Glucose levels at the site of subcutaneous insulin administration and their relationship to plasma levels

Abstract

Objective: To examine insulin's effect on the tissue glucose concentration at the site of subcutaneous insulin administration.

Research design and methods: A CMA-60 microdialysis (MD) catheter and a 24-gauge microperfusion (MP) catheter were inserted into the subcutaneous adipose tissue of fasting, healthy subjects (n = 5). Both catheters were perfused with regular human insulin (100 units/ml) over a 6-h period and used for glucose sampling and simultaneous administration of insulin at sequential rates of 0.33, 0.66, and 1.00 units/h (each rate was used for 2 h). Before and after the insulin delivery period, both catheters were perfused with an insulin-free solution (5% mannitol) for 2 h and used for glucose sampling only. Blood plasma glucose was clamped at euglycemic levels during insulin delivery.

Results: Start of insulin delivery with MD and MP catheters resulted in a decline of the tissue glucose concentration and the tissue-to-plasma glucose ratio (TPR) for approximately 60 min (P < 0.05). However, during the rest of the 6-h period of variable insulin delivery, tissue glucose concentration paralleled the plasma glucose concentration, and the TPR for MD and MP catheters remained unchanged at 83.2 +/- 3.1 and 77.1 +/- 4.8%, respectively. After subsequent switch to insulin-free perfusate, tissue glucose concentration and TPR increased slowly and reattained preinsulin delivery levels by the end of the experiments.

Conclusions: The results show the attainment of a stable TPR value at the site of insulin administration, thus indicating that insulin delivery and glucose sensing may be performed simultaneously at the same adipose tissue site.

Figure 1

Schematic of the experimental set-up for the assessment of insulin's effect on the glucose concentration at the tissue site of insulin delivery. A: A microperfusion (MP) and a microdialysis (MD) catheter were inserted into subcutaneous adipose tissue of healthy subjects (n = 5). Both catheters were perfused sequentially with an insulin-free solution (mannitol) for 2 h, a standard human insulin preparation (100 units/ml, Actrapid) for 6 h, and, again, an insulin-free solution (mannitol) for 2 h. Perfusion of each catheter was accomplished by applying two peristaltic pumps, with one pump (1, 1′) attached to the inflow tubing and another (2, 2′) to the outflow tubing (dual-pump operation mode [9]). B and C: The perfusate streamed through the inflow tubing and the inner cannula to the tip of the catheter and then entered the space between inner and outer cannula of the catheter (bold arrows). By operating the inflow pump at a higher speed than the outflow pump, a fraction of the perfusate entering the space between inner and outer cannula was forced to flow through the perforations and membrane pores of the MP and MD catheter, respectively, to the surrounding tissue (tissue-directed flows indicated by thin straight arrows). The magnitude of this flow fraction and, thus, the amount of perfusate solutes (e.g., insulin) convectively transported to the tissue was controlled by controlling the difference between the inflow and outflow rates of the catheter. In parallel to this convective tissue-directed solute transport, diffusive bidirectional solute transport took place across the perforations and membrane pores of the MP and MD catheter, respectively (wavy arrows), thereby causing ISF solutes (e.g., glucose) to enter the fluid fraction that flowed in the space between inner and outer cannula of the catheter (bold striped arrows). Along with the permeated ISF solutes, this fluid fraction was pumped continuously through catheter outlet and outflow tubing to the collecting vial. The efficiency by which glucose was transported via diffusion from the ISF of the tissue into the catheter (glucose recovery) was measured by applying the ionic reference technique (7,8).

Figure 2

Glucose dynamics at the subcutaneous tissue site of insulin delivery. A: Average time course (n = 5, means ± SE) of plasma glucose concentration (●) as well as tissue glucose concentration determined with MD (▵) and MP (▿) catheters. *P < 0.05 tissue glucose vs. corresponding plasma glucose values, one-factor repeated-measures ANOVA, and Dunnett's post hoc test. B and C: Average time course (n = 5, means ± SE) of the TPR obtained with MD (▴) and MP (▾) catheters. *P < 0.05; **P < 0.01 vs. the first basal TPR value, one-factor repeated-measures ANOVA (Huynh-Feldt corrected), and Dunnett's post hoc test. Panels also show the average time course (n = 5, means ± SE) of the insulin delivery rates (bars) of the MD (B) and MP (C) catheters.