Unidirectional transport of glucose and lactate into brain of fetal sheep and guinea‐pig

The first‐passage multiple‐indicator dilution method was used to measure blood to brain transport of D‐ and L‐glucose, D‐ and L‐lactate and sucrose relative to 22Na, an impermeable reference tracer, in fetal sheep. Fractional extraction for D‐glucose was 0.315 +/− 0.051 (S.E.M.) at normal glucose levels and fell to 0.198 +/− 0.041 at 5.2 +/− 0.4 mM‐glucose. Fractional extractions for L‐glucose, D‐ and L‐lactate and sucrose were not different from zero. No specific blood‐brain transport system was detected for L‐lactate in fetal sheep in vivo (fractional extraction = ‐0.024 +/− 0.019). Uptake of L‐lactate into isolated microvessels from fetal sheep cerebrum in vitro showed a slightly higher rate (32.2 +/− 8.9 pmol min‐1 (mg protein)‐1) than that for D‐lactate (22.6 +/− 5.6). In fetal guinea‐pigs, the carotid arterial injection method with tritiated water as the permeable reference was used to measure the brain uptake index (BUI). BUI was determined for D‐glucose (0.304 +/− 0.065) sucrose (0.008 +/− 0.001), L‐lactate (0.418 +/− 0.112) and D‐lactate (0.071 +/‐ 0.024). Unidirectional influx calculated from these measurements and estimates of cerebral blood flow showed that transport would be rate‐limiting for cerebral glucose utilization at arterial glucose levels below 0.5 mM in fetal sheep and 1.7 mM in fetal guinea‐pig. In fetal sheep, but not in fetal guinea‐pigs, lactate efflux may be limited by brain‐blood transport.