Volatile fatty acid metabolism in sheep. 3. Diurnal variation in the contribution of ruminal propionic acid production to the whole body glucose turnover of Merino sheep fed lucerne hay twice daily

Abstract

Three phases of the diurnal feeding cycle, namely, a pre-feeding phase (-2-0 h), a maximum fermentation (2-4 h) phase and a basal fermentation (8-10 h) phase, were chosen for investigation into the contribution of ruminal propionic acid production to the whole body glucose turnover, since near steady-state conditions in the ruminal volatile fatty acid (VFA) pool were obtained during these periods. Total VFA levels of 113,5-102,2; 156,1-151 , 1 and 90,8-89,8 meq /l respectively, were found during these 3 phases and parallel changes also occurred in the propionic acid pool (18,7- 17,2; 31,5-29,5 and 16,1-15 ,6 meq/ l). The acetic/propionic and propionic/butyric acid ratios revealed that propionic acid increased more than the other acids during the 2- 4 h segment. The diurnal cycle of the total VFA production rate followed the overall concentration pattern with the exception of the maximum fermentation segment (5, 67- 6,14 moles/12 h), in which an 89-105% increase over basal levels (3,00 moles/12 h) was significantly higher than the corresponding increase of 74-68% in total VFA concentration. The percentage production rate of propionic acid was statistical indistinguishable from its molar percentage concentration except for the pre-feeding segment in which the production rate (13, 6± 1,0 %) was lower than the concentration (16,5 ± 0, 9 molar %) at -2 h. The glucose entry rate remained almost constant over the course of the feeding cycle and averaged 4,07 mg/min/kg⁰’⁷⁵ over the 3 periods. However, there was a significant difference in glucose pool (392,5 ± 84,4-257,6 ± 64,6 mg/kg⁰’⁷⁵) and rate constant (1,05 ± 0,21- 1,71 ± 0,15% of pool/min) between the 1st and 2nd phases, respectively. A constant fraction of the propionic acid production in the rumen (34 ± 6 %) served as precursor for gluconeogenesis. The equation describing this linear relationship between the total (x) and that fraction (y) of the propionic acid production converted to glucose was found to be y=0,314 x + 0,02 when both x and y were expressed in moles/12 h. A direct relationship between plasma insulin levels and the respective glucose fractional entry rate (% of pool/min) was found, supporting the hypothesis that insulin acts through control of the rate of peripheral uptake.