INSULIN BINDING TO LIVER PLASMA-MEMBRANES IN SALMONIDS WITH MODIFIED PLASMA-INSULIN LEVELS

Insulin binding to the liver plasma membranes was assessed in coho salmon (Oncorhynchus kisutch) injected intraperitoneally with 6.6-mu-mol arginine/g body weight, and in rainbow trout (Oncorhynchus mykiss) fasted for 40 days and then refed for 15 days. Corresponding control groups of fish were injected with saline (coho salmon) or fed regularly (trout). Arginine injection was followed by a substantial elevation of plasma insulin titres from 12.5 to 76.8 ng/mL, an increase in the binding capacity and, consequently, an increase in specific binding of insulin to the liver plasma membranes from 3.4 to 5.5%. The binding affinity remained unchanged. Food deprivation lowered plasma insulin titres from 13.2 to 3.0 ng/mL, increased the binding capacity, but decreased the binding affinity, so the specific binding remained essentially unchanged (5.6% in fed versus 5.1% in fasted fish). Refeeding of fasted fish resulted in restoration of insulin levels and an increase in binding affinity relative to both control and fasted groups of fish. This led to a substantial elevation of the specific binding of insulin up to 7.1%. The binding of insulin to liver plasma membranes in salmonids depends both on the number of binding sites (binding capactity) and on the binding affinity. Long-term treatment, such as food deprivation, resulted in altered affinity and capacity of binding, whereas short-term treatment, such as arginine injection, affected mostly the binding capacity. Modulation of the number of binding sites in liver plasma membrane according to the insulin level was observed only in experiments on trout conducted at a higher (15-degrees-C) water temperature.