3H]Quinuclidinyl benzilate binding to muscarinic receptors and [3H]WB-4101 binding to alpha-adrenergic receptors in rabbit iris. Comparison of results in slices and microsomal fractions

The binding characteristics of ³H]-(l)-quinuclidinyl benzilate (QNB) and ³H]WB-4101 to microsomal fractions and slices from rabbit iris muscle were compared. ³H] QNB binding to both microsomal fractions and muscle slices was of high affinity and low capacity and was displaced by muscarinic ligands. The equilibrium dissociation constants (K_D) for ³H]QNB binding to microsomes and slices were 0.069 nM and 1.97nM, respectively. This shift to a higher value for the K_p of the microsomal fraction compared with that of the slices was also observed lor the association rate constants (K_I) and inhibition constants (K_J), but not for the dissociation rate constants (K_?1). Kinetic studies on the binding characteristics of ³H]WB-4101 revealed high affinity sites with K_D values of 2.33 and 10.19 nM for microsomal fractions and slices, respectively. The findings of comparable binding patterns for ³H]QNB and ³H]WB-4101 binding to microsomal fractions and intact muscle slices argue against the possibility of alterations in receptor properties following tissue disruption. It is proposed that the differences in receptor-mediated biochemical responses that are seen between intact tissue and cell-free homogenates, such as the ‘phosphoinositide effect’, are more likely to be due to alterations in receptor function, e.g. changes in ionic permeabilities, rather than to actual changes in receptor properties.