Troglitazone and vascular reactivity: role of glucose and calcium

Evidence of direct smooth muscle relaxant effects of the fibrate gemfibrozil

Fibrates are commonly employed to treat abnormal lipid metabolism via their unique ability to stimulate peroxisome proliferator-activated receptor alpha (PPARalpha). Interestingly, they also decrease systemic arterial pressure, despite recent evidence that PPAR alpha may contribute to expression of renin and related hypertension. Yet, mechanisms responsible for their potential antihypertensive activity remain unresolved. Rapid decreases in arterial pressure following bolus intravenous injections of bezafibrate strongly suggest they may relax arterial smooth muscle directly. But since bezafibrate is highly susceptible to photodegradation in aqueous media, it has never been critically tested for this possibility in vitro with isolated arterial smooth muscle preparations. Accordingly, we tested gemfibrozil which is resistant to photodegradation. We examined it over a therapeutically-relevant range (50-400 microM) for both acute and delayed relaxant effects on contractions of the isolated rat tail artery; contractions induced by either depolarizing its smooth muscle cell membranes with high potassium or stimulating its membrane-bound receptors with norepinephrine and arginine-vasopressin. We also examined these same gemfibrozil levels for effects on spontaneously-occurring phasic rhythmic contractile activity, typically not seen in arteries under in vitro conditions but commonly exhibited by smooth muscle of uterus, duodenum and bladder. We found that gemfibrozil significantly relaxed all induced forms of contraction in the rat tail artery, acutely at the higher test levels and after a delay of a few hours at the lower test levels. The highest test level of gemfibrozil (400 microM) also completely abolished spontaneously-occurring contractile activity of the isolated uterus and duodenum and markedly suppressed it in the bladder. This is the first evidence that a fibrate drug can directly relax smooth muscle contractions, either induced by various contractile agents or spontaneously-occurring. These findings are particularly relevant to both the recently renewed concern over the impact of fibrates on hypertension and a new understanding of their gastrointestinal side effects.

Rosiglitazone (peroxisome proliferator-activated receptor-gamma) counters hypertension and adverse cardiac and vascular remodeling in 2K1C hypertensive rats

Peroxisome proliferator-activated receptor-gamma (PPAR-gamma) agonists have been shown controlling blood pressure (BP) in spontaneously hypertensive rats and salt-sensitive hypertensive rats. The present study aims to test the hypothesis that PPAR-gamma agonist rosiglitazone has beneficial effects on cardiac and vascular adverse remodeling in a model of renovascular hypertension (two-kidneys-one-clip, 2K1C model). Wistar rats were divided into four groups (n=6): SHAM group, 2K1C, 2K1C+HYD (treated with hydralazine for 5 weeks) and 2K1C+ROSI (treated with rosiglitazone for 5 weeks). The left ventricle (LV), thoracic aorta (Ao) and common carotid artery (CCA) were analyzed. The BP did not show significant difference at the end of the experiment in groups 2K1C+ROSI, 2K1C+HYD and SHAM. The LV mass was smaller in 2K1C+ROSI compared with the other groups. The intima-media thickness was smaller in 2K1C+ROSI compared with untreated 2K1C ones, but not in 2K1C+HYD; 2K1C and 2K1C+HYD showed smaller Ao and CCA density of smooth muscle cell nuclei, and smaller surface density of the elastic lamellae than SHAM. The Ao and CCA circumferential wall tension and tensile stress were greater in 2K1C than in SHAM. Hypertrophied cardiomyocytes were seen in 2K1C, but not in 2K1C+ROSI and SHAM; 2K1C+ROSI had enhanced volume and length densities of intramyocardial arteries than 2K1C. The volume density of cardiac interstitium was greater in 2K1C and 2K1C+HYD than in SHAM. In conclusion, PPAR-gamma agonist rosiglitazone has beneficial effects controlling BP, reducing vascular adverse remodeling, and preserving intramyocardial vascularization in renovascular hypertensive rats (2K1C model).

A rapid, PPAR-gamma-dependent effect of pioglitazone on the phosphorylation of MYPT

Peroxisome proliferator-activated receptor (PPAR)-gamma ligands, thiazolidinediones, have been demonstrated to regulate vascular reactivity. We examined the effect of pioglitazone (PIO; 20 muM) in rat primary cultured aortic smooth muscle cells on constitutive phosphorylation of the regulatory subunit of myosin phosphatase (MYPT). PIO decreased the phosphorylation of Thr(697) on MYPT within 15 min, and the inhibition was maintained up to 6 h. The PPAR-gamma antagonist GW-9662 (5 microM) abrogated the inhibition of Thr(697) phosphorylation mediated by PIO. Because longer-term PIO treatment inhibits RhoA/Rho kinase (ROCK) signaling and Thr(697) phosphorylation, we tested the effect of the ROCK inhibitor Y-27632 (10 muM) on the inhibition of Thr(697) phosphorylation by PIO. Y-27632 alone inhibited Thr(697) phosphorylation, and there was an additive effect with PIO. In addition, up to 1 h of PIO treatment did not affect RhoA localization or decrease ROCK-dependent phosphorylation of Thr(855). These results suggest that the effect of PIO is independent of inhibition of RhoA/ROCK. PIO increased the phosphorylation of Ser(696) in the same time course as its effect on Thr(697). Ser(696) has been shown to be phosphorylated by PKA and PKG. PKA inhibitor H-89 (10 microM) and PKG inhibitor KT-5823 (0.5 microM) abrogated the effect of PIO on both Thr(697) and Ser(696) phosphorylation. The constitutive turnover of phosphorylation of Thr(697) is rapid, suggesting that the decreased phosphorylation of Thr(697) by PIO is due to enhanced phosphorylation of Ser(696). This is supported by the finding that PIO blocks ANG II-stimulated phosphorylation of Thr(697) but not ANG II-stimulated RhoA translocation. Therefore, the effect of shorter-term PIO apparently is to increase myosin light chain phosphatase activity, thereby desensitizing the vascular smooth muscle to agonist signaling.

Effects of PPAR gamma agonists on cardiovascular function in obese, non-diabetic patients

Peroxisome proliferator-activated receptors (PPARs) are nuclear receptors, and when activated by their ligands, they induce perixosome proliferation. Three receptors have been identified: PPAR gamma, PPAR delta, and PPAR alpha, all with different tissue expression. PPAR gamma is predominantly expressed in adipose tissue and regulates the formation of fat cells and their function. The effect of PPAR gamma activation is to enhance the action of insulin in insulin-sensitive tissue by increasing peripheral glucose disposal and decreasing hepatic glucose production. The thiazolidinediones (TZDs) are a class of medications used for treatment and possibly the prevention of type 2 diabetes, which are potent agonists for the PPAR gamma receptor. Because the thiazolidinediones target insulin resistance, these agents may improve many of the risk factors associated with obesity and insulin resistance including dyslipidemia, hypertension, impaired fibrinolysis, and atherosclerosis. The impact of the thiazolidinediones on cardiovascular mortality is currently unclear but it appears that the thiazolidinediones exert numerous non-glycemic effects that may improve cardiovascular outcomes. Several non-TZD PPAR gamma agonists and combined PPAR gamma/alpha effect on cardiovascular disease are also being evaluated. These drugs have anti-inflammatory and vascular properties and are currently the subject of numerous studies targeting the primary and secondary prevention of macrovascular disease in patients with diabetes and insulin resistance and might be developed as anti-atherogenic agents on the basis of their actions.

Antidiabetic and hypolipidemic potential of DRF 2519—a dual activator of PPAR-α and PPAR-γ

We investigated the biological activity of Dr. Reddy's Research Foundation (DRF) 2519, a benzoxazinone analogue of the thiazolidinedione class of compounds. In the in vitro transactivation assay, DRF 2519 showed interesting dual activation of Peroxisome Proliferator Activated Receptor (PPAR) alpha and gamma. In insulin-resistant ob/ob mouse model, DRF 2519 showed significant alleviation of insulin resistance and dyslipidemia, which is better than rosiglitazone. Fatty Zucker rats treated with DRF 2519 showed better reduction of plasma insulin, triglyceride and free fatty acid levels than those treated with rosiglitazone. In addition, these rats were able to clear plasma lipids better when challenged with exogenous lipid (i.v.). DRF 2519 treatment resulted in improved plasma lipid profiles in high-fat-fed Sprague-Dawley rats. Treated rats showed better plasma lipid clearance and hepatic triglyceride secretion. When compared to DRF 2519, fenofibrate was comparatively less efficacious while rosigltiazone showed no activity in these models. In ex vivo studies, DRF 2519 showed induction of liver acyl CoA oxidase mRNA and increase in lipoprotein lipase (LPL) protein expression and activity in adipose tissue. In the in vitro studies, DRF 2519 inhibited the lipid biosynthesis and secretion of apolipoprotein B from human hepatoma (Hep)G2 cells. It also enhanced insulin-induced relaxation of rat aortic smooth muscle. These results indicate that DRF 2519, a dual activator of PPAR-alpha and gamma, could be an interesting development candidate in the management of metabolic disorders and associated complications.

Attenuation by 4-aminopyridine of delayed vasorelaxation by troglitazone

Troglitazone and other thiazolidinediones (TZDs) are thought to relax arterial smooth muscle by directly inhibiting calcium channels in smooth muscle cell membranes. However, until recently such inhibition was only examined acutely, ie, within only seconds or minutes after administration of these agents to arterial smooth muscle preparations. Recently, a novel experiment was reported in which troglitazone caused a 2-phase relaxation of perfused resistance arteries, namely, an acute relaxation (within the first 20 minutes of treatment), which was blocked by a nonselective calcium channel blocker and a delayed relaxation (after 2 hours), which was not. We sought to determine if any of the 4 major potassium (K) channels in vascular smooth muscle play a role in the delayed relaxation. We incubated vascular contractile rings prepared from ventral tail arteries of rats with physiological buffer containing either 0 or 4 micromol/L troglitazone for 3 hours (4 micromol/L is typical of plasma levels from diabetic patients). Different K channel inhibitors (1 mmol/L 4-aminopyridine [4AP]; 1 mmol/L tetraethylammonium [TEA]; 5 micromol/L glyburide; 20 micromol/L barium) were coadministered with each level of troglitazone in additional preparations. Then these arterial rings were contracted with either norepinephrine (NE), arginine vasopressin (AVP), or high-K buffer. All contractions were significantly relaxed by troglitazone (P <.05). Only 4AP significantly attenuated troglitazone's relaxation of NE and AVP contractions (P <.05), though not high-K-induced contractions. TEA, glyburide, and barium had no such influence. Thus, for both adrenergic (NE) and nonadrenergic (AVP) contractions, the delayed arterial vasorelaxation by troglitazone may be mediated at least in part by activation of 4AP-sensitive K channels. Furthermore, the specific subtype of the channels involved is most likely those bound in the outer cell membrane where their effectiveness in terms of mediating relaxation would depend on an intact transmembrane K ion gradient.

Troglitazone and emerging glitazones: new avenues for potential therapeutic benefits beyond glycemic control

Insulin resistance is characterized as one of the major pathogeneses of type 2 diabetes and has been associated with these same cardiovascular risk factors. Troglitazone, rosiglitazone, and pioglitazone are a new class of oral antidiabetic agents which can ameliorate peripheral insulin resistance in type 2 diabetes. There is considerable evidence that trogliterazone may have beneficial effects on cardiovascular and metabolic abnormalities associated with insulin resistance. There is supportive evidence for positive effects of the other glitazones, but they have been less well studied. These potential benefits span effects ranging from molecular events in the arterial wall to amelioration and/or improvement in lipid parameters known to be associated with atherosclerosis.