Neonatal de-afferentation of capsaicin-sensitive sensory nerves increases in vivo insulin sensitivity in conscious adult rats

Sensory neuropeptides, released from the peripheral nervous system, might modulate glucose homeostasis by antagonizing insulin action. The effects of de-afferentation of functional small diameter unmyelinated C- fibres (sensory nerves) on in vivo insulin-mediated intracellular glucose metabolism were investigated by using euglycaemic insulin (6 and 18 mU/kg.min) clamps with [3-³H]-glucose infusion in 24 adult rats, treated neonatally with either capsaicin (CAP) (50 mg/kg) or vehicle (CON). Following the clamp, skeletal muscle groups, liver and adipose tissue were freeze- clamped. At plasma insulin levels of approximately 90 mU/l, CAP-rats showed a 21% increase in whole body glucose uptake compared with CON (24.4±1.6 vs 20.1±0.8 mg/kg·min, p < 0.02), which was paralleled by a 20 % increase in whole body glycolysis (12.6 ± 0.8 vs 10.5 ± 0.5 mg/ kg.min p < 0.05) (concentration of ³H₂O in plasma). Whole body skeletal muscle glycogenesis was increased by 80% in CAP-rats (5.7±0.7 vs 3.l± 0.7 mg/kg · min, p < 0.05) with increased muscle glycogen synthase activity. Whole body (muscle, liver and adipose tissue combined) de novo lipogenesis also was increased in CAP-rats compared with CON (0.69 ± 0.10 vs 0.44 ± 0.06 mg/kg · min, p < 0.05) (incorporation of [3-³H]-glucose counts into glycogen or fat). Hepatic glucose production was lower in CAP-rats compared with CON (0.6±0.6 vs 2.1 ± 0.7 mg/kg · min, p < 0.05). Plasma glucagon, corticosterone, epinephrine and norepinephrine levels were reduced in CAP-rats: 43±2 compared with 70±6 pg/ml, 855±55 compared with 1131±138 nmol/l, 513 ± 136 compared with 1048 ± 164 pmol/l and 928 ± 142 compared with 1472 ± 331 pmol/l, respectively, p < 0.05. At plasma insulin levels of approximately 400 mU/l, CAP-rats showed no differences in peripheral and hepatic insulin action compared with CON. We conclude that the removae of endogenous sensory neuropeptides, by de- afferentation of capsaicin-sensitive sensory nerves, increases in vivo insulin sensitivity, but not responsiveness: 1) primarily through an increased sensitivity of skeletal muscle glycogen synthesis to insulin; 2) through a reduction in the levels of counter-regulatory hormones, thereby creating a milieu which favours overall in vivo insulin sensitivity with respect to glucose uptake, glucose production, glycolysis, glycogenesis and lipogenesis.