Utilization of glucose, alanine, lactate and glycerol as lipogenic substrates by periuterine adipose tissue in situ in fed and starved rat.

Abstract

A technique previously used to study placental transfer in pregnant rats, consisting of labeled tracer infusion through the left uterine artery, was employed to determine the utilization oflipogenic substrates by periuterine adipose tissue in the fed and 48-hr starved female virgin rat. After 20 min infusion with either D[U- 14C]glucose, L-[U- 14C]alanine, [U14C]glycerol or L-[U- 14C]lactate, the radioactivity appearing in periuterine adipose tissue 14C-labeled lipids from the left side was always higher than that appearing in tissue from the right side. Negligible radioactivity was detected in the tissue from either side when the infusion was done with non-metabolizable derivatives such as L-[1- 14C]glucose or [1- 14C]a-aminoisobutyric acid. Simultaneous infusion ofL-[U- 14C]alanine and an alanine transaminase inhibitor (aminooxyacetic acid) into the left uterine artery completely blocked the conversion of the alanine into periuterine adipose tissue 14C-labeled lipids. The utilization of the infused substrate for fatty acids and glyceride-glycerol synthesis by the tissue was quantified by taking into account the infused radioactivity, the difference in the amount of 14C-labeled lipids appearing in periuterine adipose tissue on the left and the right sides, the arterial plasma concentration of the studied metabolite, and the uterine horn blood flow. In fed animals, the highest fatty acid synthesis was found with lactate, followed by glucose, alanine, and glycerol. This process was intensely decreased with all the substrates in 48-hr starved rats. In fed animals the synthesis of glyceride-glycerol was highest with glucose followed by lactate, alanine, and glycerol. In the starved rats it did not change with lactate and decreased with both alanine and glycerol. Besides validation of the technique for the study of periuterine adipose tissue metabolism in situ, results show the efficient utilization of lactate as lipogenic and of glucose as glycerogenic substrates. On the basis of the known capacity of adipose tissue to synthesize alanine from other amino acids, we propose that in the fed state this tissue may use different amino acids for lipogenesis. The negligible use of glycerol by periuterine adipose tissue in situ contrasts to its known utilization in vitro indicating that, whereas glycerokinase activity in the tissue may have a role in the re-utilization of glycerol released into the tissue, it does not contribute to the use of circulating glycerol.