Fig. 3. Glucose and lactate specific activities in blood and brain.

(A) Percent of total ¹⁴C dpm recovered in purified glucose (Glc; squares) and purified lactate (Lac; circles) in each of the four two-minute timed arterial blood samples (0-2, 2-4, 4-6, and 6-8 min) drawn from each of five conscious rats with bilateral K⁺-induced spreading cortical depression that were given intravenous bolus injections of [6-¹⁴C]glucose (Cruz et al., 1999). For each of these arterial blood samples the following are also plotted: glucose SA (nCi/μmol; filled diamonds), lactate SA (nCi/μmol; filled inverted triangles), and the relative specific activity (RSA) of blood lactate SA to that of blood glucose SA (triangles); the SA ratios were calculated from respective values in each rat and the mean RSA at each time point is expressed as percent, i.e., 100(SA lactate/SA glucose). Brain metabolites were not analyzed in this arteriovenous difference study (Cruz et al., 1999), but they were measured in our preceding study of brain glucose utilization during unilateral spreading cortical depression (Adachi et al., 1995) that used an identical pulse labeling procedure. For comparison to blood values, mean brain glucose SA (open diamond) and brain lactate SA (open inverted triangle) determined at 5-6 min (n=12/group; Fig. 2E) are included (for clarity, brain values are offset from the true sampling time). Values are means; vertical bars represent 1 SD (if not visible, the SD is smaller than the symbol). Note that the legend text color code identifies the y axis corresponding to different data sets; black text denotes the left ordinate and blue text identifies the right ordinate data sets. (B) Plasma lactate SA was not determined in our generalized sensory stimulation study in which brain SAs were assayed (Fig. 2B). To compare plasma lactate SA to brain lactate SA in these animals, plasma lactate specific activities were calculated from measured lactate levels in the last plasma sample at kill time, measured total ¹⁴C in the same plasma sample and the estimated ¹⁴C in blood lactate (i.e., using the mean [¹⁴C]lactate concentration of 1.1 ± 0.5% over the 8 min experimental interval; circles, Fig. 3A). Each plasma lactate SA was then expressed as a percentage of the measured brain lactate SA in the same animal (i.e., %RSA; n=6/group). (C) Arterial plasma and brain glucose specific activities were plotted from the studies by Hawkins et al. (1974) who reported normalized SAs and Cremer et al. (1978) (units are 10^-3 x dpm/μmol).