Outcome of
traumatic brain injury (TBI) is impaired by
hyperglycemia,
hypotension, and
glutamate, and improved by
insulin.
Insulin reduces
glutamate concentration, making it uncertain whether its beneficial effect accrues from euglycemia.
Glucagon decreases CNS
glutamate, lessens neuronal cell injury, and improves neurological scores in mice after TBI. In vitro,
glucagon limits
NMDA-mediated excitotoxicity by increasing cAMP and
protein kinase A (PKA).
NMDA receptor activation couples cerebral blood flow (CBF) to metabolism. Dilation induced by
NMDA is impaired after fluid percussion
brain injury (FPI) due to upregulation of endogenous tPA, which further disturbs cerebral autoregulation during
hypotension after fluid percussion injury (FPI). We hypothesized that
glucagon prevents impaired
NMDA receptor-mediated dilation after FPI by upregulating cAMP, which decreases release of tPA.
NMDA-induced pial artery dilation (PAD) was reversed to vasoconstriction after FPI.
Glucagon 30 min before or 30 min after FPI blocked
NMDA-mediated vasoconstriction and restored the response to vasodilation. PAD during
hypotension was blunted after FPI, but protected by
glucagon.
Glucagon prevented FPI-induced reductions in CSF cAMP, yielding a net increase in cAMP, and blocked FPI-induced elevation of CSF tPA. Co-administration of the PKA antagonist Rp 8Br cAMPs prevented
glucagon-mediated preservation of
NMDA-mediated dilation after FPI. The pKA agonist Sp 8Br cAMPs prevented impairment of
NMDA-induced dilation. These data indicate that
glucagon protects against impaired cerebrovasodilation by upregulating cAMP, which decreases release of tPA, suggesting that it may provide neuroprotection when given after TBI, or prior to certain neurosurgical or cardiac interventions in which the incidence of perioperative
ischemia is high.