An evolutionarily conserved feature of cellular organelles is the distinct phospholipid composition of their bounding membranes, which is essential to their identity and function. Within eukaryotic cells, two major lipid territories can be discerned, one centered on the endoplasmic reticulum and characterized by membranes with lipid packing defects, the other comprising plasma-membrane-derived organelles and characterized by membrane charge. We discuss how this cellular lipid organization is maintained, how lipid flux is regulated, and how perturbations in cellular lipid homeostasis can lead to disease.

1 To characterize the prostanoid receptors on human pulmonary smooth muscle involved in vasodilatations, isolated arteries and veins were contracted with norepinephrine (10 mM) and vessels were subsequently challenged with dierent prostanoid-receptor agonists in the absence or presence of selective antagonists. 2 Prostaglandin D 2 (PGD 2 ) and the selective DP-receptor agonist, BW245C, induced relaxations in the contracted human pulmonary venous preparations. The pD 2 values were: 6.88+0.11 (n=17) and 7.31+0.12 (n=5), respectively. The relaxant responses induced by PGD 2 were reduced by the selective DP-receptor antagonist, BWA868C, and the estimated pA 2 value was 7.84+0.16 (n=4). PGD 2 and BW245C did not relax contracted human pulmonary arteries. 3 The selective IP-receptor agonists, iloprost and cicaprost, both induced relaxations in the contracted human vascular preparations. The pD 2 values for iloprost were: 7.84+0.08 (n=6) and 8.25+0.06 (n=4) and for cicaprost: 8.06+0.12 (n=5) and 8.11+0.09 (n=5) in arteries and veins respectively. 4 Prostaglandin E 2 (PGE 2 ) and the EP 2 /EP 3 -receptor agonist, misoprostol, partially relaxed the contracted venous preparations and the pD 2 values were: 8.10+0.15 (n=15) and 6.24+0.33 (n=3), respectively. These relaxations suggest the presence of an EP receptor in the human pulmonary veins. The contracted human pulmonary arteries did not relax when challenged with PGE 2 . 5 In human pulmonary venous preparations, the PGE 2 -induced relaxations were neither modi®ed by treatment with TP/EP 4 -receptor antagonist, AH23848B (10 and 30 mM, n=6), nor by the DP/ EP 1 /EP 2 -receptor antagonist, AH6809 (3 mM, n=6). 6 These data suggest that the relaxation induced by prostanoids involved DP-, IP-receptors and to a lesser extent an EP-receptor on human pulmonary venous smooth muscle. In contrast, only the IPreceptor is involved in the prostanoid induced relaxations on human pulmonary arterial smooth muscle.

1 To characterize the prostanoid receptors (TP, FP, EP 1 and/or EP 3 ) involved in the vasoconstriction of human pulmonary veins, isolated venous preparations were challenged with di erent prostanoid-receptor agonists in the absence or presence of selective antagonists. 2 The stable thromboxane A 2 mimetic, U46619, was a potent constrictor agonist on human pulmonary veins (pEC 50 =8.60+0.11 and E max =4.61+0.46 g; n=15). The a nity values for two selective TP-antagonists (BAY u3405 and GR32191B) versus U46619 were BAY u3405: pA 2 =8.94+0.23 (n=3) and GR32191B: apparent pK B =8.25+0.34 (n=3), respectively. These results are consistent with the involvement of TP-receptor in the U46619 induced contractions. 3 The two EP 1 -/EP 3 -agonists (17-phenyl-PGE 2 and sulprostone) induced contraction of human pumonary veins (pEC 50 =8.56+0.18; E max =0.56+0.24 g; n=5 and pEC 50 =7.65+0.13; E max =1.10+0.12 g; n=14, respectively). The potency ranking for these agonists: 17-phenyl-PGE 2 4sulprostone suggests the involvement of an EP 1 -receptor rather than EP 3 . In addition, the contractions induced by sulprostone, 17-phenyl-PGE 2 and the IP-/EP 1 -agonist (iloprost) were blocked by the DP-/EP 1 -/EP 2 -receptor antagonist (AH6809) as well as by the EP 1 antagonist (SC19220). 4 PGF 2a induced small contractions which were blocked by AH6809 while¯uprostenol was ine ective. These results indicate that FP-receptors are not implicated in the contraction of human pulmonary veins. 5 These data suggest that the contractions induced by prostanoids involved TP-and EP 1 -receptors in human pulmonary venous smooth muscle.

1 Iloprost and cicaprost (IP-receptor agonists) induced relaxations in the histamine-(50 mM) contracted human bronchial preparations (pD 2 values, 6.63+0.12 and 6.86+0.08; E max values, 90+04 and 65+08% of the papaverine response for iloprost (n=6) and cicaprost (n=3), respectively).2 Prostaglandin E 2 (PGE 2 ) and misoprostol (EP-receptor agonist) relaxed the histamine-contracted human bronchial preparations (pD 2 values, 7.13+0.07 and 6.33+0.28; E max values, 67+04 and 57+08% of the papaverine response for PGE 2 (n=14) and misoprostol (n=4), respectively). In addition, both relaxations were inhibited by AH6809 (DP/EP 1 /EP 2 -receptor antagonist; 3 mM; n=5 ± 6). 3 The PGE 2 -induced relaxations of human bronchial preparations were not modi®ed by treatment with AH23848B (TP/EP 4 -receptor antagonist; 30 mM; n=4). 4 The contracted human bronchial preparations were signi®cantly relaxed by prostaglandin D 2 (PGD 2 ) or by BW245C a DP-receptor agonist. However, these responses did not exceed 40% of the relaxation induced by papaverine. In addition, the relaxations induced by PGD 2 were signi®cantly inhibited by treatment with a DP-receptor antagonist BWA868C (0.1 mM; n=3). 5 These data suggest that the relaxation of human isolated bronchial preparations induced by prostanoids involved IP-, EP 2 -and to a lesser extent DP-receptors but not EP 4 -receptor.

Reduced synthesis of nitric oxide (NO) contributes to the endothelial dysfunction and may be related to limited availability of L-arginine, the common substrate of constitutive nitric-oxide synthase (NOS) and cytosolic arginase I and mitochondrial arginase II. To determine whether arginases modulate the endothelial NO synthesis, we investigated the effects of the competitive arginase inhibitor N -hydroxy-nor-L-arginine (Nor-NOHA) on the activity of NOS, arginases, and L-arginine transporter and on NO release at surface of human umbilical vein endothelial cells (HUVECs). In unstimulated cells, Nor-NOHA dose-dependently reduced the arginase activity with maximal inhibition at 20 M. When HUVECs were stimulated by thrombin without extracellular L-arginine, Nor-NOHA dose-dependently increased the NOS activity and the NO release with maximal effects at 20 M. Extracellular L-arginine also dosedependently increased NO release and arginase activity. When HUVECs were stimulated by thrombin in the presence of 100 M L-arginine, NOS activity and NO release were similar in untreated and Nor-NOHA-treated cells. However, despite activation of L-arginine uptake, the inhibition of arginase activity by Nor-NOHA was still significant. The depletion of freely exchangeable L-arginine pools with extracellular L-lysine did not prevent Nor-NOHA from increasing the NO release. This indicates the presence of pools, which are accessible to NOS and arginase, but not exchangeable. Interestingly, the mitochondrial arginase II was constitutively expressed, whereas the cytosolic arginase I was barely detectable in HUVECs. These data suggest that endothelial NO synthesis depends on the activity of arginase II in mitochondria and L-arginine carriers in cell membrane.

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1 Acetylcholine (ACh) and the M, agonists (McN-A-343 or PD142505) relaxed human isolated pulmonary arteries which were pre-contracted with noradrenaline (10 ,gM). In preparations where the endothelium had been removed ACh induced a contractile response whereas the M1 agonists (McN-A-343 or PD142505) had no effect. 2 ACh-and McN-A-343-induced relaxations were abolished after treatment of endothelium-intact preparations with the drug combination N0-nitro-L-arginine (L-NOARG: 0.1 mM) and indomethacin (1.7 giM). 3 The affinity (pKB value) for pirenzepine was higher in human pulmonary arteries when tissues were relaxed with McN-A-343 as compared with ACh (pKB values, 7.71+0.30 (n=4) and 6.68+0.15 (n=8), respectively). In addition, the affinity for pFHHSiD against McN-A-343-and ACh-induced relaxations was 6.86+0.13 (n=3) and 7.35+0.11 (n=9), respectively. 4 The low affinities for methoctramine in human isolated pulmonary arteries with the endothelium either intact or removed, suggested the lack of involvement of M2 and M4 receptors in the ACh responses.5 Phenoxybenzamine (3 /M: 30 min) abolished both ACh contraction and relaxation in human pulmonary artery. The ACh contraction was present when the phenoxybenzamine treatment was preceded by incubation with pFHHSiD (2 gM) but not with pirenzepine (1 gM). In addition, the ACh relaxation was present when preparations were treated with either pFHHSiD (2 gM) or pirenzepine (1 gM), before exposure to phenoxybenzamine.6 These results in human isolated pulmonary arteries support the notion that only M3 receptors, on smooth muscle, mediate the ACh-induced contraction whereas M3 and M, receptors are involved in the endothelium-dependent ACh-induced relaxation.

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