Appendix A
Alpha-Cell Response to Hyperglycemia
in Health and Type 1 Diabetes
In this paper we report on the results of applying correlation analyses to the diurnal and postprandial profiles of circulating glucose, insulin, and glucagon in healthy subjects and in subjects with type 1 diabetes (T1D). We use the coefficient of determination (R-squared) to estimate the degree of linkage between variables, and we look for reoccurring patterns of deviations from the regression equation predictions (empirical models) to illuminate possible non-glucose forcings of glucagon secretion. Our results are consistent with the following conclusions about alpha- and beta-cell responses to rising blood glucose in the clinical studies evaluated:
Regardless of the meal contents and schedules, in these studies changes in circulating glucose accounted for at least 90% of the observed changes in circulating insulin based on measuring glucose and insulin at the same points in time. The remaining 10% of insulin variation displayed a reoccurring pattern of deviations from the regression predicted levels, suggesting some of it was caused by changes in beta-cell sensitivity to insulin changes.
In nondiabetics taking the simple carbohydrate meals, there is a negative correlation between circulating glucose and glucagon levels, with R-squared values of 0.59 in the diurnal study and 0.49 in the simple carb study. Thus, about half of glucagon postprandial variation was explained by alpha-cell response to hyperglycemia.
However, complex carbohydrate meals showed no correlation between postprandial glucose and glucagon in healthy subjects. The data from these studies suggest that alpha-cell suppression is not triggered unless the postprandial rate of glucose increase is high enough.
There also appear to be non-glucose regulatory mechanisms which result in relatively consistent postprandial deviations of circulating glucagon from levels predicted by the dose-response to circulating glucose. These nonglycemic forcings appear to account for about one-third of the measured diurnal variation in healthy subjects. The observation that T1D subjects show similar patterns of diurnal glucagon variation without any glycemic response suggests these deviations are unlikely to be artifacts of alpha-cell secretory delay, but rather reflect nonglycemic diurnal forcings of alpha-cell secretion.
Subjects with T1D have circulating glucagon levels that are similar to nondiabetic levels immediately before meals and depressed compared to nondiabetics between meals. Over a complete diurnal period, T1D subjects are about 18% hypoglucagonemic compared to nondiabetics.
In comparison to levels predicted by the glycemic model, T1D subjects are relatively hyperglucagonemic between meals. The excess is about 35% based on the AUCs of the diurnal profiles. This hyperglucagonemia would be expected to amplify the hyperglycemia characteristic of T1D.
In subjects with T1D there is no diurnal correlation between circulating glucose and glucagon, indicating that the mechanism by which postprandial glucose increases suppress alpha-cell secretion is completely missing:
These data are consistent with the theory that direct alpha-cell sensing of circulating glucose is NOT the primary glucagon regulatory mechanism, since alpha-cells in T1D otherwise appear normal.
These data are also consistent with the hypothesis that the primary T1D defect – loss of beta-cell function – is the cause of alpha-cell insensitivity to postprandial glucose.
The observation that complex carbohydrates stimulate an insulin response without causing a change in circulating glucagon implies that insulin is NOT a paracrine regulator of glucagon secretion.
The 18% absolute hypoglucagonemia in T1D suggests that alpha-cell depletion of glucagon is NOT a cause of counterregulatory malfunction. Rather, the counterregulatory failure must be caused by a failure of the hypoglycemia sensing mechanism.
These findings support the idea that a drug aimed at restoring glucose dependent control of postprandial alpha-cell secretion should address the primary defect caused by loss of beta-cells. Since these data indicate insulin is not a paracrine regulator of alpha-cell secretion, it makes sense to focus pharmaceutical efforts to regulate postprandial glucagon in T1D on the other beta-cell secreted hormone which is known to suppress alpha-cell secretion: the neurohormone amylin.