Interstitial Muscle Insulin and Glucose Levels in Normal and Insulin-Resistant Zucker Rats

Abstract

To study interstitial insulin and glucose concentrations, microdialysis was performed in the medial femoral muscles in normal SD rats as well as in insulin-resistant obese Zucker rats during a euglycemic insulin clamp. [14C]inulin was given (0.1 mCi/rat) as a constant subcutaneous infusion 24 h before the insulin clamp. Insulin infusion rates were 5-8 mU x kg(-1) x min(-1) (low rate) for 140 min and 10-20 mU x kg(-1) x min(-1) (high rate) for another 100 min. The relationship between insulin and [14C]inulin dialysate recoveries was evaluated in vivo and in vitro in plasma to calculate interstitial insulin concentration. Relative microdialysis recovery of interstitial insulin in vivo was 3.0 +/- 0.3% (mean +/- SE, n = 68). In normal SD rats, plasma and interstitial insulin concentrations were identical when plasma insulin was < or =250 mU/ml, whereas interstitial insulin was lower when plasma insulin was > or =350 mU/ml. Half-maximal glucose infusion rate was achieved in the presence of plasma and interstitial insulin concentrations of approximately 140 mU/ml, whereas maximal glucose disposal was seen at interstitial insulin concentrations of approximately 325 mU/ml, corresponding to approximately 500 mU/ml in plasma. In electrically stimulated and contracting (1 Hz) normal muscle with markedly increased blood flow, the dialysate insulin concentration was significantly higher at high rates, but not at low rates, of insulin infusion. In insulin-resistant obese Zucker rats, the interstitial insulin concentration was similar to that in plasma, even at pharmacological concentrations. The glucose infusion rate was significantly lower in the obese Zucker rats at both insulin infusion rates than in the lean animals. The glucose content in dialysates from skeletal muscle was equal in both obese and lean rats during the low insulin infusion rate. During the high insulin infusion rate, dialysate glucose concentrations decreased significantly in both groups but were significantly higher in the obese Zucker rats. The data suggest that transport of insulin and glucose diffusion across the capillary wall are rate limiting for insulin as well as for glucose metabolism in muscle in normal rats. This does not appear to be the case in the insulin-resistant obese Zucker rats, where the reduced insulin responsiveness in muscle is due to muscular cellular defects rather than an inhibited transcapillary delivery of insulin.

Induction of rat muscle insulin resistance by epinephrine is accompanied by increased interstitial glucose and lactate concentrations

“…Our data, obtained in experiments where each animal serves as its own control, suggest that compensation in terms of higher interstitial concentrations of glucose might already occur at physiological insulin concentrations but that this it not enough to overcome the reduction in glucose uptake. The microdialysis technique used here was validated recently in the rat muscle and was shown not to alter either the capillary permeability or interstitial fluid concentration of either glucose or insulin [26,27]. In our study (data not shown), the constant dialysis recovery of inulin and retrodialysis of labelled glucose ascertained constant and steady state microdialysis conditions throughout the study.…”

“…Insulin resistance could be induced by high glucose concentrations ambient to the cells through formation of hexosamines, interfering directly with the insulin signal transduction. Also, it is notable that the insulin-resistant Zucker rat has higher than normal glucose concentrations in muscle interstitial fluid even under normoglycaemic insulin clamping conditions [27]. In contrast to the decrease in A-I concentration difference for glucose, the lactate I-A concentration difference increased during epinephrine exposure.…”

“…In experiments where the interstitial concentrations of insulin were estimated (n = 68), inulin was used as an external reference. [ 14 C]-inulin (0.4 mCi/ml, New England Nuclear, Boston, Mass., USA) was infused subcutaneously (8.0 ml/h) for 24 h and in vitro experiments were done to correct for differences in binding and diffusion properties [27]. The in vivo relative recovery of insulin in microdialysate was 3.0 ± 0.3 % [27].…”

“…[ 14 C]-inulin (0.4 mCi/ml, New England Nuclear, Boston, Mass., USA) was infused subcutaneously (8.0 ml/h) for 24 h and in vitro experiments were done to correct for differences in binding and diffusion properties [27]. The in vivo relative recovery of insulin in microdialysate was 3.0 ± 0.3 % [27]. The interstitial lactate concentration was calculated by dividing the dialysate lactate concentration by the empirical relative recovery factor 0.37, described previously from measurements in rat muscle under identical experimental conditions (n = 20, 0.37 ± 0.02) [27].…”

“…The in vivo relative recovery of insulin in microdialysate was 3.0 ± 0.3 % [27]. The interstitial lactate concentration was calculated by dividing the dialysate lactate concentration by the empirical relative recovery factor 0.37, described previously from measurements in rat muscle under identical experimental conditions (n = 20, 0.37 ± 0.02) [27].…”