The binding of somatostatin to rat synaptosomal membranes: Effects of detergent solubilisation and of bacitracin

Specific high and low affinity binding sites for somatostatin have been demonstrated in rat brain synaptosomal membranes using ¹²⁵I [Tyr″] somatostatin tracer. When incubations were carried out in the presence of low concentrations of bacitracin (0.01% w/v), a 66% increase in association constant of the high affinity binding site to K_a = 1.71 × 10¹⁰ M^-1 and a 70% increase in the K_a of the low affinity site to 0.034 × 10¹⁰ M^-1 were observed; a similar (50%) increase in the binding capacity of the low affinity site but no increase in the binding capacity of the high affinity site was seen. These results may indicate a direct effect of bacitracin on the membrane that is distinct from its effect as an inhibitor of tracer degradation. The somatostatin-binding capacity of the membranes was solubilised by treatment with Triton X-100 and the apparent molecular weight of the receptor-ligand-detergent micelle complex was estimated by gel filtration to be approximately 250,000.