Somatostatin binding to hepatocytes isolated from rainbow trout, Oncorhynchus mykiss, is modulated by insulin and glucagon

Previously we reported somatostatin-14 (SS-14) binding in the liver of rainbow trout and that fasting enhanced SS-14-binding capacity. In this study, we used hepatocytes isolated from rainbow trout to study aspects of SS-14 processing and to evaluate the basis of fasting-associated changes in SS-14 binding. Hepatocytes specifically bound 5-8% of the total ¹²⁵I-SS- 14 added. Scatchard analysis revealed the existence of two classes of binding sites. The high-affinity site had a dissociation constant (K(d)) of 23.6 ± 1.1 nM and a binding capacity (B(max)) of 3459 ± 134 receptors/cell. The low-affinity site had a K(d) of 764 ± 27 nM and a B(max) of 17,432 ± 345 receptors/cell. ¹²⁵-SS-14 dissociation was hastened by the presence of 10^-6 M SS-14 . The internalization of ¹²⁵-SS-14 was time dependent; preincubation of hepatocytes with 10^-6 M SS-14 reduced internalization of ¹²⁵I-SS-14. The number of high-affinity binding sites was also reduced by 10^-6 M SS-14. Because plasma levels of insulin (INS) decline relative to those of glucagon (GLU) during fasting of trout, we also investigated the effects of these hormones on SS-14 binding. The number of high-affinity SS- 14-binding sites was reduced by 10^-6 M INS and was increased by 10^-6 M and 10^-8 M GLU. These results indicated that SS-14 regulates aspects of SS- 14 binding and processing and suggests that INS and GLU play a role in fasting-associated changes in SS-14 binding.