HOUSER, Dorian S.; CHAMPAGNE, Cory; CROCKER, Daniel E.; Sonoma State University; Sonoma State University; Sonoma State University: Glycerol gluconeogenesis in simultaneously fasting and lactating northern elephant seals

Female elephant seals (Mirounga angustirostris) fast while lactating creating a metabolic conflict between energy-demanding and energy-conserving physiological mechanisms. During the ~4 week lactation period, ~90% of energy needs are met through fat catabolism. It has been hypothesized that glycerol is the primary gluconeogenic substrate utilized by fasting elephant seals to meet the energetic needs of glucose dependent tissues. To test this hypothesis, lactating females in the fourth week of lactation were administered a 250 uCi prime dose of ¹⁴C-2-glycerol and maintained on a constant infusion of ~1.1 uCi/min for 3 hrs. Blood samples were collected every 5 min for the first 30 min following the prime dose and then every 15 min for the remainder of the sample period. Specific activities of ¹⁴C-glycerol and ¹⁴C-glucose in sequentially collected plasma samples were determined to estimate the contribution of glycerol to glucose production. The mean rate of appearance (Ra) of glycerol was 8.5 � 5.8 umol/kg/min. Less than 5% of glucose production was derived from glycerol late in lactation but there was a significant inverse relationship between female mass and the contribution of glycerol to glucose production (R² = 0.56, p = 0.02). Conversely, the contribution of glycerol to glucose production was directly related to the Ra of glycerol (R² = 0.70, p < 0.01). These results suggest glycerol is not the primary gluconeogenic precursor during the late fasting period of lactating elephant seals. Diminishing nutrient reserves are related to a greater Ra of glycerol and the resultant increase in the contribution of glycerol to gluconeogenesis may accommodate an increasing premium on protein stores and need to reduce amino acid contribution to glucose production.