Evidence for Prejunctionally Located Beta 2-adrenoceptors in the Cat Spleen

The number of beta-adrenoceptors in myocardium and spleen from 6-hydroxydopamine (6-OH-DA) treated cats was determined by radioligand binding with [125I]-iodohydroxybenzylpindolol (IHYP). The effectiveness of 6-OH-DA pretreatment was assessed by analyses of the tissue content of catecholamines and the contractile response of isolated splenic strips to electrical stimulation. Since no effect on the splenic strip was produced by the beta-agonist isoprenaline, whereas noradrenaline caused contraction, it is concluded that the smooth muscle of the splenic capsule is controlled by postjunctional alpha-adrenoceptors. The number of specific IHYP binding sites were reduced by 70% in whole spleen tissue and totally abolished in the splenic capsule by pretreatment with 6-OH-DA. Subclass analysis revealed that the reduction in total splenic beta-adrenoceptor number was due to a loss of beta 2-adrenoceptors. However, the 6-OH-DA induced chemical sympathectomy did not produce any alteration either in beta-adrenoceptor density or the relative distribution of the beta-adrenoceptor subtype in the myocardium. It is suggested that a loss of prejunctional beta-adrenoceptors, due to chemical sympathectomy, might be compensated for by an increased number of postjunctional beta-adrenoceptors in the myocardium due to the development of denervation supersensitivity in this tissue. In conclusion, the findings provide direct biochemical evidence for existence of prejunctional beta 2-adrenoceptors on the sympathetic nerve terminals of the cat spleen.