Characterization of pancreatic somatostatin binding sites with a 125I-somatostatin 28 analog

Somatostatin binding to guinea pig pancreatic acinar cell plasma membranes was characterized with an iodinated stable analog of somatostatin 28 (S 28): 125I-[Leu 8, DTrp 22,Tyr 25] S 28. The binding was highly dependent on calcium ions. In 0.2 mM free Ca 2+ medium, binding at 37°C was saturable, slowly reversible and exhibited a single class of high affinity binding sites (K D=0.05±0.01 nM, B max=157±33 fmol/mg protein). Dissociation of bound radioactivity occurred with biphasic kinetics. Rate of dissociation increased when dissociation was measured at a time before equilibrium binding was reached. In 30 nM free Ca 2+ medium, binding affinity and maximal binding capacity were decreased by about 4-fold. Decreasing calcium concentrations increased the amount of rapidly dissociating form of the receptor. Somatostatin 14 antagonist, Des AA 1,2[AzaAla 4–5,DTrp 8,Phe 12–13]-somatostatin was active at the membrane level in inhibiting the binding. We conclude that using 125I-[Leu 8,DTrp 22,Tyr 25]S 28 as radioligand allows us to characterize a population of specific somatostatin receptors which are not different from those we previously described with the radioligand 125I-[Tyr 11]-somatostatin. Somatostatin receptors could exist in two interconvertible forms. Calcium ions are an essential component in the regulation of the conformational change of somatostatin receptors.