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 (S28): 125I-[Leu8,DTrp22,Tyr25]S28. The binding was highly dependent on calcium ions. In 0.2 mM free Ca2+ medium, binding at 37 degrees C was saturable, slowly reversible and exhibited a single class of high affinity binding sites (KD = 0.05 +/- 0.01 nM, Bmax = 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 Ca2+ 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 AA1,2[AzaAla4-5,DTrp8, Phe12-13]-somatostatin was active at the membrane level in inhibiting the binding. We conclude that using 125I-[Leu8,DTrp22,Tyr25]S28 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-[Tyr11]-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.
D000818:Animals; D001667:Binding, Competitive; D002118:Calcium / Q000494:pharmacology; D002462:Cell Membrane / Q000378:metabolism; D006168:Guinea Pigs; D007457:Iodine Radioisotopes; D007700:Kinetics; D008297:Male; D010179:Pancreas / Q000378:metabolism*; D017981:Receptors, Neurotransmitter / Q000378:metabolism*; D017481:Receptors, Somatostatin; D013004:Somatostatin / Q000031:analogs & derivatives / Q000378:metabolism*; D053561:Somatostatin-28
D007457:Iodine Radioisotopes; D017981:Receptors, Neurotransmitter; D017481:Receptors, Somatostatin; D013004:Somatostatin; D053561:Somatostatin-28; D002118:Calcium