Development of microdialysis methodology for interstitial insulin measurement in rodents

IntroductionAccurate assessment of muscle insulin sensitivity requires measurement of insulin concentration in interstitial fluid (ISF), but has proved difficult. We aimed to optimise measurement of ISF insulin concentrations in rat muscles in vivo using microdialysis.MethodsFactorial experimental design experiments were performed in vitro to determine optimal conditions for insulin recovery with microdialysis probes. These conditions were tested in vivo, adjusted appropriately and used in lean and obese Zucker rats to compare ISF insulin concentrations basally and during hyperinsulinaemic-euglycaemic (HE) clamp.ResultsOptimal conditions in vivo were: a 100 kDa microdialysis probe inserted in muscle, perfused with 1% BSA, 1.5 mM glucose in 0.9% sodium chloride at 1 μl/min. Samples were collected into siliconised glass microvials. As a reference for insulin, we established a protocol of inulin infusion, beginning at − 80 min and reaching equilibrium within 60 min. HE clamp, beginning at 0 min, increased ISF insulin concentration from 122 ± 56 basally to 429 ± 180 pmol/l (P < 0.05) in lean rats and from 643 ± 165 to 1087 ± 243 pmol/l (P = 0.07) in obese rats; ISF insulin concentrations were significantly higher throughout in obese rats. The difference between ISF and plasma insulin concentration (ISF:plasma ratio) was substantially higher in obese rats, but fell to similar values in obese and lean rats during HE clamp.DiscussionOptimising insulin recovery with microdialysis allowed accurate measurement of basal ISF insulin in muscle of lean and obese Zucker rats and indicates insulin transport across capillaries is impaired in obese rats, basally and during hyperinsulinaemia.