Functional characterisation of receptors for cysteinyl leukotrienes in smooth muscle

Abstract

The cysteinyl leukotrienes (leukotriene C4, D4 and E4) have potent biological actions which significantly contribute to the airway obstruction in asthma. Several of these effects are blocked by drugs known as CysLT1-receptor antagonists, which have been introduced as new treatment for asthma. However, there are actions of leukotrienes which are not sensitive to these antagonists, suggesting the presence of additional receptor subtypes. It was the aim of this Thesis to extend the knowledge about receptors for cysteinyl leukotrienes.

Three different isolated smooth muscle preparations kept in organ baths under non-flow conditions were characterised with respect to responsiveness to cysteinyl leukotrienes and sensitivity to purported CysLT1-receptor antagonists. In addition, the study involved evaluation of a leukotriene E4 analogue, BAY u9773, suggested to inhibit responses which cannot be blocked by CysLT1-receptor antagonists. Responses sensitive to BAY u9773 have provisionally been considered to be mediated by CysLT2-receptors.

In the guinea pig ileum, BAY u9773 but not the selective CysLT, receptor antagonist ICI 198,615 concentration-dependently inhibited the contractile response to leukotriene C4 in a fashion suggesting competitive antagonism. In sheep trachealis muscle, BAY u9773 antagonised contractions induced by leukotriene C4 and leukotriene D4 in a similar manner, whereas ICI 198,615 did not. The observations support that leukotriene C4 in the guinea pig ileum, and leukotriene C4 as well as leukotriene D4 in sheep trachealis muscle, mediated contractions via activation of CysLT2-receptors.

In guinea pig lung parenchyma, however, the effects of BAY u9773 and conventional cysteinyl leukotriene receptor antagonists (ICI 198,615, FPL 55712) were more complex. First, a contraction was evoked when BAY u9773 was added to the preparation. This contraction could be inhibited by antagonists of CysLT1- and TP-receptors, suggesting that BAY u9773 acted as an agonist at these two receptors. Second, pretreatment with BAY u9773 inhibited a distinct but relatively minor component of the contractile response to leukotriene C4 and D4. The effects of BAY u9773 and ICI 198,615 were similar in guinea pig lung parenchyma. The findings suggest that the receptor mediating the major part of the contractile response to exogenous cysteinyl leukotrienes in guinea pig lung parenchyma was different from the currently defined CysLTr receptor. The data suggested that BAY u9773 was a partial agonist at cysteinyl leukotriene receptors, which presumably contributed to its profile of activity as a combined CysLT1- and CysLT2-receptor antagonist.

In addition to contracting guinea pig lung parenchyma, leukotriene C4 and lipoxin A4 also evoked release of thromboxane A2. This release was sensitive to CysLT1-receptor antagonists and contributed to part of the contractile response. Finally, the investigations included a characterisation of the role of leukotrienes and other mediators in antigen-induced contractions of lung parenchyma from actively sensitised guinea pigs. Combination of antihistamines and drugs which inhibit the formation of leukotrienes or antagonise CysLT1-receptors, blocked the major component of the antigen-induced contraction. The findings are similar to those observed in isolated human bronchi and support that guinea pig lung parenchyma may be used as a model to investigate mediator mechanisms of relevance to asthma.