EFFECTS OF INSULIN AND THE COMBINATION OF INSULIN PLUS METFORMIN (GLUCOPHAGE) ON MICROVASCULAR REACTIVITY IN CONTROL AND DIABETIC HAMSTERS

The purpose of this study was to determine the in vivo microvascular r eactivity of arterioles (mean internal diameter range: 16.0 to 106.4 m u m) and venules (mean internal diameter range: 24.0 to 117.3 mu m) in the hamster cheek pouch to insulin and to the mixture insulin + metfo rmin. Experiments were performed using an intravital microscope couple d to a closed-circuit TV system and a videotape. The TV monitor displa y was used to obtain arteriolar and venular internal diameter measurem ents by an image-shearing device. The studied drugs were applied topic ally, added to the superfusion solution, to avoid systemic effects tha t would complicate the analysis of the results. In control animals (gl ycemia 7.7 +/- 0.4 mmol/L), application of insulin (10 to 500 mu U/mL/ min) evoked vasodilatation in a dose-dependent fashion in arterioles ( 4.9 +/- 3.2% to 50.9 +/- 6.5%, smallest and largest concentration, res pectively, values expressed in percent of the initial diameter as mean +/- SE) and venules (-2.1 +/- 3.1% to 14.3 +/- 5.1%), decreased and f inally abolished the spontaneous vasomotion frequency (from 9.5 +/- 0. 3 cycles per minute [cpm] to 0.0 +/- 0.0 cpm) and amplitude (from 8.6 +/- 0.3 to 0.0 +/- 0.0 mu m). Addition of metformin, 0.2 mg/ml/min, di d not significantly change either the observed vasodilatation in arter ioles and venules or the vasomotion frequency and amplitude curves. Tw o types of diabetic hamsters were studied: severely diabetic, induced with three intraperitoneal injections of streptozotocin, diluted in ph ysiological saline, 50 mg/kg/dose, given in three consecutive days, an d mildly diabetic, induced by a single dose of streptozotocin. All dia betic animals were studied four weeks after the onset of diabetes and no specific treatment for diabetes was given. In severely diabetic ham sters (glycemia 18.0 +/- 2.2 mmol/L), application of insulin, in the s ame concentration range, evoked a significantly reduced vasodilatation in arterioles as compared with control animals (5.9 +/- 1.3% to 18.9 +/- 3.5%) and did not change the vasodilatation observed in the venule s (5.9 +/- 1.4% to 21.3 +/- 2.5%). In these preparations no spontaneou s arteriolar vasomotion could be detected. Addition of metformin did n ot significantly improve the impaired vasodilatation. In mildly diabet ic hamsters (glycemia 12.1 +/- 0.8 mmol/L), application of insulin, in the same concentration range, evoked vasodilatation, in a dose-depend ent fashion, equivalent to the one observed in control animals, in art erioles (3.1 +/- 2.5% to 53.4 +/- 10.0%) and venules (7.1 +/- 3.0% to 29.9 +/- 4.8%) and also reduced the vasomotion frequency (from 10.1 +/ - 0.3 to 0.1 +/- 0.1 cpm) and amplitude (from 9.2 +/- 0.6 to 0.2 +/- 0 .2 mu m). Addition of metformin tended to increase the observed arteri olar dilatation (6.6 +/- 3.0% to 67.8 +/- 5.5%), did not change the ve nular dilatation (6.7 +/- 4.8% to 28.0 +/- 3.3%), and tended to preser ve vasomotion frequency and amplitude. These experiments show that (1) insulin has a direct dilatatory effect on arterioles and venules; (2) the vasodilatation evoked by insulin is impaired in severe diabetes, and (3) no significant abnormality could be detected on microvascular reactivity in mild diabetes. Further addition of metformin helped to m aintain the spontaneous arteriolar vasomotion even during moderate vas odilatation and tended to augment the arteriolar dilatation evoked by insulin in mildly diabetic animals.