Demonstration of α‐Adrenoceptors in the Rabbit Bladder Base and Urethra with 3H‐Dihydroergocryptine Ligand Binding

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Abstract
The presence of .alpha.-adrenoceptors in a crude membrane preparation made from rabbit bladder base and urethra was demonstrated. This was achieved by radioligand binding studies, using 3H-dihydro-.alpha.-ergocryptine (3H-DHE) as the radioligand. The specific binding, i.e., the binding that could be inhibited by 10-5 M phentolamine, was saturable with 73 f[femto]mol 3H-DHE bound per mg membrane protein. Binding was at steady state after 60 min, and was reversible. Rate constants for association and dissociation were 3 .times. 107 M-1 min-1, and 2 .times. 10-2 min-1, respectively. A number of compounds were tested for their abilities to compete with 3H-DHE for the binding sites. The relative affinity of some adrenoceptor agonists was: (-)-adrenaline [epinephrine] > (-)-noradrenaline [norepinephrine] .mchgt. (.+-.)-isoprenaline. Stereoselectivity was shown, since (-)-noradrenaline had 42 times higher affinity than (+)-noradrenaline. Adrenoceptor antagonists inhibited 3H-DHE binding in the following order of potency: DHE > phentolamine .mchgt. (.+-.)-propranolol. Kd for DHE to the binding sites were estimated in 3 different ways. The constants were derived from saturation, competition and kinetic studies, and gave Kd values of 1.1, 1.4 and 0.7 nM, respectively. Apparently, .alpha.-adrenoceptors were labeled by 3H-DHE in the tissue homogenates.