5 years ago



I 2.60 o ? 2.0O 1.50

I 2.60 o ? 2.0O 1.50 1.00 0.50 0.00 1.60 0.00 468 GLYCEROLIPID SYNTHESIS IN BOVINE FATTY LIVER 81 TIME (WEEKS) 468 TIME (WEEKS) 4 6 TIME (WEEKS) free access restricted 10 12 FIGURE 3 Ante- and postpartum concentrations of he patic microsomal (A) and cytosolic CPCT (B) and DGAT (C) in cows with either restricted (n = 7) or free access to feed (n = 8 or 10) during the dry period. Values are means ±SEM. 0.5-12 wk postpartum) in the test group when com pared with the control group (Fig. 4B). Group mean postpartum serum activities of aspartate aminotransferase were higher (P = 0.114; 0.5-12 wk postpartum) in the test group given free access to feed (Fig. 4C). Before calving, serum total bilirubin concentrations were not different in the two groups. After calving, total bilirubin concentrations were higher in the test group than in the control group (P = 0.028; 0.5-12 wk postpartum) (Fig. 4D). Blood variables of lipid metabolism. Before parturition, neither plasma NEFA nor serum 3-hy- droxybutyrate concentrations differed between the groups. Plasma NEFA and serum 3-hydroxybutyrate concentrations rose after calving, rising more in the test group than in the control group (NEFA: P = 0.001; 0.5-5 wk postpartum and 3-hydroxybutyrate: P = 0.090; 0.5-5 wk postpartum) (Fig. 5A and B). Serum total and free cholesterol, triacylglycerols and phospholipids were not significantly different for the two groups before parturition; the values dropped signif icantly (P < 0.001) at 0.5 wk after calving and then rose steadily, the rise being slowest for the triacylgly cerols. Postpartum serum triacylglycerol concentra tions were significantly (P = 0.038; 0.5-12 wk post partum) lower in the test group (Fig. 6C). Postpartum concentrations of serum phospholipids (P = 0.228) (Fig. 6D), total (P = 0.583) (Fig. 6A) and free cholesterol (P = 0.640; 0.5-12 wk postpartum) (Fig. 6B) were sim ilar in the test and control groups. Plasma glucose and serum insulin concentra tions. When compared with the values at 0.5 wk be fore parturition, mean plasma glucose concentrations fell slightly after parturition, but there was no group difference. Average ante- and postpartum plasma glu cose concentrations were 3.8 mmol/L (SEM0.27) and 3.1 mmol/L (SEM0.07). When compared with values at 0.5 wk before parturition (1877.0 pmol/L, SEM 203.20; n = 17), serum insulin concentrations were significantly lower (P = 0.004) at 0.5 wk after calving (1257.1 pmol/L, SEM111.93; n = 15). Throughout the period after parturition, serum insulin concentrations were not significantly different between the two groups (P = 0.126), but overall group means were lower in the test (1136.5 pmol/L, SEM75.77) than in the re stricted control cows (1308.7 pmol/L, SEM 79.21; 1 pmol/L corresponds to 5.8072 X IO'6 IU/L). DISCUSSION As would be anticipated (Fronk et al. 1980, Johannsen et al. 1988, Reid et al. 1986), the cows given free instead of restricted access to feed during the dry pe riod developed one- to twofold higher postpartum he patic triacylglycerol concentrations. The degree of fatty liver in the overfed cows was of the same order of magnitude as that seen in practice in patients with fatty liver syndrome (Herdt et al. 1982, Herdt et al. 1983). Accumulation of hepatic triacylglycerols was associated with liver damage and/or depressed liver function as indicated by the rise in group mean serum activity of alkaline phosphatase, lactate dehydrogenase and aspartate aminotransf erase and the raised concen trations of serum bilirubin. In both the overfed and the control group, various cases of clinical disease oc curred. There were two cows with downer syndrome in the test group, but the relatively small number of cows studied does not allow conclusions as to differ- Downloaded from by guest on December 18, 2012

82 VAN DEN TOP ET AL. 3 3000 „ 2SOO 1 2000 ï 1500 e 1000 S 600P*rtua 120 partus I ..I 60 30 restricted.VH^ ^-Ã-€” ^ free access I I f - —" — 468 TIME (WEEKS) 4 6 TIME (WEEKS) free access restricted 10 12 ISO 160 140 120 100 80 60 40 20 O 15 10 partus partus 468 TIME (WEEKS) 10 12 4 6 TIME (WEEKS) free access restricted 10 free access restricted 10 12 FIGURE 4 Ante- and postpartum activities of serum láclate dehydrogenase (A), alkaline phosphatase (B), aspartate aminotransferase (C) and concentrations of total bilirubin (D) in cows with either restricted (n = 7) or free access to feed (n = 8 or 10) during the dry period. Values are means ±SEM.Significant diet effects: total bilirubin, P = 0.028 (0.5-12 wk post partum); lactate dehydrogenase, P = 0.029 (0.5-12 wk postpartum). enees in frequency and severity of the clinical symp toms between the two groups. Plasma NEFA and liver triacylglycerol concentra tions rose after parturition, showing more or less the same time course and both reaching significantly higher values in the test group than in the control group. This observation is in agreement with the pre cursor-product relationship between the two variables as discussed earlier (Van den Top et al. 1995, Veenhuizen et al. 1991). The higher postpartum plasma NEFA levels in the group given free access to feed dur ing the dry period are most likely caused by a greater rate of lipolysis. The greater body weight loss and the tendency toward lower group mean serum insulin concentrations during the first 4 wk postpartum in the overfed group are consistent with more lipolysis. Milk production was similar in the test and control groups, and in the test group daily dry matter intake during the first 0.5 wk after calving was only slightly lower. Nevertheless, it appears that the overfed cows had a greater, negative energy balance postpartum than had the restricted control cows. This view is supported by the observation that postpartum hepatic glycogen levels had fallen to lower values in the test vs. the restricted control group. Postpartum plasma glucose concentrations were not different between the two groups. Thus, the higher postpartum plasma NEFA concentration in the overfed cows may relate to a greater, negative energy balance and possibly also to greater adipose tissue stores releasing more fatty acids. Microsomal GPAT catalyzes a step in the first part of the pathway leading to triacylglycerol and phospholipid formation, and its activity could be expected to be raised during postpartum fatty liver develop ment, i.e., to be higher in the test than in the control group. However, we found the opposite. Microsomal and cytosolic PAP activities had increased after calv ing, but there was no difference between the overfed and control group. Hepatic DGAT activities also rose after parturition, but again there was no significant difference between the two groups. Thus, it is reason able to suggest that the high postpartum plasma level of NEFA in overfed cows and the resulting high amounts of NEFA taken up by the liver is the main determinant of triacylglycerol production, and that the activities of the measured enzymes involved in tria cylglycerol formation are controlled by substrate availability only. A high influx of fatty acids into the liver can by itself be responsible for accumulation of triacylglycerols, but it is possible that in the bovine liver this process is enhanced by the inability to rapidly upregulate VLDL secretion. Downloaded from by guest on December 18, 2012

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