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To assess the potential role of polyol-pathway activity in diabetic neuropathy, we measured the effects of sorbinil--a potent inhibitor of the key polyol-pathway enzyme aldose reductase--on nerve conduction velocity in 39 stable diabetics in a randomized, double-blind, cross-over trial. During nine weeks of treatment with sorbinil (250 mg per day), nerve conduction velocity was greater than during a nine-week placebo period for all three nerves tested: the peroneal motor nerve (mean increase [+/- S.E.M.], 0.70 +/- 0.24 m per second, P less than 0.008), the median motor nerve (mean increase, 0.66 +/- 0.27, P less than 0.005), and the median sensory nerve (mean increase, 1.16 +/- 0.50, P less than 0.035). Conduction velocity for all three nerves declined significantly within three weeks after cessation of the drug. These effects of sorbinil were not related to glycemic control, which was constant during the study. Although the effect of sorbinil in improving nerve conduction velocity in diabetics was small, the findings suggest that polyol-pathway activity contributes to slowed nerve conduction in diabetics. The clinical applicability of these observations remains to be determined, but they encourage further exploration of this approach to the treatment or prevention of diabetic neuropathy.

A B S T R A C T The in vivo hepatic metabolism of connecting peptide (C-peptide) in relation to that of insulin has not been adequately characterized. A radioimmunoassay for dog C-peptide was therefore developed and its metabolism studied in conscious mongrel dogs, with sampling catheters chronically implanted in their portal and hepatic veins and femoral artery. The hepatic extraction of endogenous C-peptide under basal conditions was negligible (4.3±4.5%) and was similar to the hepatic extraction of C-peptide measured during the constant exogenous infusion of C-peptide isolated from dog pancreas. Simultaneously measured hepatic extraction of endogenous and exogenously infused insulin were 43.8±7.6 and 47.5±4.4%, respectively. The metabolic clearance rate of infused C-peptide was 11.5±0.8 ml/kg per min and was constant over the concentration range usually encountered under physiological conditions. In additional experiments, the effect of parenteral glucose administration on the hepatic extraction of C-peptide and insulin was investigated. The hepatic extraction of C-peptide (6.2±4.0%) was again negligible in comparison with that of insulin (46.7±3.4%). Parenteral glucose administration did not affect the hepatic extraction of either peptide irrespective of whether it was infused peripherally, intraportally, or together with an intraportal infusion of gastrointestinal inhibitory polypeptide. The fasting C-peptide insulin molar ratio in both the portal vein (1.2±0.1) and femoral

The role of the brain in directing counterregulation during hypoglycemia induced by insulin infusion was assessed in overnight-fasted conscious dogs. Concomitant brain and peripheral hypoglycemia was induced in one group of dogs (n = 5) by infusing insulin peripherally at a rate of 3.5 mU.kg-1.min-1. In another group (n = 4), insulin was infused as described above to induce peripheral hypoglycemia, and brain hypoglycemia was minimized by infusing glucose bilaterally into the carotid and vertebral arteries to maintain the brain glucose level at a calculated concentration of 85 mg/dl. Glucose was also infused peripherally as needed so that the peripheral glucose levels in both of the protocols were similar (45 +/- 2 mg/dl with and 48 +/- 3 mg/dl without brain glucose infusion, both P less than .05). The responses (in terms of change of area under the curve) of epinephrine, norepinephrine, cortisol, and pancreatic polypeptide when brain glycemia was controlled during insulin infusion were only 14 +/- 6, 39 +/- 12, 17 +/- 8, and 9 +/- 4%, respectively, of those present during insulin infusion without concomitant brain glucose infusion (all P less than .05). Of particular interest was the glucagon response that occurred when head hypoglycemia was minimized; the glucagon level was only 21 +/- 8% of that present when marked brain hypoglycemia accompanied insulin infusion (P less than .05). During hypoglycemia resulting from insulin infusion, endogenous glucose production (EGP), as assessed with [3-3H]glucose, rose from 2.6 +/- 0.1 to 4.4 +/- 0.5 mg.kg-1.min-1 (P less than .05). In contrast, EGP decreased from 2.7 +/- 0.2 to 2.0 +/- 0.3 mg.kg-1.min-1 when brain hypoglycemia was minimized. In an additional set of studies, when insulin was infused at 3.5 mU.kg-1.min-1 and glucose was infused peripherally to maintain both the head and peripheral glucose concentrations at 88 +/- 6 mg/dl, EGP decreased from 2.6 +/- 0.1 to 1.2 +/- 0.2 mg.kg-1.min-1. These results suggest that under marked hyperinsulinemic conditions the brain is the primary director of glucagon release and that it is responsible for approximately 75% of the life-sustaining glucose production.

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