Glucagon and the Fasting Liver

The mirror image of insulin

If insulin says “store it,” [[glucagon|glucagon]] says “release it.” Secreted by the islet's [[alpha-cell|alpha cells]] when glucose falls, glucagon is the master hormone of the fasting state. Where insulin works mainly on muscle and fat, glucagon's main audience is the liver — the body's glucose warehouse and factory. The two hormones move in opposite directions: a low glucose level both raises glucagon and lowers insulin, so the storage signal fades just as the release signal grows.

Glucagon is a peptide hormone, so it cannot enter cells; it works from outside. It binds a [[endo-gpcr|G protein-coupled receptor]] on liver cells, which switches on the enzyme adenylyl cyclase to make the second messenger [[endo-cyclic-amp|cyclic AMP]]. That cyclic AMP flips the liver from storing glucose to exporting it.

Two ways the liver makes glucose

The liver has two distinct ways to put glucose back into the blood, and the timing differs:

1. [[glycogenolysis|Glycogenolysis]] — breaking down stored glycogen. The liver keeps glucose chained together as glycogen; glucagon orders it chopped back into free glucose. This is the fast response, the first few hours of a fast, but the glycogen tank is limited and runs low overnight.
2. [[gluconeogenesis|Gluconeogenesis]] — building brand-new glucose from non-sugar materials such as amino acids, lactate and glycerol. This is the slow but renewable response that carries you through a longer fast once glycogen is spent. Quite literally, the liver manufactures sugar from scratch.

The backup team: counter-regulatory hormones

Recall the asymmetry from the first guide: only insulin lowers glucose, but several hormones raise it. Glucagon leads that group, the [[counter-regulatory-hormone|counter-regulatory hormones]] — so named because they counter insulin. When glucose falls sharply or stress hits, three more pitch in: [[epinephrine|epinephrine]] (adrenaline) for the fast fight-or-flight surge, [[cortisol|cortisol]] for the sustained stress response, and [[growth-hormone|growth hormone]] over longer fasts. Together they form a layered defense, which is why a healthy person almost never crashes from low sugar — there is redundancy.

Timeline of defending against falling glucose:

  minutes 0-30     glucagon  + epinephrine
                   -> glycogenolysis (spend the liver's stash)

  hours 1-12       glucagon keeps pushing
                   -> gluconeogenesis ramps up (make new glucose)

  hours 12+        cortisol + growth hormone
                   -> spare glucose for the brain;
                      shift other tissues to burning fat

  Net effect: blood glucose held near set point even while fasting,
  with the brain always served first.