Glucose autoregulation is the dominant component of the hormone-independent counterregulatory response to hypoglycemia in the conscious dog

Abstract

The contribution of hormone-independent counterregulatory signals in defense of insulin-induced hypoglycemia was determined in adrenalectomized, overnight-fasted conscious dogs receiving hepatic portal vein insulin infusions at a rate 20-fold basal. Either euglycemia was maintained ( group 1) or hypoglycemia (≈45 mg/dl) was allowed to occur. There were three hypoglycemic groups: one in which hepatic autoregulation against hypoglycemia occurred in the absence of sympathetic nervous system input ( group 2), one in which autoregulation occurred in the presence of norepinephrine (NE) signaling to fat and muscle ( group 3), and one in which autoregulation occurred in the presence of NE signaling to fat, muscle, and liver ( group 4). Average net hepatic glucose balance (NHGB) during the last hour for groups 1–4 was −0.7 ± 0.1, 0.3 ± 0.1 ( P < 0.01 vs. group 1), 0.7 ± 0.1 ( P = 0.01 vs. group 2), and 0.8 ± 0.1 ( P = 0.7 vs. group 3) mg·kg−1·min−1, respectively. Hypoglycemia per se ( group 2) increased NHGB by causing an inhibition of net hepatic glycogen synthesis. NE signaling to fat and muscle ( group 3) increased NHGB further by mobilizing gluconeogenic precursors resulting in a rise in gluconeogenesis. Lowering glucose per se decreased nonhepatic glucose uptake by 8.9 mg·kg−1·min−1, and the addition of increased neural efferent signaling to muscle and fat blocked glucose uptake further by 3.2 mg·kg−1·min−1. The addition of increased neural efferent input to liver did not affect NHGB or nonhepatic glucose uptake significantly. In conclusion, even in the absence of increases in counterregulatory hormones, the body can defend itself against hypoglycemia using glucose autoregulation and increased neural efferent signaling, both of which stimulate hepatic glucose production and limit glucose utilization.