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

In this study, beta -adrenoceptors of intact equine lymphocytes were identified and subclassified by (-)-[I-125]-iodocyanopindolol (ICYP) binding. ICYP binding to intact equine lymphocytes was rapid, saturable (maximal number of binding sites 320 +/- 20 ICYP binding sites/cell, n = 12) and of high affinity (K-D value for ICYP 14.4 +/- 1.7 pmol/l, n = 12). Binding was stereospecific as shown by the 10 times greater potency of (-)-propranolol to inhibit binding than its (+)-isomer. beta -adrenoceptor agonists inhibited ICYP binding with an order of potency: (-)-isoprenaline >(-)-adrenaline >(-)-noradrenaline; the same order of potency was obtained for agonist-induced stimulation of lymphocyte cyclic AMP content. The selective beta (2)-adrenoceptor antagonist ICI 118,551 was about 1000 times more potent in inhibiting ICYP binding than was the beta (1)-selective adrenoceptor antagonist CGP 20712A. It is, therefore, concluded that in intact equine lymphocytes, ICYP labels a class of functional beta -adrenoceptors 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 model 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-adrenoreceptor subtypes present on equine lymphocytes.