Metabolism of [2-14C]prostaglandin E1 on passage through the pulmonary circulation

Recent advances in studies of SLCO2A1 as a key regulator of the delivery of prostaglandins to their sites of action

Solute carrier organic anion transporter family member 2A1 (SLCO2A1, also known as PGT, OATP2A1, PHOAR2, or SLC21A2) is a plasma membrane transporter consisting of 12 transmembrane domains. It is ubiquitously expressed in tissues, and mediates the membrane transport of prostaglandins (PGs, mainly PGE₂, PGF_2α, PGD₂) and thromboxanes (e.g., TxB₂). SLCO2A1-mediated transport is electrogenic and is facilitated by an outwardly directed gradient of lactate. PGs imported by SLCO2A1 are rapidly oxidized by cytoplasmic 15-hydroxyprostaglandin dehydrogenase (15-PGDH, encoded by HPGD). Accumulated evidence suggests that SLCO2A1 plays critical roles in many physiological processes in mammals, and it is considered a potential pharmacological target for diabetic foot ulcer treatment, antipyresis, and non-hormonal contraception. Furthermore, whole-exome analyses suggest that recessive inheritance of SLCO2A1 mutations is associated with two refractory diseases, primary hypertrophic osteoarthropathy (PHO) and chronic enteropathy associated with SLCO2A1 (CEAS). Intriguingly, SLCO2A1 is also a key component of the Maxi-Cl channel, which regulates fluxes of inorganic and organic anions, including ATP. Further study of the bimodal function of SLCO2A1 as a transporter and ion channel is expected to throw new light on the complex pathology of human diseases. Here, we review and summarize recent information on the molecular functions of SLCO2A1, and we discuss its pathophysiological significance.

Development of a high-affinity inhibitor of the prostaglandin transporter

Prostaglandin E(2) (PGE(2)) triggers a vast array of biological signals and physiological events. The prostaglandin transporter (PGT) controls PGE(2) influx and is rate-limiting for PGE(2) metabolism and signaling termination. PGT global knockout mice die on postnatal day 1 from patent ductus arteriosus. A high-affinity PGT inhibitor would thus be a powerful tool for studying PGT function in adult animals. Moreover, such an inhibitor could be potentially developed into a therapeutic drug targeting PGT. Based on structure-activity relationship studies that built on recently identified inhibitors of PGT, we obtained N-(2-(2-(2-azidoethoxy)ethoxy)ethyl)-4-((4-((2-(2-(2-benzamidoethoxy)ethoxy)ethyl)amino)-6-((4-hydroxyphenyl)amino)-1,3,5-triazin-2-yl)amino)benzamide (T26A), a competitive inhibitor of PGT, with a K(i) of 378 nM. T26A seems to be highly selective for PGT, because it neither interacts with a PGT homolog in the organic anion transporter family nor affects PGE(2) synthesis. In Madin-Darby canine kidney cells stably transfected with PGT, T26A blocked PGE(2) metabolism, resulting in retention of PGE(2) in the extracellular compartment and the negligible appearance of PGE(2) metabolites in the intracellular compartment. Compared with vehicle, T26A injected intravenously into rats effectively doubled the amount of endogenous PGE(2) in the circulation and reduced the level of circulating endogenous PGE(2) metabolites to 50%. Intravenous T26A was also able to slow the metabolism of exogenously injected PGE(2). These results confirm that PGT directly regulates PGE(2) metabolism and demonstrate that a high-affinity inhibitor of PGT can effectively prevent PGE(2) metabolism and prolong the half-life of circulating PGE(2).

2,3-Dinor-5,6-dihydro-15-F(2t)-isoprostane: a bioactive prostanoid metabolite

15-F(2t)-isoprostane (15-F(2t)-IsoP), also termed 8-isoprostaglandin F(2alpha), is one of a series of prostanoids formed by free radical-mediated peroxidation of arachidonic acid and exerts potent biological actions such as vasoconstriction. We recently demonstrated that 15-F(2t)-IsoP is metabolized in humans to a major metabolite, 2,3-dinor-5,6-dihydro-15-F(2t)-IsoP (15-F(2t)-IsoP-M). 15-F(2t)-IsoP-M can also potentially be formed as a product of free radical-induced oxidation of the low abundance fatty acid gamma-linolenic acid. We confirmed that 15-F(2t)-IsoP-M is generated during oxidation of gamma-linolenic acid and explored whether it may exhibit biological activity. 15-F(2t)-IsoP-M caused marked constriction of porcine surface retinal and intraparenchymal brain microvessels, comparable to that observed with 15-F(2t)-IsoP. These effects were associated with increased thromboxane A(2) (TXA(2)) formation and were virtually abolished by TXA(2)-synthase and -receptor inhibitors (CGS-12970 and L-670596). Vasoconstriction induced by either 15-F(2t)-IsoP or 15-F(2t)-IsoP-M on perfused ocular choroid was also abrogated by TXA(2)-synthase inhibition as well as by removal of endothelium. Similar to 15-F(2t)-IsoP, 15-F(2t)-IsoP-M evoked vasoconstriction and TXA(2) generation by activating Ca(2+) influx from nonvoltage-gated channels (SK&F96365 sensitive) in the retina and from both nonvoltage- and N-type voltage-gated Ca(2+) channels (omega-conotoxin MVIIA sensitive), respectively, in brain endothelial and astroglial cells; smooth muscle cells were unresponsive to both agents. Cross-desensitization experiments further suggest that 15-F(2t)-IsoP and 15-F(2t)-IsoP-M act on the same receptor mechanism. Findings reveal a novel concept by which a beta-oxidation metabolite of 15-F(2t)-IsoP that can also be formed by nonenzymatic oxidation of gamma-linolenic acid is equivalently bioactive to 15-F(2t)-IsoP and may prolong the vascular actions of F(2)-IsoPs.

Successful weaning from cardiopulmonary bypass with central venous prostaglandin E1 and left atrial norepinephrine infusion in patients with acute pulmonary hypertension

OBJECTIVE

Postoperative pulmonary hypertension increases the mortality risk in cardiac surgery. We have used central venous prostaglandin E1 (PGE1) and left atrial norepinephrine (NE) infusion to wean from cardiopulmonary bypass (CPB) patients with refractory postoperative pulmonary hypertension.

DESIGN

Observational, nonrandomized study.

SETTING

Department of Cardiac Surgery in a university hospital.

PATIENTS

We studied 10 nonconsecutive American Society of Anesthesiologists III and New York Heart Association class III-IV patients with postoperative pulmonary hypertension and low cardiac output syndrome preventing separation from CPB.

INTERVENTIONS

Patients received right atrial PGE1 (31.5 +/- 6.26 ng/kg/min) and left atrial NE (0.11 +/- 0.02 microg/kg/min) infusion. Hemodynamic data were obtained before CPB (T0), after CPB under maximal inotropes and vasodilator infusion (T1), 10 mins (T2) and 12 hrs (T3) after PGE1 and NE infusion, and 48 hrs after withdrawal of PGE1 and NE (T4).

MEASUREMENTS AND MAIN RESULTS

All patients were successfully weaned from CPB and survived. The biatrial infusion of PGE1 and NE caused a dramatic reduction in mean pulmonary artery pressure (from 42.8 +/- 5.1 mm Hg at T1 to 28.5 +/- 2.6 mm Hg at T2 and 20.5 +/- 2.0 mm Hg at T4), pulmonary vascular resistance index (from 1158 +/- 269 dyne x sec/cm5 x m2 at T1 to 501 +/- 99 dyne x sec/cm5 x m2 at T2 and 246 +/- 50 dyne x sec/cm5 x m2 at T4), and pulmonary-to-systemic vascular resistance index ratio (from 0.61 +/- 0.17 at T1 to 0.20 +/- 0.04 at T2 and 0.11 +/- 0.03 at T4). Cardiac index increased from 1.7 +/- 0.2 L/min/m2 at T1 to 2.3 +/- 0.2 L/min/m2 at T2 and 2.9 +/- 0.1 L/min/m2 at T4.

CONCLUSIONS

In patients with refractory postoperative pulmonary hypertension, the combined administration of low-dose PGE1 in the right atrium and NE in the left atrium is an effective means to wean patients from cardiopulmonary bypass.

Molecular mechanisms of prostaglandin transport

Despite the fact that prostaglandins (PGs) have low intrinsic permeabilities across the plasma membrane, they must cross it twice: first upon release from the cytosol into the blood, and again upon cellular uptake prior to oxidation. Until recently, there were no cloned carriers that transported PGs. PGT is a broadly-expressed, 12-membrane-spanning domain integral membrane protein. When heterologously expressed in HeLa cells or Xenopus oocytes, it catalyzes the rapid, specific, and high-affinity uptake of PGE2, PGF2 alpha, PGD2, 8-iso-PGF2 alpha, and thromboxane B2. Functional studies indicate that PGT transports its substrate as the charged anion. The PGT substrate specificity and inhibitor profile match remarkably well with earlier in situ studies on the metabolic clearance of PGs by rat lung. Because PGT expression is especially high in this tissue, it is likely that PGT mediates the membrane step in PG clearance by the pulmonary circulation. Evidence is presented that PGT may play additional roles in other tissues and that there may be additional PG transporters yet to be identified molecularly.

Improvement of arterial oxygenation by selective infusion of prostaglandin E1 to ventilated lung during one-lung ventilation

BACKGROUND

One-lung anesthesia provides a better surgical field for thoracic procedures but also impairs the arterial oxygenation and venous admixture. During one-lung ventilation, pulmonary vasoconstriction is assumed to be present within both ventilated and collapsed lungs. We propose that arterial oxygenation could be optimized by offsetting the vasoconstriction within the microcirculation of ventilated lung.

METHOD

In an anesthetized dog model, incremental doses of prostaglandin E1 (PGE1) were selectively infused into the main trunk of the pulmonary artery of the ventilated lung after one-lung ventilation for 60 min (PGE1 group, n = 9). Arterial oxygenation and calculated venous admixture (Qs/Qt) was also assessed in a time-course control group (Control group, n = 5). During two-lung ventilation (FIO2: 0.66), arterial PO2 and venous admixture was 44.2 +/- 3.5 kPa and 10.7 +/- 2.3%, respectively. One-lung ventilation (FIO2: 0.66) with left lung collapsed reduced arterial PO2 to 11.6 +/- 1.7 kPa and increased venous admixture to 40.7 +/- 5.8% (P<0.001). Venous O2 tension also decreased from 6.3 +/- 0.7 kPa to 5.0 +/- 0.6 kPa with a slight increase in mean pulmonary artery pressure and pulmonary vascular resistance (P<0.05).

RESULTS

During selective infusion of PGE1 at a dose of 0.04 to 0.2 mu g kg-1 min-1, there was a dose-dependent improvement in arterial PO2 with a parallel reduction of venous admixture during one-lung ventilation. Arterial PO2 increased to a maximum of 23.0 +/- 4.3 kPa, and the venous admixture decreased significantly to a minimum of 27.4 +/- 4.2% by PGE1 at a dose of 0.04-0.4 mu g kg-1 min-1 (P<0.01). PGE1 resulted in a small increase in cardiac output and decreases of pulmonary pressure and pulmonary vascular resistance at a relatively high dose of 0.4 mu g kg-1 min-1 during selective infusion (P<0.05).

CONCLUSIONS

These results suggest that a selective pulmonary artery infusion of PGE1 to the ventilated lung within the dose range of 0.04-0.4 mu g kg-1 min-1 is practical and effective to improve arterial oxygenation and reduce venous admixture during one-lung ventilation.

The effect of bleomycin on lung metabolism of prostaglandin E2 in hamster

Lung is a major site of prostaglandin synthesis and degradation. One site of metabolism has been shown to be the endothelial cell. Metabolism of prostaglandins has been shown to be influenced by both physiological and pathological mechanism. Furthermore, it has been suggested that a relationship might exist between pulmonary disease and the lung's ability to synthesize and/or degrade prostaglandins. Therefore, we evaluated if bleomycin-induced fibrosis, a model of human pulmonary fibrosis, affects the ability of lung to metabolize prostaglandins. Single pass metabolism of prostaglandin E2 was evaluated in an isolated, perfused and ventilated lung of hamsters at 5 and 500 nM concentrations 4,7,14,21 and 28 days after intratracheal bleomycin. The metabolism of prostaglandin E2 was not changed at the 5 nM level, but was significantly decreased at 500 nM level on day 14 and day 28 after intratracheal bleomycin. The results suggest that intratracheal bleomycin causes alterations in prostaglandin metabolism; the mechanism(s) is unknown but may be related to endothelial cell damage and possible changes in alveolar-capillary surface area.

Influence of plasma protein on the inhibitory effects of indocyanine green and bromcresol green on pulmonary prostaglandin E1 extraction

The purpose of this study was to examine the influence of plasma protein on the inhibitory effects of the anionic dyes indocyanine green and bromcresol green on prostaglandin E1 (PGE1) uptake by the lungs. Dog lung lobes were isolated and perfused with either autologous plasma or Krebs-Ringer bicarbonate solution (KRB) containing no protein but with dextran used as a colloid. PGE1 uptake was determined by injecting a bolus, containing radiolabelled PGE1 into the lobar artery and then analysing ethanolic extracts of the venous effluent for radioactivity in PGE1 and PGE1 metabolites by thin layer chromatography and scintillation counting. When the lobes were perfused with KRB, bromcresol green at an average initial concentration of 28.5 microM, reduced PGE1 by an average of 56%. When the lobes were perfused with plasma, similar concentrations of bromcresol green reduced the uptake by less than 2%. A similar result was obtained with indocyanine green, which at an average initial concentration of 17.5 microM reduced uptake by about 70% when the lobes were perfused with KRB, but when the lobes were perfused with plasma similar concentrations of the dye reduced uptake by less than 3.5%. The results suggest that plasma protein binding interferes with the inhibitory effects of these dyes on PGE1 uptake in the lungs.

Comparison of the vasodepressor action of ZK 36 374, a stable prostacyclin derivative, PGI2 and PGE1 with their effect on platelet aggregation and bleeding time in rats

ZK 36 374, a new, chemically stable prostacyclin derivative, was compared with PGE1 and PGI2 with respect to its action on platelet aggregation in vitro, on bleeding time and on arterial blood pressure in conscious rats. The time of occlusion of a hole in a polyethylene (PE) tube of an AV-shunt between the left carotid artery and the right jugular vein by a microthrombus was considered as an index of bleeding time. All three substances inhibited the ADP-induced aggregation of human and rat platelets. In human PRP, ZK 36 374 was 17 times more active than PGE1 and 2 - 5 times as potent as PGI2. In contrast, in rat PRP, PGI2 was 9.2 and 3.4 times as potent as PGE1 and ZK 36 374, respectively. Similar differences in potency were found in the in vivo experiments where these substances given by an intravenous infusion to conscious rats prolonged bleeding time and depressed mean arterial pressure in a dose-dependent manner. ZK 36 374 was also orally active. At oral doses of 1, 2 and 3 mg/kg this new compound caused a dose-dependent prolongation of bleeding time and a fall in arterial blood pressure. In conclusion, the results show that ZK 36 374 is an intravenously and orally active prostacyclin derivative which may be of therapeutic value for occlusive peripheral vascular diseases.

Uptake of metabolism of norepinephrine in isolated perfused fetal, newborn and adult rabbit lungs

We compared the ability of isolated perfused lungs from previable, 26-day gestation, fetal rabbits; newborn rabbits (within 12 hours of birth) and 3 month old adult rabbits to metabolize a 20-second bolus of norepinephrine (NE). The concentration of NE infused was much below the Km for the NE uptake process to assure first order uptake kinetics. At these low concentrations no vasoactivity was observed. The retention time of a vascular marker dye was monitored as an index of pulmonary vascular surface area. In all three sizes of lungs perfusate flow was adjusted to produce an approximately 7 second dye retention time. At these flow adult and newborn lungs inactivate about 50 to 60 percent of the infused NE. In contrast, fetal rabbit lungs inactivate about 80 percent of the infused NE. We conclude that circulating NE is most avidly taken up and metabolized during fetal lung development. The physiologic significance of this fetal NE inactivation remains unknown.

Uptake and metabolism of prostaglandin E1 in isolated perfused fetal, newborn and adult rabbit lungs

PGE1 has been used to maintain the patency of the ductus arteriosus in newborns with pulmonary hypertension. However, it is known that adult lungs avidly take up and metabolize circulating PGE1. We compared the ability of isolated perfused lungs from previable, 26-day gestation, fetal rabbits; newborn rabbits (within 12 hours of birth) and 3 month old adult rabbits to metabolize a 20-second bolus of PGE1. The concentration of PGE1 infused was approximately 2 orders of magnitude below the Km for the PGE1 uptake process so that first order uptake kinetics are assured. The retention time of a vascular marker dye was monitored as an index of pulmonary vascular surface area. In all three sizes of lungs perfusate flow was adjusted to produce an approximately 7 second dye retention time. At these flows the adult lungs inactivate about 45 percent of the infused PGE1. In contrast, fetal and newborn rabbit lungs both inactivate about 22 percent of the infused PGE1. We conclude that the ability of the lung to take up and metabolize circulating PGE1 is markedly reduced in premature and term newborns.

Effect of sulphasalazine on pulmonary inactivation of prostaglandin F2 alpha in the pig

1 The metabolism of prostaglandin F2 alpha (PGF2 alpha) 15 nm in 100,000 g supernatant fractions from piglet lung homogenates was inhibited by sulphasalazine with an IC50 value of 25 micrometers. 2 The piglet isolated lung perfused with Krebs solution, containing either albumin or Ficoll 70 to prevent oedema and vascular damage, efficiently metabolized PGF2 alpha given as a bolus injection (1 ng in 0.1 ml; 30 nm). 3 In Krebs solution containing Ficoll 70, sulphasalazine inhibited the pulmonary inactivation of PGF2 alpha in a dose-dependent manner with an IC50 value of 110 micrometers. No inhibition of inactivation by sulphasalazine was found when the perfusion fluid contained albumin, which is known to bind this drug effectively. 4 Analysis of the separated efflux profiles for PGF2 alpha and its metabolites with reference to the dilution curve for an extracellular marker provided evidence that sulphasalazine inhibited PGF2 alpha uptake into lung cells. 5 We conclude that the effect of sulphasalazine on pulmonary prostaglandin inactivation is primarily due to inhibition of prostaglandin transport, and not to inhibition of prostaglandin metabolism.

Effect of gestational age on pulmonary metabolism of prostaglandin E1 & E2

The fetus and prematurely delivered newborn lamb have high concentrations of circulating PGE2 that may play a hormonal role, particularly in maintaining the patency of the ductus arteriosus. We studied the ability of the isolated, perfused lung from immature (100 +/- 2 days gestation, +/- SEM n = 8) and near term (142 +/- 1 days, n = 10; term is 150 days) lamb fetuses to metabolize PGE2 as a function of PGE2 concentration in the perfusate. After an intra-arterial infusion of 3H-PGE2 and 14C-inulin (to act as a marker of extracellular space), the bulk of the 14C-inulin was rapidly cleared through the isolated lung and the majority of the 3H activity appeared after the 14C activity had fallen to negligible values. The 3H activity that was retained longer in the lung was primarily associated with the 15-keto prostaglandin E2 and 15-keto-13,14 dihydro prostaglandin E2 metabolites. Lungs from immature fetal lambs metabolized 25% less PGE2 than did lungs from animals near term. This is consistent with our prior observation that premature lambs have decreased plasma clearance rates (in vivo) and elevated circulating concentrations of PGE2 when compared with term newborn lambs.

Elimination of low steady-state concentrations of [5,6-3H2]prostaglandin E1 in the pulmonary and the systemic circulations of anaesthetized rats

The elimination of [3H]prostaglandin E1 in anaesthetized rats was studied by continuous intravenous or intraarterial infusions, producing steady-state concentrations at the level of endogenous prostaglandin E2 in mixed venous blood. Blood samples (0.5 ml) were collected from the carotid artery or the right atrium, respectively. The levels of [3H]prostaglandin E1 were measured at different infusion time intervals and the 3H-labeled hydrophobic metabolites characterized. Cardiac output was estimated by a modification of the dye injection method, using 125I-labelled albumin as the marker. From the cardiac output and the rate of infusion, the fractional clearance of the lung and the systemic beds in the steady-state situation were estimated to 88.3 +/- 3.2% and 54.1 +/- 15.2% (mean +/- S.D.), RESPECTIVELY. The hydrophobic metabolites were characterized chromatographically on Sephadez LH-20 columns, using synthetically prepared [14C]prostaglandin metabolites as internal standards and markers. The identities of some metabolites were further established by derivative formation to a constant [3H]/[14C] ratio. The major metabolite was 15-keto-13,14-dihydro-[3H]prostaglandin E1, while 15-keto-[3H]prostaglandin E1 and 13,14-dihydro-[3H]prostaglandin E1 could not be demonstrated.

The role of the blood in metabolism of prostaglandin E1 in the cat lung

We found that when 15-keto-PGE1 was added to cat blood, it was converted to 13, 14-dihydro-15-keto-PGE1 (dihydro-keto-PGE1) by a NADH-dependent enzyme associated with some formed element(s) in the blood. When PGE1 was injected into the pulmonary artery of blood-perfused lungs, the only metabolite detectable in the pulmonary venous blood was the dihydro-keto-PGE1. However, when the lungs were perfused with an artificial perfusate containing no blood cells, a small amount of 15-keto-PGE1 was detected in the venous effluent. Therefore it would appear that a blood-borne delta13 reductase was partially responsible for the conversion of PGE1 to dihydro-keto-PGE1 on passage through blood-perfused cat lungs.

Dependence of pulmonary prostaglandin metabolism on carrier-mediated transport processes

Inhibitors of prostaglandin (PG) transport (probenecid, indomethacin, or bromcresol green) were found to eliminate the difference between the pulmonary transit time of 3H and 14C when [3H]PGF2alpha and E114C]sucrose were injected as a single intra-arterial bolus into the isolated perfused rat lung. Similar results were obtained with PGE1. The transit time of [3H]PGA1 was not significantly different from that of [14C]sucrose even in the absence of an inhibitor. These inhibitors increased the amount of [3H]PGF2alpha or [3H]PGE1 and decreased the amount of [3H]PG metabolites found in the venous effluent: these agents also inhibited the pulmonary metabolism of continously infused, nonradioactive PGF2alpha. One of the three inhibitors, bromcresol green, was shown not to be an effective inhibitor of PG metabolism in cell-free preparations of rat lung homogenates. These results indicated that under normal conditions, PG's are rapidly transported into intracellular compartment(s) where they are metabolized. Inhibition of this transport process prevents rapid access of PG's to the cytoplasmic enzymes and therefore inhibits pulmonary PG metabolism. This implies that inhibitors of PG transport, including anti-inflammatory organic acids, and some PG antagonists, metabolites, and analogues, can be expected to inhibit the pulmonary metabolism of PG's and thus could potentiate the systemic effects endogenous or exogenous PG's.