Identification and characterisation of beta-adrenoceptors on intact equineperipheral blood lymphocytes with the radioligand (-)-[I-125]-iodocyanopindolol

In this study, beta -adrenoceptors of intact equine lymphocytes were identi fied and subclassified by (-)-[I-125]-iodocyanopindolol (ICYP) binding. ICY P binding to intact equine lymphocytes was rapid, saturable (maximal number of binding sites 320 +/- 20 ICYP binding sites/cell, n = 12) and of high a ffinity (K-D value for ICYP 14.4 +/- 1.7 pmol/l, n = 12). Binding was stere ospecific as shown by the 10 times greater potency of (-)-propranolol to in hibit binding than its (+)-isomer. beta -adrenoceptor agonists inhibited IC YP binding with an order of potency: (-)-isoprenaline >(-)-adrenaline >(-)- noradrenaline; the same order of potency was obtained for agonist-induced s timulation of lymphocyte cyclic AMP content. The selective beta (2)-adrenoceptor antagonist ICI 118,551 was about 1000 t imes more potent in inhibiting ICYP binding than was the beta (1)-selective adrenoceptor antagonist CGP 20712A. It is, therefore, concluded that in in tact equine lymphocytes, ICYP labels a class of functional beta -adrenocept ors that belong predominantly (> 90%) to the beta (2)-adrenoceptor subtype; a small (< 10%) beta (1)-adrenoceptor component, however, cannot be ruled out completely. ICYP binding to equine lymphocytes might be a suitable mode l to study function and regulation of the beta -adrenoceptor system in the horse in vivo. The aim of this study was to characterise the P-adrenorecept or subtypes present on equine lymphocytes.