Interaction between α1B - and other α1 - and α2 -adrenoceptors in producing contractions of mouse spleen.

We have investigated the interaction of α1 - and α2 -adrenoceptor subtypes in producing isometric contractions to NA in mouse whole spleen. The α1 -adrenoceptor antagonist prazosin (10-8  M) or the α2 -adrenoceptor antagonist yohimbine (10-6  M) alone produced only small shifts in NA potency in wild type (WT) mice, but the combination produced a large shift in NA potency. In spleen from α1A/D -KO mice, the effects of prazosin and the combination of prazosin and yohimbine were similar to their effects in WT mice. Hence, in α1A/D -KO mice, in which the only α1 -adrenoceptor present is the α1B -adrenoceptor, prazosin still antagonized contractions to NA. The α1A -adrenoceptor antagonist RS100329 (3 × 10-9  M) produced significant shifts in the effects of higher concentrations of NA (EC50 and EC75 levels) and the α1D -adrenoceptor antagonist BMY7378 (3 × 10-8  M) produced significant shifts in the effects of lower concentrations of NA (EC25 and EC50 levels). The effects of BMY7378 and RS00329 demonstrate α1D -adrenoceptor and α1A -adrenoceptor components and suggest that the α1B -adrenoceptor interacts with an α1D -adrenoceptor, and to a lesser extent an α1A -adrenoceptor, at low, and an α1A -adrenoceptor at high, NA concentrations. This study demonstrates the complex interaction between α1 - and α2 -adrenoceptor subtypes in producing contractions of mouse spleen and may have general implications for α-adrenoceptor mediated control of smooth muscle.