Species dependent relaxation of intrapulmonary arteries (IPA) of rabbits, dogs and humans by prostacyclin

Relaxant actions of nonprostanoid prostacyclin mimetics on human pulmonary artery

The specific prostacyclin (IP) receptor agonist cicaprost relaxed human pulmonary artery preparations precontracted with phenylephrine [50% inhibitory concentration (IC50) approximately 0.6 nM], U-46619 (IC50 approximately 0.9 nM), and K+ (approximately 40% maximal relaxation); endothelium removal had little effect on relaxant activity. Ranking of relaxant potencies for prostacyclin and five of its analogs was 17 alpha, 20-dimethyl-delta 6,6a-6a-carba PGI1 (TEI-9063) > or = cicaprost > iloprost > prostacyclin > taprostene > benzodioxane prostacyclin > 15-deoxy-16 alpha-hydroxy-16 beta,20-dimethyl-delta 6,6a-6a-carba PGI1 (TEI-3356). The potency of the isocarbacyclin TEI-3356 may have been under-estimated because of its contractile (EP3 receptor agonist) activity. The potency ranking of four nonprostanoid prostacyclin mimetics was 3-[4-(4,5-diphenyl-2-oxazolyl)-5-oxazolyl]phenoxy] acetic acid (BMY 45778; IC50 approximately 2.5 nM) > > 2-[3-[2-(4, 5-diphenyl-2-oxazolyl)ethyl]phenoxy]acetic acid (BMY 42393) > octimibate > CU 23 (a novel diphenylindole). From IP receptor binding affinities obtained on human platelet membranes, it is suggested that the slightly shallower log concentration-response curves for BMY 45778, BMY 42393, and CU 23 may reflect the near-maximal receptor occupancy required for complete relaxation. A fifth nonprostanoid, CU 602, had much shallower log concentration-response curves than cicaprost against phenylephrine tone but not against U-46619 tone; this may indicate IP receptor partial agonism coupled with TP receptor antagonism. The relaxant actions of the nonprostanoid mimetics were more persistent than those of the prostacyclin analogs on washout of the organ bath; by the inhalation route, this type of compound may be retained within pulmonary tissue and thus afford greater pulmonary/systemic selectivity than currently used pulmonary vasodilators.

Potent contractile actions of prostanoid EP3-receptor agonists on human isolated pulmonary artery

1. In 13 of 15 experiments, prostaglandin E2 (PGE2) and sulprostone (a prostanoid EP1/EP3-receptor agonist) contracted isolated rings of human pulmonary artery at low concentrations (> or = 5 and > or = 0.5 nM respectively). Tissue was obtained from patients undergoing surgery mainly for carcinoma of the lung. Characterization of the receptors involved was complicated by loss of sensitivity to the contractile PGE action over the experimental period. In contrast, contractile responses to KCl, phenylephrine and the specific thromboxane (TP-) receptor agonist, U-46619, did not decrease with time. 2. The relative contractile potencies for seven PGE analogues, measured during the first few hours after setting up the preparations, were as follows: sulprostone > misoprostol = gemeprost > or = PGE2 > or = GR 63799X > 17-phenyl-omega-trinor PGE2 > or = 11-deoxy PGE1. This ranking indicates that an EP3-receptor is involved. 3. The contractile action of sulprostone was not blocked by the TP-receptor antagonists, EP 169 and GR 32191, and the EP1-receptor antagonist, AH 6809. 4. In two experiments, PGE2 (50 nM) reduced basal tone and sulprostone was a weak contractile agent. Phenylephrine-induced tone was also inhibited by PGE2 (EC50 = 5-20 nM), 11-deoxy PGE1 and butaprost (a selective EP2-receptor agonist); the latter prostanoids were about 2 and 4 times less potent than PGE2 respectively. Interactions with phenylephrine were different in experiments where PGE2 alone was contractile: PGE2 induced contraction superimposed on the phenylephrine response and 11-deoxy PGE1 induced either further contraction or had no effect. Butaprost produced relaxation at high concentrations;this may not be an EP2 action since preparations were highly sensitive to relaxant actions of prostacyclin (IP-) receptor agonists (cicaprost and TEI-9063).5 The study has shown that in the majority of experiments on the human isolated pulmonary artery,the contractile EP3 system outweighed the relaxant EP2 system. However, in two experiments the reverse was true. It is not clear to what extent these differences are due to disease processes affecting the tissues.The findings are discussed in relation to the adverse cardiovascular responses occasionally encountered during treatment of postpartum haemorrhage with sulprostone, and more generally to the clinical use of EP-receptor agonists in man.

Maintenance of functional activity of human pulmonary arteries after cryopreservation

1. Human intrapulmonary arteries have been investigated in vitro in fresh tissue or after storage at -190 degrees C in foetal calf serum containing 1.8 M dimethyl sulphoxide. 2. After cryopreservation of the arteries, maximal contractile force was reduced to 76%. This was assessed by the responses (in g) to 10 nM of the thromboxane analogue, U 46619. 3. Constricting agonists such as noradrenaline, 5-hydroxytryptamine, histamine and U 46619 stimulated fresh and frozen/thawed arteries producing pD2 values similar to the respective values determined on fresh tissues. 4. Endothelium-independent relaxant responses of U 46619-precontracted arteries to prostacyclin (PGI2), aminophylline and papaverine were generally unchanged after storage. The same was true for relaxant response to the potassium channel activator P-1075 whereas the pD2 values for SDZ PCO 400, RP 49356 and cromakalim were somewhat diminished. 5. Nevertheless, a significant correlation was obtained when the apparent pD2 values for all agonists on fresh and frozen/thawed tissues were compared (P less than 0.001). 6. The evidence suggests that after cryopreservation of human intrapulmonary arteries at -190 degrees C, mechanisms of both contraction and relaxation are well-maintained.

Histamine releases PGI2 from human pulmonary artery

Histamine caused a triphasic response of human pulmonary artery strips in vitro, consisting of a small initial contraction followed by pronounced relaxation preceding a second contractile response. These characteristics were not seen with other contractile stimuli including 5-hydroxytryptamine, leukotriene D4, and KCl. The relaxant component of this response was ablated by removal of endothelium from the vascular strips or by pretreatment of the tissue with 1 microM indomethacin. Measurement of the PGI2 degradation product 6-keto-PGF1 alpha in supernatants from histamine-challenged tissues confirmed the synthesis of PGI2. Supernatants from unstimulated or leukotriene-challenged tissues contained no detectable amounts of 6-keto-PGF1 alpha. The histamine H1 antagonist diphenydramine inhibited both the contractile and relaxant responses to histamine whereas the H2 antagonist cimetidine affected neither component. The released PGI2 significantly altered the dose-response curve to histamine without inhibiting the maximal contractile responses. We conclude that histamine induces PGI2 formation from pulmonary arterial endothelium via an H1 receptor.

Effects of indomethacin, prostaglandins E2 and I2 on the tone of human isolated femoral arteries

Strips of endothelium-denuded femoral arteries from operated patients were set up for isometric recording. Indomethacin (IND, 3 mumol 1(-1] enhanced basal tension of the arteries from 0.13 +/- 0.04 to 0.86 +/- 0.12 mN (P less than 0.001; n = 16) and potentiated the contractile responses of the strips to noradrenaline (NA); EC50 for NA was 1.5 +/- 0.3 mumol 1(-1) (n = 6) in the absence, and 0.4 +/- 0.07 mumol 1(-1) (n = 6) in the presence of IND (P less than 0.01). PGI2 produced dose-related relaxation in IND-treated vessels, its IC50 being 15.0 +/- 1.3 nmol 1(-1). Low concentrations of PGE2 (0.85-8.5 nmol 1(-1] reduced whereas its higher concentrations (28-85 nmol 1(-1] increased smooth muscle tone in the presence of IND. These results indicate that human femoral arteries--unlike femoral arteries of some laboratory animals--are highly sensitive to cyclooxygenase inhibition as well as to PGI2 and PGE2.