Caumo, Andrea

The present studies were undertaken to determine whether 1) the cold- and hot-GINF techniques used with Steele's model provide equivalent estimates of the rates of glucose appearance (R(a)) and disappearance (R(d)) in the presence of physiological changes in glucose and insulin concentrations, 2) the conditions for the best estimation of R(a) are the same as those for R(d), 3) the magnitude of error (if present) differs in diabetic and nondiabetic subjects, and 4) situations exist in which the knowledge of R(d) allows inferences to be made on whole body glucose uptake. To do so we performed experiments in non-insulin-dependent diabetes mellitus and nondiabetic subjects using simultaneous infusions of [6-3H]glucose and [6-14C]glucose; glucose and insulin were infused to mimic normal postprandial glucose and insulin profiles; the infused glucose contained [6-14C]glucose but not [6-3H]glucose. Compared with the hot-GINF method, the traditional cold-GINF method underestimated (P < 0.05) R(a) and R(d) by 10-15% and hepatic glucose release by 25-50% during the 1st h of the study, with the magnitude of error being the same in both diabetic and nondiabetic subjects. Error analysis demonstrated that errors in R(a) and R(d) have different analytic expressions containing common structural but different volume errors. Both R(a) and R(d) can be accurately measured in diabetic and nondiabetic subjects if glucose specific activity is kept constant and the volume of the accessible pool is used to calculate glucose disappearance. The relationship between R(d) and whole body glucose uptake was also derived. Although R(d) can be determined by relying on measurements in the accessible pool only, the assessment of whole body glucose uptake requires a model of the nonaccessible portion of the glucose system. However, knowledge of R(d) can provide useful insights into the behavior of whole body glucose uptake.

The isotope dilution technique has been extensively used to assess insulin action in humans. To determine if nonsteady state (NSS) has led to erroneous estimates of hepatic and extrahepatic insulin sensitivity, we measured glucose turnover in healthy subjects during infusion of insulin at rates of 0.25, 0.6, and 2.0 mU.kg-1.min-1. Turnover was calculated using Steele's traditional NSS equations [fixed-effective volume (pV) method] as well as with methods [radioactive infused glucose (hot-GINF) or variable pV] designed to minimize NSS error. In contrast to the fixed-pV method, both the hot-GINF and variable-pV methods indicated that several hours were required for suppression of hepatic glucose release at all insulin concentrations and that small increases in plasma insulin (approximately 100 pmol/l) had comparable effects on glucose disappearance and hepatic glucose release. Nevertheless, despite these differences, when turnover during the final hour of the insulin infusions was plotted vs. the prevailing insulin concentration, all three methods yielded similar insulin dose-response curves for suppression of hepatic glucose release. Thus despite previous errors in measurement of glucose turnover, the widely accepted belief that the human liver is exquisitely sensitive to small changes in insulin is correct.

Measurement of glucose turnover under non-steady-state conditions has proven problematic. When the mass of the glucose pool is not changing (i.e., glucose concentrations are constant) non-steady-state error can be minimized if all glucose entering the circulation has the same specific activity as plasma [radioactive infused glucose (hot-GINF) method]. Alternatively, a second tracer can be used to measure the effective volume of glucose [variable-pV method of Issekutz (T. Issekutz, R. Issekutz, and D. Elahi. (Can. J. Physiol. 52:215-224, 1974)]. To determine whether these techniques provide concordant assessments of insulin action under non-steady-state conditions, glucose turnover was measured in six subjects. After initiation of insulin (0.6 mU.kg-1 x min-1), both methods indicated similar rates of suppression of hepatic glucose release, which was complete by approximately 100-120 min. In contrast, the traditional fixed-pV method of Steele (R. Steele, J. Wall, R. DeBodo, and N. Altszuler. Am. J. Physiol. 187:15-24 1956) underestimated turnover (P < 0.01) resulting in apparent complete suppression of glucose release within approximately 40 min (P < 0.01 vs. other methods). The hot-GINF and variable-pV methods also yielded similar estimates of turnover after discontinuation of insulin. Both indicated that resumption of hepatic glucose release was slower (P < 0.01) and fall of glucose uptake faster (P < 0.01) than suggested by the fixed-pV method. Thus both the hot-GINF and variable-pV methods avoid non-steady-state error introduced by the fixed-pV method and provide concordant assessments of the rate of onset and offset of insulin action.

People with NIDDM are resistant to insulin. The present studies sought to determine whether the ability of glucose to regulate its own metabolism in the presence of basal insulin concentrations is impaired. To address this question, basal insulin concentrations were maintained constant with an exogenous insulin infusion, while endogenous hormone secretion was inhibited by somatostatin. The integrated glycemic response above baseline during identical prandial glucose infusions was greater (1,411 ± 94 vs. 938 ± 45 mmol/l per 5 h; P &lt; 0.01) in the diabetic subjects than in the nondiabetic subjects, indicating a decrease in net glucose effectiveness. [6-3H]glucose also was infused to determine whether the decrease in net glucose effectiveness was due to a decrease in the ability of glucose to stimulate its own uptake and/or to suppress its own production. Despite identical rates of tracer infusion, the increment in plasma concentration of [6-3H]glucose was higher (4.50 ± 0.29 vs. 3.16 ± 0.21 × 105 dpm/ml per 5 h; P &lt; 0.05) in the diabetic subjects than in the nondiabetic subjects. This was due to both a decrease (P &lt; 0.05) in the ability of glucose to stimulate its own disappearance via mass action and to a greater (P &lt; 0.01) inhibitory effect of glucose on its own clearance. The increase in glucose concentration resulted in prompt and comparable suppression of endogenous glucose production in both groups. Under these optimized conditions, indexes of glucose effectiveness calculated with both the “cold” and “hot” minimal models also were lower (P &lt; 0.05) in the diabetic subjects than in the nondiabetic subjects and were highly correlated (r = 0.94–0.99; P &lt; 0.001) with the indexes of glucose effectiveness calculated from the increments above baseline of glucose and [6-3H]glucose concentration. We conclude that the ability of glucose to regulate its own metabolism in the presence of basal insulin concentrations is abnormal in people with NIDDM.

Both glucose and insulin are important regulators of glucose uptake and hepatic glucose release. Because insulin concentrations rarely if ever increase under daily living conditions, unless glucose concentrations also increase, we sought to determine whether hepatic and extrahepatic responses to changes in insulin and glucose concentration are impaired in patients with non-insulin-dependent diabetes mellitus (NIDDM). To address this question, glucose metabolism was measured in diabetic and nondiabetic subjects. A computer-driven infusion system was used to produce a nondiabetic postprandial insulin profile in both groups while sufficient exogenous glucose was infused to mimic nondiabetic postprandial glucose concentrations. Although NIDDM was associated with greater (P &lt; 0.05) hepatic glucose release both before and during the prandial insulin infusion, suppression did not differ in the diabetic and nondiabetic subjects (−1.06 ± 0.20 vs. −0.86 ± 0.15 mmol/kg every 4 h). In contrast, stimulation of both glucose disappearance (0.77 ± 0.27 vs. 1.68 ± 0.27 mmol/kg every 4 h) and forearm glucose uptake (187 ± 81 vs. 550 ± 149 μmol/dl every 4 h) was lower (P &lt; 0.05) in diabetic than in nondiabetic subjects. Thus, despite increased basal rates of glucose production, obese individuals with NIDDM had decreased stimulation of glucose disappearance but normal suppression of hepatic glucose release in response to nondiabetic prandial glucose and insulin concentrations. These data indicate that the increase in glucose that occurs with carbohydrate ingestion is likely to compensate for hepatic but not extrahepatic insulin resistance.