Bicarbonate conservation during incomplete cerebral ischemia with superimposed hypercapnia

Area of Science:

• Neuroscience
• Biochemistry
• Physiology

Background:

• Intracellular pH (pHi) and bicarbonate ([HCO3-]i) play critical roles in cellular function during ischemic events.
• Distinguishing the independent contributions of pHi and [HCO3-]i to postischemic recovery is essential for understanding brain injury mechanisms.

Purpose of the Study:

• To investigate the differential roles of intracellular pH (pHi) and bicarbonate ([HCO3-]i) in postischemic metabolic and electrophysiological recovery.
• To compare recovery patterns under extreme hypercapnia-induced ischemia versus previously studied hyperglycemic ischemia.

Main Methods:

• Induction of incomplete global cerebral ischemia in anesthetized dogs using intracranial hypertension.
• Superimposition of extreme hypercapnia (400-450 mmHg Pco2) before and during ischemia.
• Measurement of ATP, phosphocreatine (PCr), and pHi using 31P magnetic resonance spectroscopy.
• Calculation of [HCO3-]i using the Henderson-Hasselbalch equation.

Main Results:

• Extreme hypercapnia significantly reduced cerebral blood flow and pHi during ischemia.
• Despite low pHi, rapid and complete recovery of ATP, PCr, and O2 consumption was observed during normocapnic reperfusion.
• Somatosensory-evoked potentials (SEP) recovered to 43% of control amplitude.
• Results contrasted sharply with poor recovery in hyperglycemic ischemia, where [HCO3-]i was significantly lower.

Conclusions:

• Bicarbonate depletion, rather than end-ischemic pHi alone, appears to be a more critical determinant of postischemic metabolic and electrophysiological recovery.
• Elevated [HCO3-]i may enhance glial cell buffering capacity or reduce hydroxyl radical production, thereby improving recovery.
• Findings suggest therapeutic strategies targeting bicarbonate levels could be beneficial in managing ischemic brain injury.