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 α_1D-adrenoceptor antagonist BMY7378 (10^-6 M), the α_1A-adrenoceptor antagonist RS100329 ((3 × 10^-8 M), and the putative α_1B-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 α_1A- and α_1B-adrenoceptors produce approximately parallel shifts in Phe potency.

Conclusions: Contractions of rat spleen to adrenergic agonists involve α₂- and α_1B-adrenoceptors, with a lesser role for α_1A-adrenoceptors. This confirms the suggestion that smooth muscle contractions commonly involve multiple subtypes.