Splanchnic and renal metabolism of insulin in human subjects: a dose-response study

Kinetic analyses of insulin metabolism were performed in 32 healthy subjects with hepatic-venous or renal-venous catheters, in whom steady-state conditions of hyperinsulinemia or hyperglycemia were achieved with the use of the glucose-insulin clamp technique. In the basal state, the splanchnic bed removed 42 +/- 2% of the insulin influx. After graded insulin infusions with maintenance of euglycemia, stable arterial insulin levels of 35-1,430 microU/ml were attained. Splanchnic insulin extraction was constant (approximately 60%) at physiological insulin levels but fell (to 29 +/- 1%, P less than 0.02) at supraphysiological (greater than 500 microU/ml) concentrations. The metabolic clearance rate of infused insulin was essentially constant within the physiological concentration range. Hyperglycemia (+125 mg/100 ml) did not alter splanchnic insulin extraction. Basally, the kidneys extracted 0.04 +/- 0.01 mU X min-1 X kg-1 or 25 +/- 5% of arterial insulin. Renal insulin clearance (3.9 +/- 0.4 ml X min-1 X kg-1) represented over 80% of the extrasplanchnic insulin clearance. Hyperglycemia (+125 mg/100 ml) had no effect on renal insulin extraction. In conclusion, a) both splanchnic and renal insulin removal are independent of glycemia and increase in proportion to plasma insulin concentration within the physiological range; b) splanchnic uptake is the dominant mechanism of removal of insulin from the circulation whether the route of delivery is portal or peripheral; and c) the kidneys account for the greater part of extrasplanchnic insulin metabolism.