Highlights
- •Recurrent hypoglycemia (RH) increases adrenergic sensitivity in liver and visceral fat.
- •RH enhances hepatic gluconeogenesis by facilitating lactate uptake.
- •RH increases lipogenesis without affecting systemic lipolysis.
- •RH accelerates glucose disposal into visceral adipose tissue via upregulated GLUT4.
Abstract
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.
Abbreviations:
3dRH (3-day recurrent hypoglycemia), ADRA1B (alpha-1B-adrenergic receptor), ADRB (beta-adrenergic receptor), AGL (amylo-1,6-glucosidase), AQP (aquaporin), ARRB (beta arrestin), ATGL (adipose triglyceride lipase), BG (blood glucose), EGP (endogenous glucose production), ETT (epinephrine tolerance test), eWAT (epidydimal white adipose tissue), FABPpm (plasma membrane-associated fatty acid-binding protein), FATP (fatty acid transport protein), FBP1 (fructose 1,6-bisphosphatase 1), G6PC (glucose 6-phosphatase), GCGR (glucagon receptor), GK (glycerol kinase), GLUT (glucose transporter), GOT2 (glutamic-oxaloacetic transaminase 2), GPCRs (G-protein coupled receptors), GRKs (GPCR kinases), GYS (glycogen synthase), HSL (hormone-sensitive lipase), IIT (intensive insulin therapy), ITT (insulin tolerance test), i.p. (intraperitoneally), iWAT (inguinal white adipose tissue), LDH (lactate dehydrogenase), LTT (lactate tolerance test), MCT (monocarboxylic acid transporter), PC (pyruvate carboxylase), PCK (phosphoenolpyruvate carboxylase), PTT (pyruvate tolerance test), PYGL (glycogen phosphorylase (liver)), PYGM (glycogen phosphorylase (muscle)), RE (recurrent hyperinsulinemic-euglycemia), RH (recurrent hypoglycemia), STZ (streptozotocin), TA (tibialis anterior)Keywords
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Article info
Publication history
Published online: September 02, 2022
Accepted:
August 31,
2022
Received:
May 8,
2022
Identification
Copyright
© 2022 Elsevier Inc. All rights reserved.
