A novel function of insulin in rat dermis

Abstract

In this study we present a novel function of insulin in rat dermis. We investigated local effects of insulin on interstitial fluid pressure (Pif), and capillary albumin leakage and pro-inflammatory cytokine production in skin and serum after intravenous lipopolysaccharide (LPS), tumour necrosis factor-alpha (TNF-alpha) and interleukin-1beta (IL-1beta) challenge treated with a glucose-insulin-potassium regimen (GIK). The main objective for this study was to investigate anti-inflammatory effects of insulin. Work by others shows that insulin stimulates cell adhesion, and that this effect is dependent upon phosphatidylinositol 3-kinase (PI3K) activity. Cytokines like platelet-derived growth factor BB (PDGF-BB) attenuate lowering of Pif, possibly via PI3K. LPS and pro-inflammatory cytokines contribute to oedema development during acute inflammation by lowering the Pif. Intravenous injection of LPS, TNF-alpha or IL-1beta to Wistar Møller rats caused a lowering of Pif, but after local injection of insulin in the paw, Pif increased back to control values. IL-1beta caused a lowering in control from -0.5 +/- 0.2 mmHg to -3.0 +/- 0.2 mmHg after 20 min (mean +/- S.E.M.) (P < 0.05). Within 50 min after insulin injection the pressure was increased to -0.6 +/- 0.2 mmHg (P > 0.05 compared with control). Insulin was given together with a PI3K inhibitor (wortmannin) locally in the skin, almost abolishing the effect of insulin on Pif. A GIK regimen was given as a continuous intravenous infusion, significantly attenuating the oedema formation after LPS or TNF-alpha/IL-1beta challenge. The same GIK regimen caused a significant reduction in pro-inflammatory cytokines in serum and in interstitial fluid in skin of endotoxaemic rats. These experiments show a possible role for insulin in the interstitium during inflammation induced by LPS and TNF-alpha/IL-1beta. Insulin can attenuate a lowering of Pif possibly via PI3K, and it has an anti-inflammatory effect by inhibiting production of pro-inflammatory cytokines.

Figure 1. Interstitial fluid pressure after i.v. injection of IL-1β (upper), TNF-α (middle) and LPS (lower)

A control group (▾) receiving saline i.v. and s.c. is shown on all three graphs. ○, recordings done in experimental animals receiving insulin after 20 min; •, experimental animals receiving saline s.c. after 20 min. Recordings are done for 90 min. Data are presented as means ± s.e.m. *P < 0.001, ^aP = 0.003, ^bP = 0.002 and ^cP = 0.007 compared with own control. †P < 0.001 compared with own control and experimental group receiving insulin s.c.

Figure 2

A, extravasation of albumin (E_alb) in skin of paw and back after i.v. injection of LPS, LPS + GIK, TNF-α/IL-1β, TNF-α/IL-1β + GIK or saline + GIK (control). B, total tissue water (TTW) in skin of paw and back after i.v. injection of LPS, LPS + GIK, TNF-α/IL-1β, TNF-α/IL-1β + GIK or saline + GIK (control). Data are presented as means ± s.e.m. LPS had a significantly higher E_alb compared with corresponding GIK group and control, described with † (P < 0.001). TNF-α/IL-1β had a significantly higher E_alb compared with control (P = 0.002) but not to GIK treatment (P > 0.05). LPS increased TTW compared to control P = 0.009), but not GIK treatment (P > 0.05). TNF-α/IL-1β increased TTW both compared with control and GIK treatment (P = 0.007 for both).

Figure 3

A, concentrations of TNF-α and IL-1β in interstitial fluid in skin after i.v. injections of LPS, LPS + GIK or saline + GIK (control). B, concentrations of TNF-α and IL-1β in serum after i.v. injections of LPS, LPS + GIK or saline + GIK (control). Data are presented as means ± s.e.m. Concentrations of TNF-α or IL-1β were significantly higher in both interstitial fluid and serum for LPS challenged rats compared with GIK treated and control. This is described as † on the figure. P ≤ 0.001 for all groups, except for IL-1β in serum (LPS compared with GIK-treated), with a value of 0.042. The lowering in IL-1β concentration in GIK treated was only from 203 to 106 pg ml⁻¹ explaining the high P-value. In GIK-treated rats the concentration of TNF-α or IL-1β was not different from control for interstitial fluid, but significantly different from control for serum, described as * on the figure (P < 0.001 and P = 0.007, respectively).