Introduction: Recurrent hypoglycemia (RH) impairs secretion of counterregulatory hormones. Whether and how RH affects responses within metabolically important peripheral organs to counterregulatory hormones are poorly understood.
Objective: To study the effects of RH on metabolic pathways associated with glucose counterregulation within liver, white adipose tissue and skeletal muscle.
Methods: Using a widely adopted rodent model of 3-day recurrent hypoglycemia, we first checked expression of counterregulatory hormone G-protein coupled receptors (GPCRs), their inhibitory regulators and downstream enzymes catalyzing glycogen metabolism, gluconeogenesis and lipolysis by qPCR and western blot. Then, we examined epinephrine-induced phosphorylation of PKA substrates to validate adrenergic sensitivity in each organ. Next, we measured hepatic and skeletal glycogen content, degree of breakdown by epinephrine and abundance of phosphorylated glycogen phosphorylase under hypoglycemia and that of phosphorylated glycogen synthase during recovery to evaluate glycogen turnover. Further, we performed pyruvate and lactate tolerance tests to assess gluconeogenesis. Additionally, we measured circulating FFA and glycerol to check lipolysis. The abovementioned studies were repeated in streptozotocin-induced diabetic rat model. Finally, we conducted epinephrine tolerance test to investigate systemic glycemic excursions to counterregulatory hormones. Saline-injected rats served as controls.
Results: RH increased counterregulatory hormone GPCR signaling in liver and epidydimal white adipose tissue (eWAT), but not in skeletal muscle. For glycogen metabolism, RH did not affect total content or epinephrine-stimulated breakdown in liver and skeletal muscle. Although RH decreased expression of phosphorylated glycogen synthase 2, it did not affect hepatic glycogen biosynthesis during recovery from hypoglycemia or after fasting-refeeding. For gluconeogenesis, RH upregulated fructose 1,6-bisphosphatase 1 and monocarboxylic acid transporter 1 that imports lactate as precursor, resulting in a lower blood lactate profile during hypoglycemia. In agreement, RH elevated fasting blood glucose and caused higher glycemic excursions during pyruvate tolerance test. For lipolysis, RH did not affect circulating levels of FFA and glycerol after overnight fasting or upon epinephrine stimulation. Interestingly, RH upregulated the trophic fatty acid transporter FATP1 and glucose transporter GLUT4 to increase lipogenesis in eWAT. These aforementioned changes of gluconeogenesis, lipolysis and lipogenesis were validated in streptozotocin-diabetic rats. Finally, RH increased insulin sensitivity to accelerate glucose disposal, which was attributable to upregulated visceral adipose GLUT4.
Conclusions: RH caused metabolic adaptations related to counterregulation within peripheral organs. Specifically, adrenergic signaling was enhanced in liver and visceral fat, but not in skeletal muscle. Glycogen metabolism remained unchanged. Hepatic gluconeogenesis was augmented. Systemic lipolysis was unaffected, but visceral lipogenesis was enhanced. Insulin sensitivity was increased. These findings provided insights into mechanisms underlying clinical problems associated with intensive insulin therapy, such as high gluconeogenic flux and body weight gain.
Keywords: Adrenergic sensitivity; Gluconeogenesis; Glycogen; Insulin sensitivity; Monocarboxylic acid transporter; Recurrent hypoglycemia.
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