The influence of alpha 1- and alpha 2-adrenoceptor agonists on cardiac output in rats and cats.

In pithed rats, but not in pithed cats, the alpha 2-/dopamine agonist B-HT 920 (10-300 micrograms kg-1) increased cardiac output. The increase was inhibited by yohimbine (1 mg kg-1), but not by sulpiride (0.3 mg kg-1) and nifedipine (1 mg kg-1). Both in pithed rats and cats, the alpha 1-adrenoceptor agonist methoxamine (30-300 micrograms kg-1 and 30-1000 micrograms kg-1, respectively) increased cardiac output. This increase (rats) was inhibited by prazosin (0.1 mg kg-1). The results indicate the existence of functional venous alpha 2-adrenoceptors in pithed rats, and their absence in pithed cats. Furthermore, alpha 2-adrenoceptor-mediated venoconstriction appears independent of extracellular calcium.