Effects of short-term pulsatile and continuous insulin delivery on glucagon secretion and insulin secretion and action

Pattern of insulin delivery affects hepatic insulin sensitizing substance (HISS) action and insulin resistance

Hepatic insulin sensitizing substance (HISS) action accounts for 55% of the glucose disposal effect of a bolus of insulin in the fed state. To determine the effect of continuous versus pulsatile insulin delivery on HISS action in male Sprague–Dawley rats, insulin sensitivity was assessed using the rapid insulin sensitivity test (RIST) before and after a continuous, pulsatile, or bolus insulin (60 mU/kg i.v.) delivery. There was a significant difference in the RIST index after a continuous insulin infusion (247.9 mg/kg before, 73.2 mg/kg after) but not after 3 pulses where insulin action returned to baseline between pulses (211.6 mg/kg before, 191.0 mg/kg after) or single bolus (205.8 mg/kg before, 189.9 mg/kg after) insulin infusion. If a 3-pulse infusion was timed so that insulin action did not return to baseline between pulses, HISS action was suppressed. Continuous insulin infusion (10–30 min) showed progressive postinfusion blockade of HISS action. To maintain HISS-dependent insulin action, continuous insulin infusions should be avoided.Key words: pulsatile, glucose uptake, RIST, euglycemic clamp, insulin sensitivity.

Pulsatile intravenous insulin replacement in streptozotocin diabetic rats is more efficient than continuous delivery: effects on glycaemic control, insulin-mediated glucose metabolism and lipolysis

Short-term exposure of tissues to pulses of insulin generally leads to an enhancement of insulin action. We have investigated the possible beneficial effects of long-term near-physiological continuous vs pulsatile intravenous insulin treatment of insulin-deficient streptozotocin (70 mg/kg) diabetic rats on blood glucose control, in vivo insulin action and in vitro insulin action in isolated adipocytes. First, we determined the 24-h peripheral plasma insulin profiles in normal rats under precisely controlled mealfeeding conditions. Basal plasma insulin levels (40 +/- 9 microU/ml) oscillate with a periodicity of 11.9 +/- 0.9 min (p < 0.05), and an amplitude of 60 +/- 10%. Subsequently, the 24-h insulin profile was mimicked in diabetic (D) rats by a continuous (c) or pulsatile (p) (6-min double, 6-min off) insulin infusion rate for 2 weeks, using a programmable pumpswivel unit. Control (C) rats received vehicle treatment. In Cc, Dc, Cp and Dp daily urinary glucose loss and average plasma glucose levels were 0 +/- 0, 7.5 +/- 4.4, 0 +/- 0, 0.8 +/- 0.4 mmol and 6.7 +/- 0.2, 11.5 +/- 2.7, 6.6 +/- 0.1, 5.9 +/- 1.4 mmol/l, respectively. Hypoglycaemia (< 3 mmol/l) was observed in 10 and 20% of the blood samples collected from Dc and Dp rats, respectively. After 2 weeks of treatment, in vivo peripheral and hepatic insulin action was measured by the hyperinsulinaemic euglycaemic (6 mmol/l) clamp with [3-3H]-glucose infusion. Pre-clamp counter-regulatory hormone levels were similar among rats. Compared to Cc and Cp, Dc showed a reduction in insulin sensitivity and responsiveness for peripheral glucose uptake whereas Dp only showed a reduction in insulin sensitivity. Suppression of hepatic glucose production by insulin was similar among rats. After 2.5 weeks of treatment, epididymal adipocytes were isolated. Specific [125I]-insulin binding, basal and insulin-stimulated [U-14C]-glucose uptake and isoproterenol-stimulated glycerol output were comparable among rat adipocytes. The inhibition of glycerol output by insulin was identical in Cp and Dp (V(max) = 48.6 +/- 6.1 and 42.3 +/- 4.6%) but blunted in Dc vs Cc (V(max) = 8.2 +/- 4.6 vs 44.0 +/- 7.2%, p < 0.01) adipocytes, suggesting a post-binding defect in the antilipolytic action of insulin in Dc rats. In conclusion, long-term near-physiological pulsatile intravenous insulin replacement in insulin-deficient diabetic rats is more efficient than continuous delivery in reducing blood glucose, lowering glucosuria, increasing insulin sensitivity and inhibiting lipolysis.

Gas chromatographic-mass spectrometric method for measuring stable isotope enrichments of underivatized cortisol from small plasma volumes

Isotope dilution methods utilizing quantification of stable enrichment by mass spectrometry has been used recently to determine cortisol production rates in humans. Studies of steroid production and utilization rates require frequent blood sampling, and while stable isotopes are safe for pediatric and perinatal use, the required blood volumes may be problematic using published methodologies. We have developed methods to isolate and quantify the plasma enrichment of cortisol with its stable isotope, [9,12,12-2H3]cortisol using a simple four-step isolation procedure and gas chromatography-mass spectrometry. Isolation is semi-quantitative, but reproducible (mean recovery; 51.5 +/- 5.4% coefficient of variation) and the method does not require derivation. In human studies, this method can determine plasma enrichments between 1 and 10 mol% at plasma concentrations of 2 micrograms/dl or more; only 1 ml of plasma is required. The concentrations of labelled cortisol added to plasma are low (0.2-0.5 micrograms/dl) and are not expected to interfere with the sensitive hypothalamic-pituitary-adrenal feedback system. We conclude that reproducible quantification of stable cortisol isotope enrichment can be achieved from small plasma volumes.

Insulin infusion stimulates daily food intake and body weight gain in diabetic rats

Current theories state that physiological levels of insulin inhibit daily food intake and reduce body weight. To test whether insulin induces satiety, systematically increasing doses of insulin from 2.0 to 5.0 U/day were infused intravenously into streptozotocin-induced diabetic rats. Food intake increased significantly from 70.0 +/- 1.4 kcal/day during the saline baseline up to 102.2 +/- 1.9 kcal/day in the 3.5 U/day insulin infusion and then stabilized at 95.9 +/- 0.5 kcal/day for the remaining doses (p less than 0.05). Retained energy values (kcal of food intake minus kcal of urinary glucose loss) also increased from 69.9 +/- 1.4 kcal/day to stabilize at 95 kcal/day (p less than 0.001). Food intake and retained energy of normal controls remained unchanged at 75.4 +/- 1.6 kcal/day for the duration of the study. With elevated food intake and retained energy values after the 3.5 U/day insulin dose, the diabetic rats gained more weight than the normal controls (p less than 0.01). Contrary to expectations, increasing the amount of insulin infused through the physiological range results in a 40% increase in daily food intake and a rapid gain in body weight.