Substrates of Energy Metabolism of the Pituitary and Pineal Glands

Abstract

The capability of the neurohypophysis, the adenohypophysis, and the pineal gland to oxidize nonesterified fatty acids and glucose as energy sources was studied in vivo. Fed and 48‐h‐starved rats had catheters placed in their femoral vessels. After they became conscious, an intravenous injection of one of the following was given: [1‐¹⁴C]acetate, [1‐¹⁴C]octanoate, [1‐¹⁴C]‐palmitate, or [2‐¹⁴C]glucose. After 5 min the rats were sacrificed. These metabolites produce [¹⁴C]acetyl‐CoA within the mitochondria when they are oxidized as metabolic fuels. On passage through the Krebs cycle a considerable portion of the ¹⁴C is trapped in large amino acid pools closely associated with the Krebs cycle; the appearance of ¹⁴C in these amino acids was taken as evidence of oxidation. As expected, brain structures behind the blood‐brain barrier (cerebral cortex and caudate) showed considerable labeling of Krebs cycle‐associated amino acids in both nutritional states when [2‐¹⁴C]glucose was the substrate. Surprisingly, however, no label was detected in amino acids of the neurohypophysis or the pineal gland in starved rats and very little in fed rats. On the other hand, ¹⁴C from acetate and palmitate was extensively incorporated into amino acids of the pineal gland and the neurohypophysis, while little ¹⁴C labeling was found in the cerebral cortex and the caudate. Octanoate, which passes the blood‐brain barrier readily, labeled amino acids of all tissues. The experiments demonstrated conclusively that the neural structures studied, which have no blood‐brain barrier, do not rely heavily upon glucose as a fuel for oxidative energy metabolism, in contrast to the rest of the brain. The results also showed that nonesterified fatty acids may supply at least some of their energy requirements.