α1-adrenoceptor subtypes and signal transduction mechanisms in the rat thoracic aorta

The α1-adrenoceptors are a heterogeneous family of receptors consisting of the cloned α1a/d-, α1b-, α1c-adrenoceptors and the 'classical' tissue α1A- and α1B-subtypes. This thesis describes a [ 3H]-prazosin binding study comparing the pharmacological profiles of the cloned mammalian α1a/d-, α1b-, α1c-adrenoceptors to those of the proposed tissue α1A-, α1B- and α1C-subtypes described in rat and rabbit tissues. On the basis of agonist and antagonist binding affinities, the cloned α1a/d-subtype did not resemble the tissue α1A-subtype. The cloned α1b-adrenoceptor closely matched its tissue counterpart and the cloned α1c-subtype correlated well to the proposed tissue α1C-adrenoceptor and also correlated closely with the rat tissue α1A-subtype indicating that this cloned receptor may represent the 'classical' α1A-subtype. The α1-adrenoceptor agonist-induced contractions of the rat aorta have been suggested to be mediated via either a homogeneous or heterogeneous population of α1-adrenoceptors. In this thesis, the subtype(s) of α1-adrenoceptor involved in the contraction of isolated rat thoracic aortic rings has been investigated by using a range of α1-adrenoceptor agonists and selective antagonists. Contractions to noradrenaline were highly sensitive to the α1A/α1C-selective antagonists (WB-4101 and 5-methyl-urapidil) and the α1B/α1c-alkylating agent CEC. However, contractions to oxymetazoline were insensitive to CEC but highly sensitive to WB-4101. It was concluded that a heterogeneous population of α1-adrenoceptors are responsible for the contractile response in the rat thoracic aorta. The source of Ca2+ mobilised during α1-adrenoceptor mediated contractions of the rat thoracic aorta was also investigated. Noradrenaline- and oxymetazoline-induced contractions were differentially affected by the L-type Ca2+-channel blocker, nifedipine or by extracellular Ca2+ removal, since the responses to noradrenaline were partially sensitive to these treatments and those to oxymetazoline were almost totally abolished. Further, the initial phasic component of the noradrenaline-induced contraction was least sensitive to extracellular Ca2+ removal compared to the latter tonic component and in addition, IP3 production only occurred during this early component of the contraction. These results suggest that noradrenaline and oxymetazoline may couple differentially to different sources of Ca2+ mobilisation and that intracellular Ca 2+ mobilisation might be primarily involved in the rapid phasic component of the contraction to noradrenaline. These studies suggest that the pharmacology of the cloned rat α1a/d-adrenoceptor does not correlate to that of the 'classical' α1A-subtype but instead, the cloned α1c-subtype may represent the recombinant counterpart to this receptor. The α1-adrenoceptor agonist-induced contractions of the rat thoracic aorta appear to be mediated by a heterogeneous population of α1-adrenoceptors and further, both intracellular and extracellular sources of Ca2+ seem to be involved with the contractile response