Effects of verapamil on activation of arachidonic acid metabolism in guinea-pig lungs.

The effects of (+/-)-verapamil and its optical isomers on the activation of arachidonic acid (AA) in guinea-pig isolated perfused lungs were investigated. The calcium ionophore A23187 (6-15 nmol), histamine (20-50 nmol) and leukotriene E4 (1-2 nmol) induced the release of thromboxane A2 (TxA2), which was detected by bioassay and by radioimmunoassay of the stable metabolite TxB2. Racemic (+/-)-verapamil (0.4-40 microM) caused concentration-dependent inhibition of A23187-induced release of TxA2 without affecting the conversion of exogenous AA to TxA2. Both (+)-verapamil and (-)-verapamil (1-10 microM) caused concentration-dependent inhibition of histamine-induced release of TxA2. In contrast, racemic (+/-)-verapamil did not inhibit leukotriene E4 (LTE4)-induced release of TxB2. Calcium depletion (with 2 mM ethylenediamine tetra acetate) significantly reduced both histamine-induced release of TxB2 from 8.6 +/- 2.6 to 1.8 +/- 0.8 ng/min (P less than 0.05) and LTE4-induced release of TxB2 from 4.9 +/- 0.9 to 0.5 +/- 0.2 ng/min (mean +/- S.E.M.) (P less than 0.05). Since histamine stimulates phospholipase A2 in guinea-pig lungs, these results suggest that (+/-)-verapamil inhibits phospholipase A2 and that A23187 activates AA metabolism by stimulating phospholipase A2. Although all three agents activate AA metabolism by calcium-dependent processes, LTE4 may stimulate calcium entry via separate mechanisms because it is not inhibited by (+/-)-verapamil.

Different immunological sensitizing regimens and airway responsiveness in vitro to contractile agonists in guinea-pigs.

Groups of guinea-pigs were sensitized to ovalbumin, using different regimens to induce either IgG-like antibodies or IgE-like and IgG-like antibodies and the responsiveness to histamine and carbachol in vitro of tracheal and lung parenchymal strips was determined. EC50 values for histamine and carbachol in both tracheal and lung parenchymal strips were not significantly different for either group of sensitized guinea-pigs or their respective controls. The maximal tension developed in parenchymal strips was not significantly different for either sensitized or control groups. For tracheal strips, maximal tension was not significantly different in each group except in animals with IgG-like antibodies, which developed less maximal tension with histamine than controls. The absence of any increase in maximal tension or any change in EC50 to histamine or to carbachol in tracheal or parenchymal strips from immunized guinea-pigs indicates that immunological sensitization does not increase the responsiveness of smooth muscle to these contractile agonists.

Studies on alpha adrenoceptors in guinea-pig peripheral lung strips.

Contractile responses of guinea-pig peripheral lung strips to noradrenaline were determined in the presence of propranolol (2.5 X 10(-6) mol/l). All strips (n = 44) contracted to noradrenaline and a geometric mean EC50 of 1.4 X 10(-6) mol/l (1.1 X 10(-6) mol/l, 1.8 X 10(-6) mol/l 95% confidence limits) was obtained. Contractions were antagonised by phentolamine (5 X 10(-7)-10(-5) mol/l) and by prazosin (10(-8)-10(-7) mol/l). Dose-ratios (DR) were calculated and log (DR-1) was plotted against log concentration of antagonist to yield slopes (+/- SE) of 0.84 +/- 0.14 and 0.73 +/- 0.22 respectively which were not significantly different from unity. A pA2 value (+/- SE) of 6.7 +/- 0.2 was obtained for phentolamine and 7.5 +/- 0.1 for prazosin. Yohimbine (10(-7)-10(-5) mol/l) did not significantly affect the maximal tension generated or the EC50 values for noradrenaline. These results suggest that alpha 1 adrenoceptors are mediating the contractile responses to noradrenaline. In the presence of cocaine (10(-5) mol/l, n = 18), normetanephrine (2 X 10(-5) mol/l, n = 15), hydrocortisone (2.5 X 10(-5) mol/l, n = 15) and normetanephrine (2 X 10(-5) mol/l) plus cocaine (10(-5) mol/l, n = 15) pA2 values for phentolamine of 6.9, 6.7, 6.6, and 6.3 respectively were obtained. Since these pA2 values are not significantly different from 6.7, it is unlikely that this original pA2 value, which is lower than values obtained with phentolamine at alpha-adrenoceptors in other tissues, may be explained by neuronal or extraneuronal uptake of noradrenaline.(ABSTRACT TRUNCATED AT 250 WORDS)

Side-effects of corticosteroid agents.

Anti-inflammatory corticosteroid drugs are powerful therapeutic agents for a wide range of disorders. However, they do have recognized side-effects, most of which are related to the dose and the duration of therapy. Thus, short courses of even high doses of corticosteroid drugs have very few adverse effects. A detailed knowledge of the long-term side-effects of corticosteroid agents and their incidence will assist the physician in making informed judgements on the potential benefits of treatment with these drugs.

Effects of aerosol antigen challenge in vivo on responsiveness of guinea pig tracheal and parenchymal strips in vitro to contractile agonists.

Guinea pigs sensitized to produce IgG-like antibodies were challenged with aerosol antigen in vivo. The following day responses in vitro to histamine and carbachol of tracheal and lung parenchymal strips (LPS) from challenged animals and unchallenged controls were compared. Responses of LPS to both histamine and carbachol were increased in challenged animals but tracheal strips from challenged guinea pigs did not display increased sensitivity to histamine or carbachol compared with controls. Moreover, the sensitivity of tracheal strips to carbachol decreased in challenged animals. The increased sensitivity of LPS from challenged animals may be due to the release of chemical mediators and down regulation of postsynaptic muscarinic receptors may account for the decreased sensitivity of tracheal strips.

A possible mechanism in the anti-anaphylactic effect of beta-adrenoceptor agonists in guinea-pig lungs.

The anaphylactic mediators, histamine and leukotrienes, stimulate arachidonic acid (AA) metabolism in the guinea-pig lungs, leading to the synthesis and release of thromboxane A2 (TxA2) and other cyclo-oxygenase products. Since TxA2 is a potent bronchoconstrictor, it is possible that the activation of AA metabolism by histamine may contribute to the pulmonary manifestations of anaphylaxis. In the present experiments, histamine-induced release of TxA2 was inhibited by mepyramine (10(-8)-10(-6) M) but not by cimetidine (5 X 10(-5) M) indicating that the release was mediated by H1-receptors. The beta-adrenoceptor agonists, fenoterol (10(-6) M) and isoprenaline (10(-6) M) inhibited the histamine-induced release of TxA2. This effect was partially reversed by propranolol. These results suggest that if histamine-induced TxA2 release is involved in guinea-pig pulmonary anaphylaxis then inhibition of this release may be a factor in the anti-anaphylactic effect of beta-adrenoceptor agonists.

Effect of sensitization and aerosol antigen challenge in guinea-pigs--studies of airway receptor function and characteristics.

Immunological sensitization of guinea-pigs and subsequent antigen inhalation challenge has provided an animal model which has several features in common with human asthma. Impairment of beta-adrenoceptor-mediated function and mechanisms have been postulated to contribute to the hyperreactivity to contractile agonists demonstrated in vivo and in vitro in these animals. Functional and receptor radioligand binding studies were carried out on airway tissue from: non-sensitized; sensitized; sensitized saline challenged; sensitized antigen challenged guinea-pigs. Sensitization did not alter responsiveness of airway tissue to carbachol, although subsequent antigen challenge did increase carbachol sensitivity of peripheral airway tissue six-fold. Neither sensitization itself nor subsequent antigen challenge altered binding characteristics of the muscarinic cholinoceptor ligand [3H]quinuclidinyl benzilate ([3H]QNB) to peripheral airway tissue, suggesting that mechanisms responsible for increases in carbachol sensitivity are distal to these receptors. Relaxation of airway preparations to isoprenaline was not altered by sensitization or further antigen challenge of the animals. However, sensitization significantly reduced affinity but not the total number of binding sites in peripheral airway tissue for the beta-adrenoceptor ligand [3H]dihydroalprenolol ([3H]DHA). Antigen challenge of the animals did not further alter beta-adrenoceptor ligand binding characteristics. These results suggest that airway hyperreactivity in this model is not a function of alteration in receptor characteristics, or impairment of relaxation mechanisms.