P-adrenoceptors in the Rat

I The effects of thyroid state on the properties of adrenoceptors mediating inotropic and chronotropic responses of the rat heart were assessed on the basis of the relative potencies of aand /5adrenoceptor agonists, the effects of aand ,B-adrenoceptor antagonists and the tissue uptake of PH]phenoxybenzamine (PHI-PB). 2 In isolated, electrically driven left atria the ratio of the inotropic potencies of isoprenaline and phenylephrine and the inhibitory potency of propranolol (40nM-4 giM) were significantly reduced after thyroidectomy and were moderately increased after thyroxine treatment of control rats. 3 Block of inotropic responses to noradrenaline and to phenylephrine by PB (7.3nM-7.3 gM) and the tissue uptake of [3HI-PB were significantly greater in preparations from thyroidectomized than in those from control or from thyroxine treated rats. a-Adrenoceptor inhibition by phentolamine (0.26-2.6 gM) also increased after thyroidectomy, and phentolamine effectively protected aadrenoceptors from block by and binding of P3HI-PB. 4 The /,i-receptor antagonist H 93/26 (0.1 gM) significantly potentiated a-adrenoceptor blockade by PB in hypothyroid but not in control preparations.