Abstract
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 alpha(1)-adrenoceptor agonist phenylephrine (Phe).
Results: Contractions to NA were antagonized by both the alpha(1)-adrenoceptor antagonist prazosin (10(-8) M) and the alpha(2)-adrenoceptor antagonist yohimbine (10(-6)M), 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 alpha(1D)-adrenoceptor antagonist BMY7378 (10(-6) M), the alpha(1A)-adrenoceptor antagonist RS100329 ((3 x 10(-8) M), and the putative alpha(1B)-adrenoceptor antagonist cyclazosin (10(-8) M) also produced parallel shifts in the Phe concentration-response curve. BMY7378 at the selective concentration of 3 x 10(-8) M had no effect on responses to Phe, but RS100329 in the selective concentration of 3 x 10(-9) M produced a marked shift in the effects of high concentrations of Phe. Hence, antagonists in concentrations that block both alpha(1A)- and alpha(1B)-adrenoceptors produce approximately parallel shifts in Phe potency.
Conclusions: Contractions of rat spleen to adrenergic agonists involve alpha(2)- and alpha(1B)-adrenoceptors, with a lesser role for alpha(1A)-adrenoceptors. This confirms the suggestion that smooth muscle contractions commonly involve multiple subtypes.