An investigation into the direct and indirect effects induced by platelet-activating factor receptor activation in rat isolated lung tissue.

Abstract

This study looked at the effects of platelet-activating factor (PAF) on rat isolated lung strips and tracheal spirals. Previous studies have shown that PAF induces hyperresponsiveness to the contractile effects of acetyicholine (ACh) and 5- hydroxytryptamine (5-HT) in rat isolated tracheal spirals. In this study it was hoped that previous experiments on tracheal spirals could be repeated and then be extended to investigate similar responses in lung tissue. It would then be possible to make a direct comparison between the effects of PAP on the larger airways with the effects of PAF on the smaller airways.

It has also been observed in previous studies that PAP induces a desensitization of its receptors to the bronchoconstrictor effects of PAF. This study looked at whether this PAF receptor desensitization occurred in rat isolated airway tissue. This was done by adding PAP to tissue preparations and observing its effects on the tissues, then adding a subsequent dose of PAP to see if responses were repeated. With the use of inhibitors of protein kinase C (sphingosine), phospholipase A2 (AACOCF3) and lipoxygenase (MK886) which have all been implicated as mediating the actions of PAF, it was possible to determine possible mechanisms responsible for the desensitization of PAP receptors. This was done by introducing these inhibitors into the experimental protocol before addition of PAF and then removing them before subsequent addition of PAP. One theory behind PAP receptor desensitization is receptor internalisation, and this was investigated with the use of an iminunofluorescence (IF) technique. This IF technique was then echnique was then developed to investigate the possibility of eosinophils entering lung tissue as a result ofthe method of killing the animal.

In this study hyperresponsiveness of tracheal and lung tissue to the contractile effects of ACh and 5-HT in response to PAF receptor activation was not observed. Desensitization of tissues to the bronchoconstrictor effects of PAF was however observed. The use of IF technique indicated that in the case of rat isolated airway tissue, receptor intemalisation was not responsible for PAF receptor desensitization. The use of MK886 indicated that desensitization was also not the result of leukotriene production, whilst the use of sphingosine indicated that protein kinase C activation was not responsible for desensitization. The results suggested that desensitization of tissues to the effects of PAF after an initial PAF application was the result of arachidonic acid production or, more likely, either down-regulation of receptors or receptor/G-proteinleffector uncoupling after phospholipase A2 activation.