Activation of guinea pig pulmonary adenylate cyclase by prostacyclin

Effects of phosphodiesterase inhibitors on vasoactive intestinal peptide-induced relaxation of isolated guinea-pig trachea

The relaxant effect of vasoactive intestinal peptide (VIP) was investigated in isolated guinea-pig trachea in the presence of the phosphodiesterase (PDE) inhibitors, papaverine and 3-isobutyl-1-methylxanthine (IBMX), and the results were compared to those obtained with the cyclic AMP-dependent bronchodilators, isoproterenol and prostaglandin E2 (PGE2). The relaxant effect of VIP was greater when the magnitude of the leukotriene D4 (LTD4)-induced contraction was smaller. A similar effect was also observed for the relaxation induced by isoproterenol but not by PGE2. In the presence of papaverine (1 microM) and IBMX (3 microM), which reduced the 30 nM LTD4-induced contraction to the same extent, the relaxant effect of VIP was not changed, whereas the relaxant effects of isoproterenol and PGE2 were significantly potentiated. The potentiating effect of PDE inhibitors was also observed for the relaxation induced by the adenylate cyclase activator, forskolin, but not for the relaxation induced by the guanylate cyclase activator, sodium nitroprusside. These results suggest that the relaxation induced by VIP is different from that induced by cyclic AMP-dependent bronchodilator in the guinea-pig trachea.

Direct modulation of secretory chloride channels by arachidonic and other cis unsaturated fatty acids

The effect of fatty acids on Cl- channels and transepithelial Cl- secretion is investigated. Patch-clamp experiments show that arachidonic acid blocks Cl- channels in a dose-dependent manner. Kinetic analysis shows that the mean open time is decreased 10-fold with 25 microM arachidonic acid. There is a linear relationship between the reciprocal of mean open time and blocker concentration within the range of 1 to 25 microM. The reciprocal of mean blocked time does not change with arachidonic acid concentration. Other cis unsaturated fatty acids, including oleic, linoleic, and ricinoleic acids, demonstrate similar blocks. Trans unsaturated acids such as elaidic acid and saturated fatty acids, including stearic, palmitic, and myristic acids, do not inhibit the channel at 20 microM. Ricinoleic acid decreases short circuit current in T84 cells, a colonic carcinoma cell line that secretes Cl-. Our results suggest that the direct effect of arachidonic and other fatty acids on Cl- secretion is to block Cl- channel current.

Bronchodilator-mediated relaxation of normal and ovalbumin-sensitized guinea-pig airways: lack of correlation with lung adenylate cyclase activation

Isoprenaline, vasoactive intestinal peptide (VIP), prostaglandin E2 (PGE2) and forskolin caused a dose-dependent relaxation of normal and ovalbumin-sensitized guinea-pig tracheal spirals and lung parenchymal strips in vitro. There was no difference in magnitude of relaxation or sensitivity to these relaxants between normal and sensitized tissues. The rank order of potency (concentration of each drug at which 50% of the maximum is obtained) for these relaxants on both trachea and parenchyma was VIP greater than isoprenaline greater than PGE2 greater than forskolin, although the parenchyma was more sensitive than the trachea. The rank order of efficacy of the drugs used in relaxing both the trachea and lung parenchyma was isoprenaline (10 microM) greater than forskolin (30 microM) greater than VIP (0.1 microM) greater than PGE2 (10 microM). PGE2 at concentrations greater than 1 microM sometimes contracted the lung strip. Pretreatment with indomethacin (8.5 microM), a cyclo-oxygenase inhibitor, reduced the resting tone of tracheal spirals, but did not significantly affect the tone of lung strips. Indomethacin-pretreatment did not affect drug-induced relaxations of either normal or sensitized tracheal spirals. However, both normal and sensitized indomethacin-pretreated lung strips relaxed significantly less (P less than 0.05) to isoprenaline, PGE2 and forskolin. Indomethacin-pretreatment did not affect sensitivity of normal and sensitized trachea or parenchyma to the relaxant drugs. All the relaxant drugs used stimulated adenylate cyclase activity in normal or sensitized lung parenchyma membrane preparations. The rank order of efficacy (maximal activation) was forskolin greater than isoprenaline = VIP greater than PGE2. There was no difference in response between normal and sensitized lungs. Adenylate cyclase activity of normal lung was stimulated as follows: forskolin (100 microM), 500.0 +/- 50.0%; isoprenaline (100 microM), 186.0 +/- 29.0%; VIP (10 microM), 213.0 +/- 19.0% and PGE2 (100 microM), 155.0 +/- 23.0% of basal activity. Similar values were obtained for sensitized lung parenchyma. Indomethacin-pretreatment did not significantly affect normal or sensitized lung adenylate cyclase stimulation by isoprenaline, VIP, forskolin or PGE2. It was concluded that: Immunological sensitization to ovalbumin does not induce hypoactivity of relaxant drug receptors and/or the adenylate cyclase system of the airway tissues of the guinea-pig. (b) There is an apparent lack of correlation between tissue relaxation in vitro and adenylate cyclase activity since the rank order of the efficacy of a range of relaxants was different for the two effects and furthermore indomethacin-treatment of airway tissues yielded differential results.

Desensitization of prostacyclin receptors in a neuronal hybrid cell line

1 Prostacyclin and its stable analogue, carbacyclin, bind competitively to a single population of receptors, and activate adenylate cyclase of the NCB-20 neuronal somatic cell hybrid (Kact = 40.1 nM and 96.1 nM respectively). 2 Culture of NCB-20 cells in the presence of 1 microM carbacyclin for 4 to 16 h results in a progressive decrease in the prostacyclin-dependent activation of adenylate cyclase in cell homogenates with an increase at 16 h of the Kact from 64.1 nM to 174.0 nM and decrease in the maximum adenylate cyclase activation from 41.2 to 15.1 pmol cyclic AMP min-1 mg-1 protein. 3 The prediction that the apparent decrease in affinity in the prostacyclin-dependent activation of adenylate cyclase was secondary to a reduction in receptor numbers was tested directly by measuring binding of [3H]-prostacyclin to membranes of cells exposed to carbacyclin for 16 h. This showed an actual decrease in affinity of the prostacyclin-receptor interaction, as well as a decrease in the total receptor numbers. Thus prolonged exposure of NCB-20 cells to carbacyclin caused reductions in both receptor numbers and affinity, reflected by measurements both of binding and adenylate cyclase activation.

Prostacyclin binding to guinea pig pulmonary receptors

[3H] Prostacyclin bound to membranes of guinea pig lung. Specific binding was saturable, and the results revealed two receptor classes (Kd =16 nM and 258 nM). The binding capacity of the high affinity site was 105 fmol [3H]prostacyclin per mg of membrane protein, and that of the low affinity site was 1257 fmol per mg membrane protein. A comparison of selected prostaglandins as inhibitors of [3H] prostacyclin binding revealed some of the structural requriements of the ligand for occupation of the high affinity receptor.

Enhancement by prostacyclin of the contractility of the guinea-pig airways smooth muscle

Aerosolized prostacyclin (PGI2) potentiated the increase in pulmonary resistance to inflation induced by serotonin, prostaglandin F2 alpha (PGF2 alpha), acetylcholine and histamine in the guinea-pig. This was not due to a reflex, nor to further production of PG cyclooxygenase derivatives. PGI2 and PGF2 alpha induced contraction of the parenchyma lung strip of the guinea-pig, which could be inhibited by polyphloretin phosphate and by PGE1. Since PGF2 alpha failed to potentiate the bronchial responses to acetylcholine, histamine or serotonin, under conditions where PGI2 was effective, the in vitro similarities between the two PGs cannot explain the in vivo results. The ability of PGI2 to potentiate bronchial responses was not shared by the other PGs. Since the latter are either bronchoconstrictor agents by themselves (PGF2 alpha and PGD2), or bronchodilators (PGE1, PGE2), our hypothesis is that PGI2 potentiates the responses of the bronchi to various agonists by a mechanism similar to that which accounts for the potentiation of acute inflammation and pain by PGE1 and PGE2, the latter being ineffective in enhancing the bronchial responses because of the associated bronchodilator activity.