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Relaxation of porcine smooth muscle by peroxisome proliferator-activated receptor gamma (PPARγ ) agonists is time- and concentration-dependent, but independent of PPARγ Peroxisome proliferator-activated receptors (PPARs) are members of the nuclear hormone receptor family. PPARγ is the molecular target of the thiazolidinediones pioglitazone and rosiglitazone, which act as agonists to restore insulin sensitivity in type II diabetes mellitus (1). Both pioglitazone and rosiglitazone have been reported to have direct effects on vascular smooth muscle to induce relaxation of isolated vascular and airway smooth muscle (2-4). The aim of this study was to determine whether PPARγ agonists induced relaxation responses in blood vessels and bronchioles from the pig. Porcine coronary, mesenteric, and pulmonary arteries, and porcine small bronchioles were set up for isometric tension recording, in a manner similar to those previously reported (5,6). Tissues were pre-contracted with the thromboxane mimetic U46619 (arteries) or the cholinergic agonist carbachol (bronchioles). A single concentration of pioglitazone or rosiglitazone was then added to each preparation and the relaxation responses measured for up to 90 min. Relaxation responses were expressed as a percentage of spasmogen-induced contraction and presented as mean ± S.E.M. Data were analysed by a two-way ANOVA followed by a Bonferroni post-hoc test with P<0.05 considered significant. Rosiglitazone and pioglitazone produced concentration- and time-dependent relaxations of the arteries and bronchioles. For example, in the coronary artery at 60 min, the vehicle control was 105 ± 5 %, while responses in the presence of 3, 10 and 30 µM rosiglitazone were 61 ± 19, 43 ± 11 and -15 ± 12 % control, respectively. In parallel experiments in the coronary artery at 60 min, the vehicle control was 89 ± 1 % control, while responses in the presence of 3, 10 and 30 µM pioglitazone were 80 ± 3, 57 ± 8 and -2 ± 5 % control, respectively. The structurally distinct PPARγ agonist GW1929 also evoked a time- and concentration-dependent relaxation in the coronary artery: at 60 min: control 100 ± 6; 3 µM 85 ± 6; 10 µM 83 ± 8; 30 µM 59 ± 10 % control. The presence of 1 µM GW9662, a PPARγ-selective antagonist, failed to alter responses to pioglitazone in any tissue. For example at 60 min in the pulmonary artery, responses to pioglitazone in the absence and presence of GW9662 were 67 ± 15 and 63 ± 9 % control, respectively. Relaxation responses were not inhibited by blocking K+ channels with tetraethylammonium (10 mM), nitric oxide synthase with L-NAME (300 µM), or removal of the endothelium or epithelium, but were inhibited by removal of extracellular calcium. For example, in the coronary artery, responses in normal or calcium-free Krebs solution were 44 ± 12 % and 78 ± 6 % control in the presence of 10 µM pioglitazone. Pre-incubation with rosiglitazone, but not pioglitazone, inhibited calcium-induced contractions in the coronary artery, whereas both rosiglitazone and pioglitazone inhibited calcium-induced contractions in the small bronchioles. For example, contractile responses to 300 µM CaCl2 in the absence and presence of 30 µM pioglitazone were 46 ± 7 and 20 ± 1 % KCl response. These data demonstrate that PPARγ agonists evoke PPARγ-independent relaxations of isolated arteries and bronchioles that appear to be dependent, at least in part, on extracellular calcium. 1. Ahmadian M et al. (2013) Nat.Med. 19: 557-566 PM:23652116 2. Benson S et al. (2000). Am J Hypertens 13: 74-82. PM:10678274 3. Honda K et al. (2004). J Allergy Clin Immunol 113: 882-888. PM:15131570 4. Law RE et al. (2000). Circulation 101: 1311-1318. PM:10725292 5. Rashid S et al. (2013). Br J Pharmacol 168: 1902-1910. PM:23215842 6. Roberts RE et al. (2013). Toxicol Appl Pharmacol 272: 797-805. PM:23845591
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