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 (Kd) of 23.6 +/- 1.1 nM and a binding capacity (Bmax) of 3459 +/- 134 receptors/cell. The low-affinity site had a Kd of 764 +/- 27 nM and a Bmax of 17,432 +/- 345 receptors/cell. 125I-SS-14 dissociation was hastened by the presence of 10(-6) M SS-14 . The internalization of 125I-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.