum of properties which suggests their potential for treating a number of degenerative inflammatory diseases, including NASH. However, full-scale, long-term clinical trials are needed with TZDs to test their potential to treat NASH, not least because of the (hepatotoxic) legacy of the prototype TZD, troglitazone, but also in view of the escalating burden of liver disease which is accompanying the increasing global prevalence of clinical obesity and type 2 diabetes.

Mechanical stretch induces monocyte chemoattractant activity via an NF-kappaB-dependent monocyte chemoattractant protein-1-mediated pathway in human mesangial cells: inhibition by rosiglitazone

Hemodynamic abnormalities are important in the pathogenesis of the glomerular damage in diabetes. Glomerular macrophage infiltration driven by the chemokine monocyte chemoattractant protein-1 (MCP-1) is an early event in diabetic nephropathy. The thiazolidinedione rosiglitazone ameliorates albumin excretion rate in diabetic patients with microalbuminuria and has anti-inflammatory properties, raising the possibility of a relationship between its renoprotective and anti-inflammatory activity. Investigated was whether mesangial cell stretching, mimicking in vitro glomerular capillary hypertension, enhances MCP-1 expression and monocyte chemoattractant activity. The effect of the combination of stretch with high glucose on MCP-1 production was studied and, finally, the effect of rosiglitazone on these processes was assessed. Stretching of human mesangial cells significantly enhanced their monocyte chemoattractant activity. This effect was mediated by MCP-1 as it was paralleled by a significant rise in both MCP-1 mRNA and protein levels and was completely abolished by MCP-1 blockade. Combined exposure to both stretch and high glucose further increased MCP-1 production. Stretch activated the IkappaB-NF-kappaB pathway, and NF-kappaB inhibition, with the use of the specific inhibitor SN50, completely abolished stretch-induced MCP-1, indicating that stretch-induced MCP-1 was NF-kappaB dependent. The addition of rosiglitazone significantly diminished stretch-induced NF-kappaB activation, MCP-1 production, and monocyte chemotaxis. In conclusion, stretching of mesangial cells stimulates their monocyte chemoattractant activity via an NF-kappaB-mediated, MCP-1-dependent pathway, and this effect is prevented by rosiglitazone.

Rapid reversal of hepatic steatosis, and reduction of muscle triglyceride, by rosiglitazone: MRI/S studies in Zucker fatty rats

AIM

This study aimed to chart the time course and durability of the effects of rosiglitazone, a potent thiazolidinedione-based peroxisome proliferator-activated receptor gamma agonist, on hepatic steatosis and intramyocellular lipid in an animal model of obesity, the Zucker Fatty (ZF) rat.

METHODS AND RESULTS

Rosiglitazone (3 mg/kg/day p.o.) significantly reduced both liver fat content (by 59%; p < 0.05) and size (11.5%; p < 0.05) in male ZF rats that received between 3 days and 1 week of treatment, and these reductions were maintained for at least 12 weeks. Liver fat content measured by magnetic resonance spectroscopy (MRS) correlated closely and positively with plasma insulin levels (reduced by 89% within a week, r = 0.8) and with postmortem histological fat fractional volume (r = 0.89). Similarly, liver volume measured by magnetic resonance imaging (MRI) correlated closely with postmortem wet weight (r = 0.99). MRS also showed, and numbers of lipid vacuoles counted in transmission electron micrographs confirmed, that rosiglitazone significantly reduced the elevated intramyocellular lipid seen in ZF rat skeletal muscle by at least 40% (p < 0.05).

CONCLUSIONS

Localized MRS and MRI showed that rosiglitazone reversed the hepatic steatosis, hepatomegaly and intramyocellular lipid, characteristic of the ZF rat, an animal model of obesity.

Abnormal cardiac and skeletal muscle energy metabolism in patients with type 2 diabetes

BACKGROUND

It is well known that patients with type 2 diabetes have increased risk of cardiovascular disease, but it is not known whether they have underlying abnormalities in cardiac or skeletal muscle high-energy phosphate metabolism.

METHODS AND RESULTS

We studied 21 patients with type 2 diabetes with no evidence of coronary artery disease or impaired cardiac function, as determined by echocardiography, and 15 age-, sex-, and body mass index-matched control subjects. Cardiac high-energy phosphate metabolites were measured at rest using 31P nuclear magnetic resonance spectroscopy (MRS). Skeletal muscle high-energy phosphate metabolites, intracellular pH, and oxygenation were measured using 31P MRS and near infrared spectrophotometry, respectively, before, during, and after exercise. Although their cardiac morphology, mass, and function appeared to be normal, the patients with diabetes had significantly lower phosphocreatine (PCr)/ATP ratios, at 1.50+/-0.11, than the healthy volunteers, at 2.30+/-0.12. The cardiac PCr/ATP ratios correlated negatively with the fasting plasma free fatty acid concentrations. Although skeletal muscle energetics and pH were normal at rest, PCr loss and pH decrease were significantly faster during exercise in the patients with diabetes, who had lower exercise tolerance. After exercise, PCr recovery was slower in the patients with diabetes and correlated with tissue reoxygenation times. The exercise times correlated negatively with the deoxygenation rates and the hemoglobin (Hb)A1c levels and the reoxygenation times correlated positively with the HbA1c levels.

CONCLUSIONS

Type 2 diabetic patients with apparently normal cardiac function have impaired myocardial and skeletal muscle energy metabolism related to changes in circulating metabolic substrates.

Rosiglitazone, a peroxisome proliferator-activated receptor-gamma, inhibits the Jun NH(2)-terminal kinase/activating protein 1 pathway and protects the heart from ischemia/reperfusion injury

This study was conducted to evaluate whether treatment of normal and diabetic rat hearts with rosiglitazone, a high-affinity ligand of the peroxisome proliferator-activated receptor-gamma (PPAR-gamma) used for the treatment of type 2 diabetes, improves postischemic functional recovery. The effects of acute rosiglitazone administration were investigated using working hearts isolated from normal rat or rats diabetic for 4 weeks after streptozotocin (STZ) injection. Hearts were subjected to 30 min of normothermic, zero-flow ischemia followed by 30-min reperfusion. Rosiglitazone (1 micromol/l) administered before ischemia had no effect on cardiac function during baseline perfusion, but it significantly improved aortic flow during reperfusion in both normal and diabetic hearts. In a chronic protocol in which rosiglitazone was given by daily gavage (10 micromol/kg body wt) immediately after STZ injection, rosiglitazone also prevented postischemic injury and significantly improved functional recovery. Using Western immunoblotting, it was demonstrated that the acute cardioprotective effect of rosiglitazone is associated with an inhibition of Jun NH(2)-terminal kinase phosphorylation in both normal and diabetic rat hearts. Furthermore, rosiglitazone also inhibited activating protein-1 DNA-binding activity. These data, demonstrating that rosiglitazone limits postischemic injury in isolated hearts, suggest an important function for PPAR-gamma in the heart.

Thiazolidinedione treatment normalizes insulin resistance and ischemic injury in the zucker Fatty rat heart

Obesity is associated with risk factors for cardiovascular disease, including insulin resistance, and can lead to cardiac hypertrophy and congestive heart failure. Here, we used the insulin-sensitizing agent rosiglitazone to investigate the cellular mechanisms linking insulin resistance in the obese Zucker rat heart with increased susceptibility to ischemic injury. Rats were treated for 7 or 14 days with 3 mg/kg per os rosiglitazone. Hearts were isolated and perfused before and during insulin stimulation or during 32 min low-flow ischemia at 0.3 ml small middle dot min(-1) small middle dot grams wet wt(-1) and reperfusion. D[2-(3)H]glucose was used as a tracer of glucose uptake, and phosphorus-31 nuclear magnetic resonance spectroscopy was used to follow energetics during ischemia. At 12 months of age, obese rat hearts were insulin resistant with decreased GLUT4 protein expression. During ischemia, glucose uptake was lower and depletion of ATP was greater in obese rat hearts, thereby significantly impairing recovery of contractile function during reperfusion. Rosiglitazone treatment normalized the insulin resistance and restored GLUT4 protein levels in obese rat hearts. Glucose uptake during ischemia was also normalized by rosiglitazone treatment, thereby preventing the greater loss of ATP and restoring recovery of contractile function to that of lean rat hearts. We conclude that rosiglitazone treatment, by normalizing glucose uptake, protected obese rat hearts from ischemic injury.

Effects of chronic murine and human leptin infusion on plasma leptin and corticosterone levels and energy balance in lean Zucker rats

AIM

To clarify whether centrally delivered leptin can access the circulation and to determine to what extent the effects of i.c.v. h-leptin and m-leptin on body weight and plasma corticosterone are due to reduced food intake.

METHODS

Male lean Zucker rats were infused i.c.v. with recombinant m-leptin or h-leptin (42 microg/day) for 7 days. Terminal plasma leptin levels were measured using selective r-leptin, m-leptin and h-leptin RIA. Plasma h-leptin and corticosterone levels were determined on days 0, 2, 4 and 6 of h-leptin infusion. Interscapular brown adipose tissue weight and UCP-1 mRNA expression (an indicator of thermogenic capacity) were also measured.

RESULTS

The terminal plasma leptin level was elevated (from 2.2 +/- 0.4 to 42.7 +/- 20.2 ng/ml) in the h-leptin-treated lean rats to levels similar to those in vehicle i.c.v. infused fa/fa rats (72.2 +/- 4.7 ng/ml), but this was only detectable when the h-leptin radioimmunoabsorbent assay (RIA) was used. Further, both m-leptin and h-leptin infusions in lean rats elevated terminal plasma corticosterone (352 +/- 37 and 389 +/- 55 ng/ml, respectively) to levels similar to those in i.c.v. rats (386 +/- 62 ng/ml), whereas diet-restriction by pair-feeding, with the h-leptin group, in lean rats had no effect (207 +/- 45 ng/ml). The increase in plasma corticosterone level coincided with the maximum hypophagic effects of leptin and preceded the appearance and sustained elevation of exogenous human leptin in the circulation. Both m-leptin and h-leptin i.c.v. infusion reduced body weight gain (3% and 4%, respectively, compared to pair-fed group) and increased UCP-1 expression (11-fold and 16-fold, respectively) in lean rats. However, h-leptin elicited an earlier effect than m-leptin on body weight, manifested as an earlier reduction in food intake and greater increase in UCP-1 expression. h-Leptin also elicited a greater reduction in body weight gain than did pair-feeding.

CONCLUSIONS

Intracerebroventricular-infused m-leptin or h-leptin was detected in the circulation. Furthermore, m-leptin and h-leptin elevated plasma corticosterone levels and h-leptin caused some weight loss in lean rats independently of its suppression of food intake. The elevation of corticosterone levels in the lean rats may be a mechanism whereby they resist excessive weight loss in response to leptin.

In vivo myocardial protection from ischemia/reperfusion injury by the peroxisome proliferator-activated receptor-gamma agonist rosiglitazone

BACKGROUND

Diabetes is associated with increased risk of mortality as a consequence of acute myocardial infarction. This study determined whether rosiglitazone (ROSI) could reduce myocardial infarction after ischemia/reperfusion injury.

METHODS AND RESULTS

Male Lewis rats were anesthetized, and the left anterior descending coronary artery was ligated for 30 minutes. After reperfusion for 24 hours, the ischemic and infarct sizes were determined. ROSI at 1 and 3 mg/kg IV reduced infarct size by 30% and 37%, respectively (P<0.01 versus vehicle). Pretreatment with ROSI (3 mg. kg(-1). d(-1) PO) for 7 days also reduced infarct size by 24% (P<0.01). ROSI also improved ischemia/reperfusion-induced myocardial contractile dysfunction. Left ventricular systolic pressure and positive and negative maximal values of the first derivative of left ventricular pressure (dP/dt) were significantly improved in ROSI-treated rats. ROSI reduced the accumulation of neutrophils and macrophages in the ischemic heart by 40% and 43%, respectively (P<0.01). Ischemia/reperfusion induced upregulation of CD11b/CD18 and downregulation of L-selectin on neutrophils and monocytes; these effects were significantly attenuated in ROSI-treated animals. Likewise, intercellular adhesion molecule-1 expression in ischemic hearts was markedly diminished by ROSI, as was the ischemia/reperfusion-stimulated upregulation of monocyte chemoattractant protein-1.

CONCLUSIONS

ROSI reduced myocardial infarction and improved contractile dysfunction caused by ischemia/reperfusion injury. The cardioprotective effect of ROSI was most likely due to inhibition of the inflammatory response.

Insulin-sensitizing action of rosiglitazone is enhanced by preventing hyperphagia

AIM

We investigated whether pair-feeding to prevent hyperphagia would potentiate the insulin-sensitizing effect of rosiglitazone in chow-fed and insulin-resistant dietary obese rats, and studied the role of leptin and hypothalamic neuropeptide Y as mediators of weight gain during treatment.

METHODS

Dietary obese and chow-fed rats (575 +/- 10 vs. 536 +/- 7 g; p < 0.01) were given rosiglitazone (30 mg/kg p.o.) or vehicle daily for 14 days.

RESULTS

Energy intake and weight gain were greater in rosiglitazone-treated ad-lib-fed rats (body weight: chow + 24 +/- 2 g, rosiglitazone-treated + 55 +/- 2 g, p < 0.001; dietary obese + 34 +/- 2 g, rosiglitazone-treated + 74 +/- 7 g, p < 0.001). Half of each rosiglitazone-treated group were pair-fed to vehicle-treated controls. Rosiglitazone normalized circulating free fatty acids (FFAs) and insulin sensitivity in dietary obese rats (homeostasis model assessment (HOMA): chow-fed controls, 3.9 +/- 0.3; dietary obese controls, 6.7 +/- 0.7; rosiglitazone-treated, ad lib-fed dietary obese, 4.2 +/- 0.5; both p < 0.01). Insulin sensitivity improved further with pair-feeding (HOMA: 2.9 +/- 0.4; p < 0.05 vs. rosiglitazone-treated, ad lib-fed dietary obese), despite unchanged FFAs. Qualitatively similar findings were made in chow-fed rats. Pair-feeding prevented rosiglitazone-related weight gain in chow-fed, but not dietary obese rats (body weight: + 49 +/- 5 g, p < 0.001 vs. untreated dietary obese controls). Adipose tissue OB mRNA was elevated in dietary obese rats, reduced 49% (p < 0.01) by rosiglitazone treatment, and further (by 16%) with pair-feeding (p < 0.0001). Plasma leptin, however, only fell in the pair-fed group. Hypothalamic neuropeptide Y mRNA was unchanged throughout, suggesting that weight gain associated with high-dose rosiglitazone treatment is independent of hypothalamic neuropeptide Y.

CONCLUSIONS

Food restriction potentiates the insulin-sensitizing effect of rosiglitazone in rats, and this effect is independent of a fall in FFAs.

Beta-cell mass dynamics in Zucker diabetic fatty rats. Rosiglitazone prevents the rise in net cell death

The evolution of diabetes in the male leptin receptor-deficient (fa/fa) Zucker diabetic fatty (ZDF) rat is associated with disruption of normal islet architecture, beta-cell degranulation, and increased beta-cell death. It is unknown whether these changes precede or develop as a result of the increasing plasma glucose, or whether the increased beta-cell death can be prevented. Early intervention with thiazolidinediones prevents disruption of the islet architecture. To determine the specific effects of rosiglitazone (RSG) on beta-cell mass dynamics, male fa/fa (obese) and +/fa or +/+ (lean) rats age 6 weeks were fed either chow (control group [CN]) or chow mixed with rosiglitazone (RSG group) at a dosage of 10 micromol. kg(-1) body wt.day(-1). Rats were killed after 0, 2, 4, 6, or 10 weeks of treatment (at age 6, 8, 10, 12, or 16 weeks). Plasma glucose increased from 8.9 +/- 0.4 mmol/l at 0 weeks to 34.2 +/- 1.8 mmol/l (P = 0.0001) at 6 weeks of treatment in obese CN rats and fell from 8.0 +/- 0.3 to 6.3 +/- 0.4 mmol/l in obese RSG rats (P = 0.02). beta-cell mass fell by 51% from 2 to 6 weeks of treatment (ages 8-12 weeks) in obese CN rats (6.9 +/- 0.9 to 3.4 +/- 0.5 mg; P < 0.05), whereas beta-cell mass was unchanged in obese RSG rats. At 10 weeks of treatment (age 16 weeks), beta-cell mass in obese CN rats was only 56% of that of obese RSG rats (4.4 +/- 0.4 vs. 7.8 +/- 0.3 mg, respectively; P = 0.0001). The beta-cell replication rate fell from a baseline value of 0.95 +/- 0.12% in lean rats and 0.94 +/- 0.07% in obese rats (at 0 weeks) to approximately 0.3-0.5% in all groups by 6 weeks of treatment (age 12 weeks). After 10 weeks of treatment, beta-cell replication was higher in obese RSG rats than in CN rats (0.59 +/- 0.14 vs. 0.28 +/- 0.05%, respectively; P < 0.02). Application of our mass balance model of beta-cell turnover indicated that net beta-cell death was fivefold higher in obese CN rats as compared with RSG rats after 6 weeks of treatment (age 12 weeks). The increase in beta-cell death in obese CN rats during the 6-week observation period was well correlated with the increase in plasma glucose (r2 = 0.90, P < 0.0001). These results suggest that the development of hyperglycemia in ZDF rats is concomitant with increasing net beta-cell death. beta-cell proliferation compensates for the increased beta-cell loss at a time when plasma glucose is moderately elevated, but compensation ultimately fails and the plasma glucose levels increase beyond approximately 20 mmol/l. Treatment with rosiglitazone, previously shown to reduce insulin resistance, prevents the loss of beta-cell mass in obese ZDF rats by maintaining beta-cell proliferation and preventing increased net beta-cell death.

Rosiglitazone prevents the onset of hyperglycaemia and proteinuria in the Zucker diabetic fatty rat

AIM

To investigate the potential of rosiglitazone, a highly potent agonist at the nuclear peroxisome proliferator activated receptor-gamma (PPAR-gamma), to prevent the development of diabetes in the Zucker diabetic fatty (ZDF) rat or to ameliorate the condition at a later stage of the disease.

METHODS

Rosiglitazone (10 micromol/kg body weight daily) was given via the diet to ZDF rats from aged 6 weeks, before the onset of hyperglycaemia (Prevention group), or from aged 21 weeks after hyperglycaemia and proteinuria were established (Intervention group). Untreated ZDF rats and age-matched Zucker lean rats (ZL) served as controls and the experiment was terminated when the animals were aged 28 weeks.

RESULTS

Whilst the combined ZDF control and Intervention groups were already hyperglycaemic (14.6 +/- 1.6 vs. ZL 5.7 +/- 0.1 mmol/l, mean +/- s.e.m.; p < 0.05), glycosuric and polydipsic at aged 11 weeks, and thereafter had a declining plasma insulin concentration, rosiglitazone Prevention treatment maintained normoglycaemia even at aged 27 weeks (3.7 +/- 0.3 mmol/l vs. ZL 3.0 +/- 0.3 mmol/l; NS). Intervention treatment at aged 21 weeks, however, failed to ameliorate the diabetes. These functional data were supported by determinations of pancreatic insulin content (microg/mg tissue as follows: ZL, 43.1 +/- 3.9; ZDF control (28 weeks) + ZDF Intervention control (21 weeks), 6.0 +/- 0.8; Prevention, 63.6 +/- 15.8; Intervention, 6.2 +/- 0.9) and by morphological, immunohistochemical and electron microscopical examination of pancreata at the end of the study. Thus, islets from rosiglitazone Prevention rats were similar to ZL rats, whereas ZDF controls and Intervention rats exhibited islets depleted of insulin, with a disorganized architecture and an ultrastructure indicative of work hypertrophy. ZDF control rats and Intervention rats, though not rosiglitazone Prevention rats, also exhibited marked proteinuria, indicative of renal glomerular damage.

CONCLUSIONS

Our results demonstrate that in ZDF rats, rosiglitazone prevents the progression from insulin resistance to overt diabetes. These data provide a rationale for investigating whether treatment with rosiglitazone of patients with early signs of perturbed glucose metabolism (e.g. impaired fasting glucose (IGT)) may prevent the progression to type 2 diabetes and its associated complications.

Down-regulation of orexin gene expression by severe obesity in the rats: studies in Zucker fatty and zucker diabetic fatty rats and effects of rosiglitazone

Orexins (hypocretins) are lateral hypothalamic neuropeptides implicated in regulating feeding and the sleep-wake cycle. To study their possible relevance to obesity and diabetes, we measured hypothalamic prepro-orexin mRNA levels in obese, normoglycemic Zucker fatty (fa/fa) and in hyperglycemic, non-obese Zucker diabetic fatty (ZDF) rats. Hypothalamic prepro-orexin mRNA concentrations in Zucker fatty rats were 31% lower than those in lean controls (0. 69+/-0.06 vs. 1.00+/-0.10 arbitrary units, P<0.05), but did not differ between ZDF diabetic rats and non-diabetic controls. Treatment of ZDF diabetic rats with rosiglitazone (1 or 3 mg/kg body weight daily for 13 weeks) normalized plasma glucose and significantly reduced plasma insulin, while leptin levels were 67% higher than in untreated ZDF rats (20.2+/-0.5 vs. 12.1+/-2.5, P<0. 001). Rosiglitazone treatment markedly enhanced weight gain compared with untreated ZDF rats (final weight 732+/-13 g vs. 409+/-13 g, P<0. 001) even though they were restricted to the same food intake. Rosiglitazone-treated ZDF rats had significantly lower hypothalamic prepro-orexin mRNA levels (0.68+/-0.07 arbitrary units) than both non-diabetic lean controls (1.00+/-0.10, P=0.02) and untreated diabetics (1.03+/-0.14, P=0.03). Our data suggest that prepro-orexin gene expression may be suppressed by substantial weight gain. Obesity-related signals that might mediate this effect have not been identified, but plasma leptin, insulin and glucose are not obviously involved.

Therapeutic index for rosiglitazone in dietary obese rats: separation of efficacy and haemodilution

1. The blood glucose-lowering efficacy of rosiglitazone (RSG) and the mechanisms of associated weight gain were determined in dietary obese rats (DIOs). DIO and chow-fed rats received RSG 0.3-30 mg kg-1 daily for 21 days. 2. In DIOs, plasma glucose and insulin concentrations were reduced by RSG at dosages of 3 and 10 mg kg-1, respectively. Homeostasis model assessment (HOMA) indicated the threshold for a reduction of insulin resistance was 1 mg kg-1. Neither glucose nor insulin levels were affected by treatment in chow-fed rats. 3. RSG 0.3 mg kg-1 lowered free fatty acids (FFAs) in DIOs, whereas for plasma triglycerides (TGs), the threshold was 3 mg kg-1. By contrast, the threshold for reducing packed red cell volume (PCV) and increasing cardiac mass was 10 mg kg-1. Thus, the therapeutic index for RSG in DIOs was >3 and < or = 10. 4. Energy intake and weight gain increased in treated DIOs (by 20% and 50 g, at 30 mg kg-1) and chow-fed rats (by 25% and 35 g, at 30 mg kg-1). In DIOs, these increases coincided with falls in plasma leptin (40% lower at 30 mg kg-1) and insulin (43% lower at 30 mg kg-1). By contrast, in chow-fed rats, weight gain and hyperphagia occurred without changes in either leptin or insulin. However, reductions in FFAs below 0.4 - 0.3 mM were associated with hyperphagia and weight gain in DIO and chow-fed rats. 5. We conclude that increased energy intake and body weight did not attenuate the improved metabolism evoked by RSG in DIO rats, and that insulin action was enhanced at a dose >3 fold below the threshold for causing haemodilution and cardiac hypertrophy in DIO rats.

Differential distribution of orexin-A and orexin-B immunoreactivity in the rat brain and spinal cord

The orexins are recently identified appetite-stimulating hypothalamic peptides. We used immunohistochemistry to map orexin-A and orexin-B immunoreactivity in rat brain, spinal cord, and some peripheral tissues. Orexin-A- and orexin-B-immunoreactive cell bodies were confined to the lateral hypothalamic area and perifornical nuclei. Orexin-A-immunoreactive fibers were densely distributed in the hypothalamus, septum, thalamus, locus coeruleus, spinal cord, and near the ventricles, but absent from peripheral sites investigated. In contrast, orexin-B-immunoreactive fibers were distributed sparsely in the hypothalamus. Orexin cells are strategically sited to contribute to feeding regulation, but their widespread projections suggest that orexins have other physiological roles.

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.

Differences between the vasorelaxant activity of adenosine-receptor agonists on guinea-pig isolated aorta precontracted with noradrenaline or phenylephrine

The relaxant effect of adenosine and 5'-(N-ethylcarboxamido)adenosine (NECA) against alpha-adrenoceptor-mediated contractile tone in guinea-pig isolated aortic rings has been examined to determine if this A2B-receptor-mediated relaxation was dependent upon the contracting agent, and whether the contractions were dependent upon intracellular or extracellular calcium. Relaxation responses were consistently greater for aortic rings pre-contracted with phenylephrine (3x10(-6) M) than for rings pre-contracted with noradrenaline (3x10(-6) M). Maximum inhibition by NECA was significantly greater for phenylephrine-contracted aortae than for noradrenaline-contracted (81.9+/-2.8% compared with 25.0+/-1.5%). These differences persisted in the presence of beta- and alpha2-adrenoceptor blockade and could not, therefore, be attributed to stimulation of these receptors by noradrenaline. The ratio of the contractions obtained before and in the presence of adenosine or NECA was compared with the control ratio obtained before and after vehicle. Experiments were performed both in the presence of normal calcium levels and under calcium-free conditions. In normal-calcium medium, NECA inhibited phenylephrine-induced contractions (test ratio, 76.7+/-3.9%; control ratio, 133.1+/-9.8%) to a greater extent than noradrenaline-induced contractions (108.4+/-4.1 and 123.4+/-4.9%); adenosine similarly inhibited phenylephrine-induced contractions more than those induced by noradrenaline. Under calcium-free conditions, adenosine (36.7+/-11.9 and 110.7+/-26.6%) and NECA (55.2+/-9.1 and 87.1+/-14.9%) were only effective against phenylephrine-induced contractions. This suggests that activation of the A2B-receptor by these agonists inhibited intracellular mobilization of calcium for phenylephrine-induced contractions only. The effects on extracellular calcium influx were examined for phenylephrine- and noradrenaline-induced contractions in normal-calcium medium but in the presence of ryanodine to prevent intracellular calcium mobilization. NECA inhibited phenylephrine-induced contractions (77.3+/-12.4 and 111.4+/-9.3%), presumably by interfering with influx of calcium through receptor-operated calcium channels. In contrast, NECA failed to reduce noradrenaline-induced contractions (121.5+/-10.7 and 122.4+/-11.6%), suggesting that the effect on noradrenaline is predominantly via interaction with intracellular calcium. Adenosine was consistently a more effective relaxant than NECA, possibly because of an additional intracellular component of the response. We conclude that adenosine receptor agonists inhibit phenylephrine-induced contractions of guinea-pig aorta more selectively than noradrenaline-induced contractions. A2B-receptor stimulation might reveal a fundamental difference between the modes of contraction elicited by these two alpha-adrenoceptor agonists.

The thiazolidinedione rosiglitazone (BRL-49653) lowers blood pressure and protects against impairment of endothelial function in Zucker fatty rats

Human obesity is associated with insulin resistance, hyperinsulinemia, and a predisposition to hypertension and vascular disease, the origin of which may lie in impairment of endothelial function. We tested the effects of the thiazolidinedione rosiglitazone on blood pressure and endothelial function in insulin-resistant fatty Zucker rats, which display hypertension and abnormal endothelial cell function. We studied fatty Zucker rats given rosiglitazone maleate (50 micromol/kg diet; n = 8) for 9-12 weeks (treated fatty), untreated fatty rats (n = 8), and lean rats (n = 8) given diet alone. At the end of the study, systolic blood pressure was significantly higher in untreated fatty (147 +/- 5 mmHg) than in lean rats (125 +/- 2 mmHg; P < 0.05), but rosiglitazone treatment prevented the development of hypertension in fatty rats (123 +/- 1 mmHg). Fasting hyperinsulinemia in untreated fatty rats (28.7 +/- 6.0 ng/ml) was significantly lowered by rosiglitazone (7.0 +/- 1.4 ng/ml; P < 0.05 vs. untreated fatty), but remained significantly higher than the levels seen in lean rats (1.5 +/- 0.4 ng/ml; P < 0.01). Mesenteric arteries were studied in a myograph. Maximal acetylcholine chloride (1.1 micromol/l)-induced relaxation of norepinephrine hydrochloride (NE)-induced constriction was impaired in untreated fatty (62.4 +/- 3.4%) vs. lean (74.3 +/- 3.5%; P = 0.01) rats; this defect was partially prevented by rosiglitazone (66.5 +/- 3.0%; P = 0.01 vs. untreated fatty). Insulin (50 mU/l) significantly attenuated the contractile response to NE in lean rats (14.7 +/- 3.3%; P = 0.02); this vasodilator effect of insulin was absent in untreated fatty rats at concentrations of 50-5,000 mU/l, but was partially restored by rosiglitazone (9.7 +/- 2.5% attenuation; P = 0.02 vs. no insulin). Thus, rosiglitazone prevents the development of hypertension and partially protects against impaired endothelial function associated with insulin resistance. These latter effects may contribute to the drug's antihypertensive properties.

Effects of single and chronic intracerebroventricular administration of the orexins on feeding in the rat

Two novel hypothalamic neuropeptides, orexin-A and -B, are suggested to regulate feeding. A single intracerebroventricular injection of orexin-A (23.4 nmol), administered 3 h into the light phase, increased feeding in satiated rats and prolonged feeding in fasted rats; it also increased feeding when given 6 h into, but not at the start of, the dark phase. An 8-day intracerebroventricular infusion with orexin-A (18 nmol/day) increased daytime feeding on days 2 and 8, but nocturnal feeding was reduced and there was no change in 24 h intake. Orexin-B had no effects. These results demonstrate a circadian variation in feeding responses to orexin-A.

Indices of carbohydrate and lipid metabolism in vasopressin-replete and -deficient New Zealand genetically hypertensive rats

Indices of carbohydrate and lipid metabolism were investigated in male New Zealand genetically hypertensive and normotensive rats. Cross-breeding of male rats of these strains with female Brattleboro diabetes insipidus rats also provided the opportunity to examine the metabolic impact of vasopressin and its deficiency in hypertensive and normotensive rats. Hypertensive and normotensive rats, with or without diabetes insipidus, were fasted for 24 h, exsanguinated and their blood/plasma analysed for various indices of carbohydrate and lipid metabolism. Whilst each group of rats maintained fasted normoglycemia, hypertensive rats, with or without vasopressin-deficiency, were hypoinsulinaemic relative to normotensive counterparts. Moreover, hypertensive or normotensive vasopressin-deficient rats were hypoinsulinaemic relative to vasopressin-replete counterparts. In vasopressin-replete rats, the apparently improved insulin sensitivity in hypertension was associated with significant falls in plasma glucagon, triglycerides and total cholesterol. Finally, normotensive vasopressin-deficient rats were hypoglucagonaemic relative to the vasopressin-replete group. These data demonstrate that independent of vasopressin status, hypertension in the New Zealand strain and the diabetes insipidus hybrid was associated with improved insulin sensitivity. However, endogenous vasopressin exercises an influential role in carbohydrate and lipid metabolism in normotensive rats.

Characterisation of the neuropeptide Y receptor that mediates feeding in the rat: a role for the Y5 receptor?

Food intake was measured in freely fed rats following intracerebroventricular administration of neuropeptide Y (NPY) and several of its analogues and antagonists to investigate the hypothesis that the NPY Y5 receptor mediates feeding. Rat NPY (rNPY), rNPY(2-36) and rNPY(3-36) produced similar feeding responses over the dose range 0.7-7.0 nmol. Rat peptide YY (rPYY) was more potent and at least as efficacious as rNPY. [Leu31 Pro34]-rNPY (agonist potency: Y1 > Y5 > Y4 = y6) and human pancreatic polypeptide (hPP) produced flatter dose-response curves, suggesting partial agonism at the receptor(s). rNPY(13-36) (agonist potency: Y2 > Y5) had little activity and rPP was inactive. [D-Trp32]-NPY was a weak orexigenic agent given alone and, consistent with partial agonism, it markedly antagonised the response to porcine NPY (pNPY). Similarly, the receptor antagonist (Y1 > Y4) 1229U91 stimulated feeding slightly, and markedly inhibited rNPY-induced feeding. In contrast to a previous report, BIBP 3226 (70 nmol), another Y1 receptor antagonist, failed to inhibit the response to rNPY. Our data in vivo are inconsistent with findings that hPP, [Leu31 Pro34]-rNPY and [D-Trp32]-rNPY are full agonists at the rat cloned Y5 receptor. Thus, whilst the Y5 receptor may be involved, its participation as the sole receptor mediating the orexigenic action of NPY in the rat remains unproven.

Peroxisome proliferator-activated receptor-gamma agonist, rosiglitazone, protects against nephropathy and pancreatic islet abnormalities in Zucker fatty rats

Rosiglitazone (BRL 49653), a peroxisome proliferator-activated receptor-gamma (PPAR-gamma) agonist and potent insulin action-enhancing agent, was given in the diet (50 micromol/kg of diet) to male Zucker rats ages 6-7 weeks for 9 months (prevention group). In this treatment mode, rosiglitazone prolonged the time to onset of proteinuria from 3 to 6 months and markedly reduced the rate of its subsequent progression. Progression was also retarded when treatment was commenced (intervention group) after proteinuria had become established (4 months; ages 24-25 weeks). In either treatment mode, rosiglitazone normalized urinary N-acetyl-beta-D-glucosaminidase activity, a marker for renal proximal tubular damage, and ameliorated the rise in systolic blood pressure that occurred coincidentally with the development of proteinuria in Zucker fatty control rats. The renal protective action of rosiglitazone was verified morphologically. Thus in the prevention group there was an absence of the various indexes of chronic nephropathy that were prominent in the Zucker fatty control group, namely, glomerulosclerosis, dilated tubules containing proteinaceous casts, a loss of functional microvilli on the tubular epithelium, and varying degrees of chronic interstitial nephritis. An intermediate pathology was observed in the intervention group. Also, pancreatic islet hyperplasia, ultrastructural evidence of beta-cell work hypertrophy, and derangement of alpha-cell distribution within the islet were prominent features of Zucker fatty control rats, but these adaptive changes were ameliorated (intervention group) or prevented (prevention group) by rosiglitazone treatment. These data demonstrate that treatment of Zucker fatty rats with rosiglitazone produced substantial protection over a prolonged period against the development and progression of renal injury and the adaptive changes to pancreatic islet morphology caused by sustained hyperinsulinemia.

Differential vasoactive effects of the insulin sensitizers rosiglitazone (BRL 49653) and troglitazone on human small arteries in vitro

BRL 49653 (rosiglitazone) and troglitazone are thiazolidinedione insulin-sensitizing agents, which are undergoing clinical evaluation as treatments for NIDDM. Potential side effects of thiazolidinediones include edema and hemodilution. Although the underlying mechanisms are presently unclear, animal and human studies have demonstrated a vasodilator action of troglitazone, which could in theory cause fluid retention. This in vitro study compared the direct vasodilator effects of troglitazone and BRL 49653 in small arteries (n = 44) from human subcutaneous fat. In arterial rings with a functioning endothelium and preconstricted with norepinephrine (NE; 6 micromol/l), troglitazone (n = 22 vessels), but not BRL 49653 (1-100 micromol/l), caused a concentration-related relaxation (69.4 +/- 5.2% at 100 micromol/l; P < 0.01). In the presence of indomethacin (IM; 10 micromol/l; n = 12), this vasorelaxant effect of troglitazone was abolished (P < 0.01 vs. troglitazone alone) and replaced by enhanced vasoconstriction (58.5 +/- 39.5% over the NE baseline) similar in magnitude to that produced by troglitazone vehicle (ethanol) alone (n = 16; NS vs. ethanol vehicle). By contrast, BRL 49653 (100 micromol/l; n = 22) and an equivalent volume of ethanol alone (n = 12) caused similar degrees of vasoconstriction (18.7 +/- 14.6 and 22.5 +/- 8.0%, respectively; NS). In the presence of IM (10 micromol/l; n = 10), the vasoconstrictor effect of BRL 49653 was enhanced (41.5 +/- 14.4%), although not significantly (NS vs. BRL 49653 alone or ethanol alone). Additional studies in Wistar rat arteries showed a similar vasodilator effect of troglitazone that was not inhibited by L-NAME (100 micromol/l). The alpha-tocopherol moiety alone had no vasorelaxant effect at concentrations up to 300 micromol/l. Thus, in human arterial resistance vessels in vitro, BRL 49653 does not possess the direct, IM-sensitive vasorelaxant action of troglitazone. This vasodilation could, in theory, permit transmission of systemic pressure to the capillary bed.

Orexins and orexin receptors: a family of hypothalamic neuropeptides and G protein-coupled receptors that regulate feeding behavior

The hypothalamus plays a central role in the integrated control of feeding and energy homeostasis. We have identified two novel neuropeptides, both derived from the same precursor by proteolytic processing, that bind and activate two closely related (previously) orphan G protein-coupled receptors. These peptides, termed orexin-A and -B, have no significant structural similarities to known families of regulatory peptides. prepro-orexin mRNA and immunoreactive orexin-A are localized in neurons within and around the lateral and posterior hypothalamus in the adult rat brain. When administered centrally to rats, these peptides stimulate food consumption. prepro-orexin mRNA level is up-regulated upon fasting, suggesting a physiological role for the peptides as mediators in the central feedback mechanism that regulates feeding behavior.

Effects of intracerebroventricular infusion of leptin in obese Zucker rats

The obese Zucker rat (OZR) exhibits a missense mutation in the cDNA for the leptin receptor, producing a single amino acid substitution in the extracellular domain of the receptor. A mutation in the leptin receptor gene of the db/db mouse prevents the synthesis of the long splice variant of the receptor. The possibility that the OZR, like the db/db mouse, is refractory to the actions of murine leptin was tested by infusing the protein intracerebroventricularly via a minipump for 7 days. Lean Zucker rats (LZR) infused with leptin acted as positive controls, and other groups of OZR and LZR were infused with vehicle. In LZR, leptin reduced bodyweight and food intake and increased brown adipose tissue (BAT) temperature. Plasma corticosterone increased (61%) in these rats, and plasma triglycerides fell (78%). Leptin treatment improved tolerance to an oral glucose load (16% reduction in the area under the blood glucose curve) while lowering plasma insulin. In OZR, the actions of leptin were blunted. Food intake was slightly, but not significantly, reduced. Although there was a reduction in the rate of increase in body mass, the effect of leptin was about half that seen in LZR. BAT temperature and glucose tolerance were unchanged. In contrast to the elevated plasma corticosterone seen in LZR, leptin reduced the level of this hormone (27%) in OZR. In OZR and LZR treated with leptin, the plasma leptin levels were increased 24-fold and 47-fold, respectively. The results suggest that leptin retains some efficacy in OZR, although these rats are less responsive than LZR.

Impaired insulin-induced attenuation of noradrenaline-mediated vasoconstriction in insulin-resistant obese Zucker rats

1. Insulin resistance is associated with hypertension but the underlying mechanism is unclear. We tested the hypothesis that insulin-induced vasodilatation is impaired in insulin-resistant obese Zucker rats. We studied mesenteric artery (approximately 220 microns diameter) function before the development of hypertension in 3-month old obese Zucker rats and age-matched lean rats. 2. In vessels from lean rats, insulin at concentrations of 50, 500 and 5000 m-units/l attenuated the constriction in response to noradrenaline (50 m-units/l: 8 +/- 3%, P < 0.05; 500 m-units/l: 13 +/- 3%, P < 0.02; 5000 m-units/l: 13 +/- 2%, P < 0.02). 3. Vessels from obese rats failed to show any such response to insulin (2 +/- 6% increase in maximal tension with 5000 m-units/l; not significant), both in the presence and absence of L-arginine (3 mmol/l). 4. Vessels from obese rats showed slight but significant impairment in the vasodilator response to acetylcholine (5 x 10(-8)-10(-4) mol/l) (obese: 64.1 +/- 3.7% relaxation; lean: 77.3 +/- 3.7% relaxation; P < 0.05); however, relaxation in response to A23187 was not significantly different between the phenotypes (obese: 81.3 +/- 10.6% relaxation; lean: 79.1 +/- 9.7% relaxation; not significant). 5. Systolic blood pressure was not significantly different in lean (126 +/- 8 mmHg) and obese (127 +/- 7 mmHg) rats at the time of study (not significant). 6. We conclude that insulin-induced attenuation of noradrenaline-mediated vasoconstriction is impaired in the obese Zucker rat and that this defect precedes and therefore could contribute to the development of hypertension in this insulin-resistant model. The defect in insulin action could reside in the endothelial generation of nitric oxide, as endothelial function is also abnormal.

Treatment with the thiazolidinedione (BRL 49653) decreases insulin resistance in obese Zucker hindlimb

Hindlimbs of mature age obese fa/fa Zucker rats were perfused and found to be markedly insulin-resistant when compared to the hindlimbs of age-matched lean Fa/? animals. Hindlimb analysis also showed a greater content of fat and a lower content of muscle in the obese. Treatment of the obese animals for 7 days with the thiazolidinedione, BRL 49653 (3 mumol/kg/day) significantly decreased the insulin resistance of the hindlimb and significantly increased the rate of weight gain in the whole rat. However, the decreased insulin resistance due to BRL 49653 could not be accounted for by an increase in the proportion of hindlimb muscle to fat or by an increase in the hindlimb muscle mass perfused.

Potential defect in the vascular control of nonshivering thermogenesis in the obese Zucker rat hind limb

Vascular control of nonshivering thermogenesis in the perfused hind limb of obese and lean Zucker rats were compared using two vasoconstrictors, norepinephrine and serotonin. For hind limbs of both phenotypes, norepinephrine infusions resulted in a dose-dependent uninterrupted increase in perfusion pressure and a biphasic change in oxygen uptake (VO2), characterized by a stimulation at low concentrations, and an increasing inhibition at higher concentrations that gradually overcame the stimulation in a dose-dependent manner. At concentrations of norepinephrine greater than 1 microM, the inhibitory effect predominated and gave rise to values for VO2 less than basal. The obese hind limb had a lower basal VO2 and a lower maximal VO2 mediated by norepinephrine than the lean rat, but these differences appeared to relate largely to the lower muscle mass and higher content of fat of the obese hind limb. Serotonin infusions resulted in dose-dependent increase in perfusion pressure and an accompanying decrease in VO2. Pressure changes were identical for the obese and lean hind limbs, but the decrease in VO2 due to serotonin was greater in the hind limbs from the lean rats, and this difference remained when the data were expressed in terms of muscle mass perfused. It is concluded that the relatively lower content of muscle of the obese hind limb accounts for its lower basal and lower maximal norepinephrine-mediated thermogenesis. In addition, an intrinsic defect in obese hind limb muscle response to serotonin is present, which may be indicative of a decrease in the potential for vasoconstrictor-regulated thermogenesis that could have implications for whole-body energy balance by the obese phenotype.

Mitral insufficiency after pericardiectomy for constrictive pericarditis

We report the case of a 24-year-old man in whom a clinical syndrome developed while he was on active military duty in Saudi Arabia that was subsequently diagnosed as constrictive pericarditis. Phrenic nerve to phrenic nerve pericardiectomy and posterior pericardial release successfully relieved the ventricular constriction with a resultant increase in the cardiac index from 1.9 to 3.8 L.min-1.m-2. Transesophageal echocardiographic monitoring during the operation disclosed trace mitral regurgitation before median sternotomy. The severity of the regurgitation noticeably increased to the moderate level immediately after pericardial resection. This echocardiographic finding had improved 1 week later, but the regurgitation still was greater than baseline. Mitral valve function had returned to baseline by 4 weeks after the operation. Possible mechanisms of this evolving pattern of perioperative mitral valve dysfunction are discussed.

Antihypertensive and haemodynamic properties of the potassium channel activating (-) enantiomer of cromakalim in animal models

The present studies describe the blood pressure lowering, and some other haemodynamic effects, of the potassium channel activator, BRL 38227 ((-) enantiomer of cromakalim, CAS 94470-67-4) in various animal models. BRL 38227 was a potent antihypertensive agent following oral administration to conscious spontaneously hypertensive rats, SHR, (0.038, 0.075 and 0.15 mg/kg), renal hypertensive cats (0.035 and 0.05 mg/kg) and renal hypertensive dogs (0.05 and 0.1 mg/kg). The (+) enantiomer of cromakalim (BRL 38226) was without effect on blood pressure in the conscious rat and cat confirming the stereospecific mode of action of this potassium channel activator. Tachycardia accompanied the antihypertensive effect of BRL 38227 in these models and in the rat this effect could be abolished by pretreatment with atenolol (conscious SHR), diltiazem, verapamil, propranolol and alinidine (anaesthetised rats). In addition to reflex tachycardia, BRL 38227 also increased plasma renin activity and aldosterone levels in the conscious renal hypertensive cat. In both the anaesthetised normotensive cat (0.001 mg/kg/min i.v.) and dog (0.0025 to 0.02 mg/kg i.v.) BRL 38227 lowered blood pressure and total peripheral resistance while increasing cardiac output via increased heart rate and stroke volume in the cat and via increased heart rate alone in the dog. BRL 38227 reduced renal vascular resistance in both conscious (0.01, 0.015 and 0.02 mg/kg p.o.) and anaesthetised (0.001 mg/kg/min i.v.) cats and the effect was maintained despite marked reductions in blood pressure. In the anaesthetised dog, BRL 38227 was a potent coronary arterial dilator and this effect was also maintained in the face of marked blood pressure lowering activity.(ABSTRACT TRUNCATED AT 250 WORDS)

Inhibition by glibenclamide of the vasorelaxant action of cromakalim in the rat

1. In rat isolated thoracic aortic rings pre-contracted with noradrenaline (10(-6) M), cromakalim (3 x 10(-7)-3 x 10(-5) M) produced concentration-related relaxation. This effect was progressively inhibited by increasing concentrations of the anti-diabetic sulphonylurea drug, glibenclamide (10(-6)-10(-5) M). 2. In rat isolated portal veins, cromakalim (3 x 10(-8)-10(-6) M) produced concentration-related inhibition of the spontaneous contractive activity and glibenclamide (3 x 10(-7)-3 x 10(-6) M) prevented this inhibitory action in a concentration-dependent manner. 3. In both rat aortic rings and portal veins, cromakalim (10(-5) M) stimulated 86Rb efflux. Prior exposure to glibenclamide (10(-7)-10(-6) M) produced a concentration-related inhibition of this response. 4. In conscious rats, cromakalim, 0.075 mg kg-1 i.v., produced a rapid and sustained fall in arterial blood pressure which was not influenced by pretreatment (2 h) with a large oral dose of glibenclamide (100 mg kg-1). 5. In conscious rats, the hypotensive action of cromakalim, 0.075 mg kg-1 i.v., was abolished by pretreatment (30 min) with glibenclamide, 20 mg kg-1, given by the intravenous route. 6. The results suggest that the vasorelaxant and hypotensive actions of cromakalim involve a K+ channel which can be inhibited by glibenclamide, but which may be distinct from the ATP-sensitive K+ channel of the pancreatic beta-cell.

The haemodynamic profile of cromakalim in the cat

The haemodynamic profile of the novel antihypertensive agent, cromakalim (BRL 34915), has been studied in conscious and anaesthetised cats. Given orally (0.03 and 0.06 mg/kg) to conscious cats and by intravenous infusion (0.002 mg/kg/min for 7-10 min) to anaesthetised animals, cromakalim reduced arterial pressure (AP; systolic/diastolic) and total peripheral resistance (TPR) while increasing cardiac output (CO) via both its components [heart rate (HR) and stroke volume (SV)]. In both models, HR changes were minimal for reductions of AP approaching 20 mm Hg. In a second group of conscious cats, cromakalim increased mean renal blood flow (MRBF) at hypotensive doses (0.03 and 0.06 mg/kg orally); HR was only significantly raised following the higher dose of cromakalim. In anaesthetised animals, intravenously infused cromakalim had little effect on central venous pressure and increased cardiac contractility. The data indicate that cromakalim lowers AP by reducing TPR with minimal increase in HR. In addition, cromakalim has a potentially beneficial effect on MRBF.

Studies on the anti-vasoconstrictor activity of BRL 34915 in spontaneously hypertensive rats; a comparison with nifedipine

1. The blood pressure lowering and anti-vasoconstrictor effects of BRL 34915 and nifedipine were compared in female spontaneously hypertensive rats (SHR). 2. In conscious SHR, intravenous injection of BRL 34915 (0.1, 0.3 mg kg-1) produced rapid, dose-related falls in mean arterial pressure of greater than 3 h duration. Nifedipine, at the same intravenous dose levels, also evoked rapid anti-hypertensive effects, though these responses were of lesser magnitude and duration than those observed for BRL 34915. 3. In anaesthetized, ganglion-blocked SHR, BRL 34915 (0.1, 0.3 mg kg-1 i.v.) dose-dependently antagonized the pressor responses to incremental intravenous infusions of noradrenaline (3.8-28.5 ng min-1) or phenylephrine (120-907 ng min-1) but did not inhibit pressor responses to incremental infusions of methoxamine (0.47-3.63 micrograms min-1), angiotensin II (7.0-52.9 ng min-1) or vasopressin (0.27-2.0 mu min-1). 4. In anaesthetized, ganglion-blocked SHR, nifedipine (0.1, 0.3 mgkg-1 i.v.) antagonized the pressor responses to each of the infused vasoconstrictor agents, being most effective against responses to noradrenaline or angiotensin II. 5. In pithed SHR, both BRL 34915 and nifedipine (each at 0.3 mg kg-1 i.v.) reduced the basal blood pressure level and produced marked inhibition of frequency-dependent pressor responses evoked by electrical stimulation of the spinal cord sympathetic outflow (0.25-4.0 Hz). Restoration of the basal diastolic blood pressure to within the control range, using a continuous intravenous infusion of vasopressin (0.98 mu min-1), prevented the inhibitory effect of BRL 34915. In the case of nifedipine, however, even raising the basal blood pressure to a level exceeding that recorded in control rats (with vasopressin, 2.0 mu min-1), did not reverse the inhibitory effect of the drug on frequency-dependent pressor responses. 6. It is concluded that the anti-hypertensive properties of BRL 34915 in SHR are probably unrelated to an anti-vasoconstrictor action. In contrast, it is suggested that the broadly-based anti-vasoconstrictor properties of nifedipine may contribute substantially to the anti-hypertensive properties of this drug.

Alpha 1-adrenoceptor-mediated contraction of rabbit mesenteric artery: a role for intra- and extracellular calcium pools

The alpha-adrenoceptors that mediate contraction to exogenous and endogenous noradrenaline in rabbit isolated mesenteric artery were investigated. Prazosin (10(-9)-10(-7) M) antagonised contractions to noradrenaline, methoxamine, and, in particular, contractions to neuronal noradrenaline released by field stimulation. Only a high concentration (10(-5) M) of idazoxan was able to markedly antagonise the three contractile stimuli. The effects of idazoxan (at high concentrations) and prazosin were studied upon noradrenaline-evoked contractions attributable to intracellular Ca2+ release using Ca2+ -deplete medium, and, on readministration of Ca2+, upon Ca2+ influx. Both components of the response were inhibited by the two antagonists, but the contraction associated with intracellular Ca2+ release was preferentially inhibited in each case. The results demonstrate that only alpha 1-adrenoceptors are involved in the contraction of this tissue to exogenous and endogenous noradrenaline. This receptor type is linked to both extracellular and intracellular Ca2+ sources, although the latter is more sensitive to inhibition by alpha-adrenoceptor blocking drugs.

Synthesis and antihypertensive activity of 4-(cyclic amido)-2H-1-benzopyrans

The synthesis and antihypertensive activity of a series of novel 4-(cyclic amido)-2H-1-benzopyran-3-ols, administered orally to conscious spontaneously hypertensive rats, are described. The effects of lactam ring size, the presence of heteroatoms in the lactam ring, substitution at C(2) and C(3), relative stereochemistry at C(3) and C(4), and aromatic substitution pattern on the blood pressure lowering activity of this series have been determined. The key compound 2 from this work [BRL 34915; (+/-)-6-cyano-3,4-dihydro-2,2-dimethyl-trans-4-(2-oxopyrrolidin-1- yl)-2H-1-benzopyran-3-ol] has been resolved, and antihypertensive activity was found to reside primarily in the (-) enantiomer. The key step in the preparation of this class of compounds is the action of a cyclo amidic anion on an appropriate epoxide. Another approach, involving a cyclization step to the lactam was found to be more convenient in certain cases, particularly in forming the cis analogue of compound 2. Compound 2 has been shown to possess a novel mechanism of action, and it has been selected for progression to the clinic.

BRL 34915, a novel antihypertensive agent: comparison of effects on blood pressure and other haemodynamic parameters with those of nifedipine in animal models

The effects of BRL 34915, (+/-) 6-cyano-3,4-dihydro-2,2-dimethyl-trans-4-(2-oxo-1-pyrrolidyl)-2H-b enzo [b]-pyran-3-ol, on blood pressure and other haemodynamic parameters in animals have been investigated in comparison with those of nifedipine. In conscious spontaneously hypertensive rats and renal hypertensive cats and dogs the oral doses of BRL 34915 lowering blood pressure are 10 to 30 times lower than those of nifedipine. Tachycardia evoked by BRL 34915 tends to be less than that produced by nifedipine in the cat and of similar magnitude in the dog. The antihypertensive response to BRL 34915 in these models is reproducible on repeat once daily dosing without rebound hypertension on cessation of dosing. In studies using electromagnetic flow probes to measure regional blood flow in anaesthetised cats the intravenous administration of BRL 34915, unlike that of nifedipine, markedly increases renal blood flow yet BRL 34915 lacks the marked effect of nifedipine on femoral blood flow. BRL 34915, a compound structurally unrelated to existing cardiovascular drugs, is a potent new antihypertensive agent having an interesting profile of activity that renders this compound of clinical interest.

Comparison of the anti-hypertensive response to beta-adrenoceptor blocking drugs in intact and adrenal-demedullated spontaneously hypertensive rats

1 The beta-adrenoceptor blocking drugs atenolol, metoprolol, practolol, propranolol, timolol and oxrenolol (as racemates) were administered acutely at three dose levels (0.01, 0.03 and 0.1 mmol/kg i.p. or s.c.) to spontaneously hypertensive rats with intact adrenal glands (SH-rats) and following unilateral adrenalectomy and contralateral adrenal-demedullation (SHAD-rats). Changes in mean arterial pressure and heart rate were determined via an indwelling aortic catheter, with the animals placed in a quiet environment. 2 All drugs significantly lowered the blood pressure of SHAD-rats, and these responses were not always associated with changes in basal heart rate. 3 With the exception of metoprolol and atenolol, the beta-adrenoceptor blocking drugs were less effective as anti-hypertensives in SH- than in SHAD-rats. Notably, timolol and oxprenolol lowered the blood pressure of SH-rats at low doses only, whereas propranolol evoked a pressor response in this model. 4 Whilst (+)-propranolol lowered the blood pressure of SHAD-rats only at a dose which caused myocardial depression, the anti-hypertensive response to (--)-propranolol did not parallel changes in heart rate and was preceded by a pressor response. 5 The results imply that adrenal catecholamine release contributes towards masking the anti-hypertensive effects of some beta-adrenoceptor antagonists in SH-rats.

Hypotension associated with prekallikrein activator (Hageman-factor fragments) in plasma protein fraction

Thirteen lots of plasma protein fraction made by one manufacturer were implicated in 23 recent reports of hypotension in surgical patients. Four of these patients required resuscitation after rapid administration of the product in the postoperative period. All implicated lots had prekallikrein-activator activity but low levels of bradykinin and kallikrein. The prekallikrein activator was identified as Hageman-factor fragments by molecular weight (35,000 as estimated by gel chromatography), isoelectric point (4.2 to 4.4), and inhibition by antibody to Hageman factor. These data suggest that Hageman-factor fragments are potent hypotensive agents, presumably because they trigger the generation of bradykinin in recipients. Prekallikrein-activator activity, usually at levels lower than those in the initial 13 implicated lots, was frequently detected in plasma protein fraction made by other manufactures. Several of these lots were associated with additional reports of hypotension. Prekallikrein-activator activity rarely occurred in albumin.

Diminishing hypotensive effect of increasing doses of pindolol in DOCA/saline hypertensive rats

In DOCA-saline hypertensive rats, pindolol (4, 20 or 50 mg/kg orally) produced a hypotensive effect which was inversely related to dose. Following adrenal demedullation, a hypotensive response to the highest dose of pindolol was unmasked and the magnitude of the responses to lower doses was increased. The results suggest that adrenal catecholamines moderate the hypotensive effects of high doses of pindolol.

Studies on the cardiovascular effects of pindolol in DOCA/saline hypertensive rats

1 A hypotensive response to orally administered pindolol in conscious normotensive and deoxycorticosterone acetate (DOCA)/saline hypertensive rats (DS-rats) is described. In DS-rats, pindolol (10-50 mug/kg) produced a dose-dependent fall in blood pressure and elevation of resting heart rate.2 The hypotensive response and tachycardia produced by oral pindolol (50 mug/kg) in DS-rats were prevented by propranolol (5 mg/kg), suggesting that pindolol's effects are mediated by beta-adrenoceptor stimulation.3 After mecamylamine (10 mg/kg), oral pindolol (50 mug/kg) produced a further fall in blood pressure in DS-rats, suggesting that its hypotensive effects are probably mediated in the peripheral vasculature.4 Pretreatment with oral pindolol (10 or 50 mug/kg) resulted in a reduction of neuronally-induced tachycardia in pithed DS-rats; neuronally-evoked pressor effects were also antagonized by pindolol (50 mug/kg, orally).5 Whereas pindolol, 50 mug/kg orally or intraperitoneally, produced a marked and progressive hypotensive response of rapid onset (20 min) in DS-rats the same dose intravenously produced a smaller response of delayed onset (80 minutes).6 In anaesthetized DS-rats, an equivalent degree of cardiac beta-adrenoceptor blockade was produced by pretreatment with pindolol, 50 mug/kg orally (2 h previously) or intravenously (1 h previously).7 After administration of pindolol, 2 mg/kg intravenously, to conscious DS-rats, the tachycardia produced by intravenous isoprenaline, 3 mug/kg, was almost abolished for the first 60 min of the study, whereas a hypotensive response to pindolol was delayed in onset (100 minutes).8 The hypotensive response and tachycardia produced by oral pindolol 50 mug/kg, in DS-rats were prevented by inhibition of metabolic enzyme activity by pretreatment with Proadifen (SKF 525-A), 80 mg/kg.9 The results suggest that pindolol's effects on blood pressure and heart rate in the conscious DS-rat are mediated by a metabolite(s) acting by stimulation of peripheral beta-adrenoceptors.

Effect of intracerebroventricular 5,6-dihydroxytryptamine on blood pressure of spontaneously hypertensive rats

The effects of intracerebroventricular injections of 5,6-DHT on the development and maintenance of hypertension in spontaneously hypertensive rats has been investigated. 5,6-DHT, injected into 6 week old rats, retarded the development of hypertension for at least 6 weeks; this effect was not accompanied by inhibition of the pressor response produced by stimulation of the total peripheral sympathetic outflow. 5,6-DHT, injected into 14-15 week old rats with established hypertension, produced a short-lived fall in blood pressure. These findings suggest that central 5-HT neurones are involved in the development of hypertension in spontaneously hypertensive rats.

Prolonged effects of p-chlorophenylalanine on the blood pressure of conscious normotensive and DOCA/saline hypertensive rats

1. In deoxycorticosterone acetate (DOCA) saline hypertensive rats a single dose of p-chlorophenylalanine methylester (PCPAME) (400 mg/kg i.p.) produced a significant fall in blood pressure (20-43 mmHg) which lasted for at least 8 days and was accompanied by a parallel depletion of brain stem 5-hydroxytryptamine (5-ht) but not of noradrenaline (NA). 2. In normotensive rats single doses of PCPAME (200 and 400 mg/kg i.p.) produced a significant hypotension (15-20 mmHg) after a latent period of 5 days. An initial pressor response (12 mmHg) was observed at the higher dose level only on day 3. 3. The hypotensive response to PCPAME (200 mg/kg i.p.) in normotensive rats was not modified by pretreatment with 5,6-dihydroxytryptamine (5,6-DHT; 50 mug i.c.v.) or 6-hydroxydopa (6-ohda; 3 X 250 mug intracerebroventricularly). 4. It is concluded that the hypotensive response to PCPAME in normotensive rats in independent of brain stem depletion of 5-HT and is probably not mediated by the formation of a false transmitter substance acting via central noradrenergic inhibitory pathways. The mechanism involved in the antihypertensive response to PCPAME in DOVA/saline hypertensive rats has yet to be defined.