Studies of receptors and modulatory mechanisms in functional responses to cysteinyl-leukotrienes in smooth muscle

Abstract

Cysteinyl-leukotrienes, i.e. leukotriene (LT) C4, D4 and E4, are inflammatory mediators and potent airway- and vasoconstrictors. Two different cysteinyl-leukotriene receptors have been cloned, CysLT, and CysLT2. The aim of this thesis was to explore the effects of cysteinyl- leukotrienes in smooth muscle containing tissues of mainly vascular or airway origin. In order to characterise the CysLT receptors as well as the modulatory mechanisms involved in cysteinyl- leukotriene-induced responses, contractions of isolated tissue preparations were studied in organ baths and mediator release and leukotriene metabolism were analysed by enzyme immunoassay and RPHPLC, respectively.

In the guinea-pig trachea, the CysLT, receptor antagonist ICI 198,615 partially inhibited the contractions induced by LTD4, and abolished the LTE4-induced contractions. The contractions induced by LTC4 in the guinea-pig trachea and ileum were resistant to CysLT, receptor antagonism and competitively inhibited by the dual CysLT1/CysLT2 receptor antagonist BAY u9773, supporting that LTC4 activates CysLT2 receptors in these preparations. In contrast, the contractions of the human and porcine pulmonary arterial smooth muscles were resistant also to BAY u9773, suggesting the presence of another CysLT receptor subtype.

In human and porcine pulmonary arteries, cyclooxygenase products were the major modulators of cysteinyl-leukotriene responses. In the human pulmonary artery, LTC4 and LTD4 stimulated the release of prostacyclin and consequently, the contractions induced by the leukotrienes were significantly enhanced after cyclooxygense inhibition. In contrast, porcine pulmonary arterial preparations mainly generated contractile cyclooxygenase products in response to LTC4. Nitric oxide synthase inhibition unmasked contractions to LTC4 in porcine pulmonary veins, but had no effect on LTC4-induced contractions in porcine pulmonary arteries. A preferential regulation by nitric oxide in porcine pulmonary veins compared with arteries was also observed with noradrenaline, acetylcholine and bradykinin.

The metabolism of LTC4 into LTD4 and subsequently into LTE4 may modulate the responses to LTC4 and LTD4 in the guinea-pig trachea. In addition to this metabolism, it was discovered that the guinea-pig trachea also formed LTC4 from LTD4. The latter alternative metabolic pathway changed the LTD4-induced CysLT1, response into a CysLT2 receptor response.

In conclusion, the present thesis suggests the existence of a previously unrecognised receptor for cysteinyl-leukotrienes. In addition, it was demonstrated that cysteinylleukotriene responses in pulmonary vessels were regulated by the release of modulatory factors, of which cyclooxygenase products dominated in the arteries and nitric oxide was the main modulator in porcine pulmonary veins. Moreover, in tissues with a heterogeneous CysLT receptor population, the interconversion between LTC4 and LTD4 may represent a major modulatory mechanism by deciding which CysLT receptor is activated by the cysteinyl-leukotrienes.