Fig. 7.

Carbonic acid buffer peak changes after complete ischemia. pH_o recordings were made with double barrelled H⁺ selective microelectrodes placed 800 μm down in parietal cortex of anesthetized rats. P_t(CO2) was measured with a surface microelectrode. Brain carbohydrate stores were modulated by pretreatment with intraperitoneal dextrose or intravenous insulin injections. Ischemia was induced by cardiac arrest caused by intravenous injection of KCl. When changes in pH_o and P_t(CO2)reached a peak, animals were decapitated and their heads frozen in liquid nitrogen for subsequent enzyme fluorometric analysis of neocortical lactate content. Upper graph shows peak pH_o compared to tissue lactate. Triangles show data from Kraig et al., 1985a and dots represent data from Kraig et al., 1984b, 1985c. The zone between 15–17 mmol/kg lactate is shaded with diagonal lines to reflect the variability in peak pH_o there. The constant peak level of pH_o between 8–13 and 16–31 mmol/kg lactate implies that [H⁺]_o is at a steady-state, but not equilibrium, with respect to [H⁺] in other brain compartments during complete ischemia. The middle graphs shows concomitant changes in P_t(CO2). P_t(CO2) initially rises linearly with increasing lactate up to about 17 mmol/kg. Then P_t(CO2) rises abruptly and remains constant at 389 ± 9 mmHg after 19 mmol/kg lactate. The constancy of peak P_t(CO2) above 19 mmol/kg lactate implies that ${\text{HCO}}_3^{-}$ stores were exhausted from acid producing cells. If [H⁺]_o continues to be determined by [SID] and P_t(CO2) (see equation 1) during complete ischemia, then the lower graph shows the concomitant changes in ${[{\text{HCO}}_3^{-}]}_{\text{o}}$ which would be expected from the measured changes in pH_o and P_t(CO2). Notice that ${[{\text{HCO}}_3^{-}]}_{\text{o}}$ rises from 8–13 mmol/kg lactate, when pH_o is constant at 6.81 pH and then begins to fall when pH_o and P_t(CO2) change abruptly between 13–19 mmol/kg lactate. ${[{\text{HCO}}_3^{-}]}_{\text{o}}$ then remains constant at about 12.3 mmol/l above 19 mmol/kg lactate. Thus ${\text{HCO}}_3^{-}$ and H⁺ both remain heterogeneously distributed between the interstitial space and acid producing cells during complete ischemia.