AUTORADIOGRAPHIC ANALYSIS OF (‐)‐[125I]‐CYP BINDING IN MOUSE KIDNEY

1. The distribution and binding characteristics of the radioligand (‐)‐[¹²⁵I]‐cyanopindolol (CYP) have been examined in slide mounted mouse kidney sections, using the technique of in vitro labelling and autoradiography. 2. (‐)‐[¹²⁵I]‐CYP binding to sections was of high affinity (K_D=55.8 pmol/l, s.e.m. = 8. 1, n= 4) to a single population of non‐interacting sites (nH = 0. 95, s.e.m. = 0. 01, B_max= 0.74 fmol/section, s.e.m. = 0. 12, n= 4) and stereoselective with respect to the (‐)‐ and (+)‐isomers of both propranolol and pindolol. 3. Autoradiographic studies showed that (‐)‐[¹²⁵I]‐CYP binding was localized to areas in the renal cortex and medulla. Both cortical and medullary binding were abolished by the inclusion of (‐)‐propranolol (1 μmol/l) in the incubation medium, whereas (‐)‐isoprenaline (200 μmol/l) selectively abolished cortical binding. 4. Medullary binding could be prevented by the inclusion of the lipophilic compounds cinanserin (10 μmol/l), haloperidol (10 μmol/l) or phentolamine (10 μmol/l), either alone or together or by washing at 37°C. These results suggest that medullary binding sites are lipid rather than receptor‐related. 5. In conclusion, in mouse kidney sections, (‐)‐[¹²⁵I]‐CYP binds to discrete areas in the cortex and medulla. Cortical binding sites have the molecular characteristics of β‐adrenoceptors while medullary binding sites are lipid‐related. Caution should therefore be exercised when defining non‐specific binding of lipophilic radioligands. The autoradiographic technique is useful for discriminating between receptor and non‐receptor binding sites.