C. B. Dolkas

Removal of insulin by perfused rat liver: Effect of concentration

The kinetics of insulin removal by isolated rat liver were investigated by measuring the rate of disappearance of insulin from the perfusate during recycling perfusion and by comparing the extraction of insulin over a wide range of constant arterial hormone levels during nonrecycling perfusion. In the recycling studies, insulin was removed from the perfusing medium at a uniform rate between 5 and 45 min. The reaction velocity constant, or hepatic clearance, during this period of uniform disappearance averaged 1.8 ml/min and represented 34% of the volume flow through the liver. In the nonrecycling flow-through studies at constant arterial insulin concentration, an initial period of accelerated hepatic uptake of insulin was seen. This period lasted for 3 to 7 min, was seen at every level of arterial insulin concentration, and was followed by a period of constant hepatic insulin removal. The hepatic removal rate during the period of constant uptake increased in a linear fashion until arterial insulin concentration reached 500 muU/ml and attained a maximal value at concentrations over 800 muU/ml. These findings indicate that the time course of hepatic insulin uptake by the perfused rat liver consists of two phases-an initial rapid phase, possibly associated with insulin binding, followed by a sustained rate of insulin removal, which probably represents insulin utilization and degradation. The rate of hepatic insulin removal was found to be proportional to arterial insulin concentration overa range of 20 to 500 muU/ML. Above this concentration, hepatic removal processes became saturated, reaching a maximal value of 183 muU of insulin per gram of liver per minute.

The site of insulin resistance in acute uremia.

In order to define the mechanism of glucose intolerance in acutely uremic rats, various studies were carried out 24 hours after bilateral nephrectomy. Glucose removal following intravenous glucose was significantly (p < 0.001) decreased in uremic rats compared with sham-operated rats (k = 2.1 ± 0.03 per cent vs. 5.1 ±0.2 per cent). This deterioration in glucose tolerance was associated with higher insulin levels in uremic rats from one to 40 minutes after glucose administration, suggesting that insulin resistance accounted for the decrease in glucose removal by uremic rats. To identify the site of the insulin resistance, we compared the ability of insulin to enhance net glucose uptake by isolated perfused liver and muscle (hindlimb) preparations obtained from uremic and shamoperated rats. Insulin suppressed glucose outflow from perfused livers of uremic rats at least as well as it did from livers of shamoperated rats, and suppression occurred at both maximal (> 600 μU./ml.) and threshold (75 μU./ml.) perfusate insulin levels. In contrast, there was a significant decrease in the ability of insulin (mean perfusate level = 225 μU./ml.) to enhance glucose uptake of perfused hindlimbs of uremic as compared with sham-operated rats. These results suggest that the insulin resistance of acute uremia may be due primarily to decreased insulin-mediated uptake of glucose by skeletal muscle without any decrease in sensitivity of the liver to insulin.

Effect of acute uremia on insulin removal by the isolated perfused rat liver and muscle.

Abstract The effect of acute uremia on insulin removal by liver and muscle was investigated by measuring the extraction of porcine insulin during recycling perfusion of isolated liver and hindlimb from nephrectomized rats. Insulin removal by the liver was not affected by uremia, and hepatic extraction of insulin by normal and uremic livers averaged 38% at perfusate insulin concentrations ranging from 50 to 600 μU/ml. In contrast, insulin extraction by normal hindlimb averaged 13.4% at comparable insulin concentrations, and this was reduced to 4.3% as a result of acute uremia. A similar reduction in insulin extraction was noted when hindlimb from uremic animals was perfused with normal blood media or when normal hindlimb was perfused with uremic blood. These findings indicate that the efficiency of insulin removal by the liver is unaffected by uremia and is approximately threefold higher than that for hindlimb muscle. On the other hand, it is apparent that skeletal muscle, by virtue of its mass and relatively high blood flow, is also an important organ system for insulin removal. Furthermore, the data suggest that the 68% reduction in insulin extraction by hindlimb from uremic rats may contribute to the prolongation of insulin removal from plasma that is characteristic of patients with renal failure.

Insulin Sensitivity of Isolated Perfused Rat Liver

The responsiveness of the isolated perfused rat liver to different metabolic effects of insulin was investigated during recycling perfusion. Infusion of porcine insulin at rates of 6,9,16 and 33 mU/hr. resulted in stable perfusate insulin levels averaging 41, 72, 120 and 229 μU/ml., respectively. Since the portal vein insulin concentration in the intact rat averaged 48 μU/ml. after a twenty-six-hour fast and 125 μU/ml. two hours after removal of food, the studies were conducted at insulin levels within the physiological range. The effect of each insulin concentration on the net accumulation of K+, amino acid nitrogen, urea nitrogen and glucose in the perfusing medium was assessed against the net accumulation of perfusate constituents during perfusion of control livers and livers perfused with perfusate insulin levels greater than 500 μU/ml. The results indicate that essentially maximal suppression of amino acid nitrogen outflow and retention of K+ occurred at insulin concentrations of 72 μU/ml., with lesser effects being noted at 41 μU/ml. Inhibition of ureogenesis was demonstrated at insulin levels above 120 μU/ml. However, significant effects of insulin on suppressing net glucose outflow was not observed until insulin levels had reached 500 μU/ml. due presumably to the absence of a sustained rate of glycogenolysis by control livers. The observation that perfused livers from normal rats are extremely sensitive to several metabolic effects of insulin at physiological concentrations suggests that this experimental approach can provide useful information as to the role of the liver in the pathogenesis of various insulin resistant states.

The Effect of Exercise Training on Insulin Resistance in Sedentary Year Old Rats

Abstract The purpose of this study was to determine if exercise training of 12-month-old Sprague-Dawley rats could reverse the resistance to insulin-induced glucose uptake that has been shown to occur in these animals. Twelve-month-old rats were trained to run 1½ miles/day in motorized exercise wheel cages, and the ability of insulin to stimulate glucose uptake in these rats was compared with values observed in two groups of similar aged sedentary rats — one fed rat chow ad libitum and the other a calorie-restricted diet for 4 months. Body weight increased and insulin-stimulated glucose uptake decreased as rats fed chow ad libitum grew from 12 to 16 months of age. In contrast, 4 months of either exercise training or calorie restriction prevented weight gain and loss of insulin-stimulated glucose uptake. Thus, the intensity of exercise training attained in this study did not result in an improvement in insulin action in older rats above and beyond that related to the reduction in rate of body weight gain