Both α1B- and α1A-adrenoceptor subtypes are involved in contractions of rat spleen.

Background The spleen is a reservoir for circulating blood cells, and can contract to expel them. Methods We have investigated the adrenoceptors involved in isometric contractions of rat spleen produced by noradrenaline (NA) and the α1-adrenoceptor agonist phenylephrine (Phe). Results Contractions to NA were antagonized by both the α1-adrenoceptor antagonist prazosin (10-8 M) and the α2-adrenoceptor antagonist yohimbine (10-6M), and the combination produced further shifts in NA potency. Contractions to Phe were antagonized by prazosin (10-8 M) which caused a marked parallel shift in the concentration-response curve. High non-selective concentrations of the α1D-adrenoceptor antagonist BMY7378 (10-6 M), the α1A-adrenoceptor antagonist RS100329 ((3 × 10-8 M), and the putative α1B-adrenoceptor antagonist cyclazosin (10-8 M) also produced parallel shifts in the Phe concentration-response curve. BMY7378 at the selective concentration of 3 × 10-8 M had no effect on responses to Phe, but RS100329 in the selective concentration of 3 × 10-9 M produced a marked shift in the effects of high concentrations of Phe. Hence, antagonists in concentrations that block both α1A- and α1B-adrenoceptors produce approximately parallel shifts in Phe potency. Conclusions Contractions of rat spleen to adrenergic agonists involve α2- and α1B-adrenoceptors, with a lesser role for α1A-adrenoceptors. This confirms the suggestion that smooth muscle contractions commonly involve multiple subtypes.