EXTRACELLULAR CHANGES OF INORGANIC-PHOSPHATE ARE DIFFERENT DURING SPREADING DEPRESSION AND GLOBAL CEREBRAL-ISCHEMIA OF RATS

Tissue levels of inorganic phosphate (iP-) and lactate (lac) increase during cerebral ischemia and cortical spreading depression (SD). Since cell membranes become leaky during these insults, iP- and lac were expected to leak into the extracellular space (ECS). In order to find out whether this occurs or does not, a microdialysis (MD) fiber was implanted into the cortex of anesthetized rats and extracellular lactate (lac(c)) and extracellular iP- (iP(e)-) were determined during various insults. Extracellular lactate increased to about the same extent during ischemia and SD. In contrast, iP(e)- increased during ischemia but not during SD. Instead, iP(e)- started to rise after SD and reached its maximum about 45 min later. The distinct pattern of iP(e)- in comparison to lac(e) during the above mentioned insults points to a qualitative difference of the underlying mechanisms: whereas lac appears within the ECS at any stressful situation, elevation of iP- within the ECS indicates depletion of energy stores in parallel to the lack of control of ion homeostasis.