5 years ago



2.00 1.60 1.00 0.50 0.00

2.00 1.60 1.00 0.50 0.00 2.50 2.00 • 1.80 1.00 0.50 0.00 GLYCEROLIPID SYNTHESIS IN BOVINE FATTY LIVER 83 468 TIME (WEEKS) 4 6 TIME (WEEKS) FIGURE 5 Ante- and postpartum concentrations of nonesterified fatty acids in plasma (A) and 3-hydroxybutyrate in serum (B) 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: nonesterified fatty acids, P = 0.001 (0.5-5 wk postpartum). In sheep with fatty liver induced by the adminis tration of phloridzin and epinephrine, the hepatic ac tivities of GPAT, PAP and DGAT were all raised (Herdt et al. 1988). In the overfed vs. control cows the three enzyme activities were not different. Thus, the drug-induced fatty liver in sheep may not be an ap propriate model for the overfeeding-induced, post partum fatty liver in dairy cows. After parturition, there were increases in the activities of hepatic GPAT, PAP and DGAT that were associated with accumu lation of hepatic triacylglycerols, the accumulation being much greater in the test than in the control group. The model of drug-induced fatty liver in sheep (Herdt et al. 1988) could resemble the changes in he patic lipid metabolism seen after parturition. NEFA act as feed-forward activators of hepatic DGAT in rats (Haagsman and van Golde 1981), but the higher post partum plasma NEFA concentrations in the test cows were not associated with higher hepatic DGAT activ ities. Possibly, rat and bovine hepatic DGAT are reg ulated differently. An interesting finding emerged from this study in that postpartum hepatic mitochondrial GPAT activi ties were significantly lower in the overfed cows than in their control counterparts. GPAT is present in the mitochondrial outer membrane, but mitochondrially produced monoacylglycerolphosphate can exit the organelles, be translocated to the microsomes and con verted to diacylglycerolphosphate (Haldar and Lipfert 1990). Because mitochondrial and microsomal GPAT had similar activities in bovine liver, the two enzymes could be equally important in diverting fatty acids from ^-oxidation to esterification or vice versa. Mi tochondrial GPAT prefers saturated acyl-CoA thioesters as substrate (Monroy et al. 1972). In overfed vs. control cows, postpartum mitochondrial GPAT was more depressed than was microsomal GPAT. This could relate to the adipose tissue fatty acids of cows being relatively saturated. Perhaps, lowering GPAT activities and thereby diverting a portion of the fatty acids from esterification to /3-oxidation is advanta geous because hepatic triacylglycerols otherwise will accumulate to excessive, cell-damaging levels. This notion is supported by the observation that at 0.5 wk after parturition, GPAT activities were similar in the control and test group but subsequently fell in the overfed group in which hepatic triacylglycerol con centrations were rising. The initial postpartum rise of GPAT activities in both the test and control group could serve to esterify the extra NEFA taken up by the liver. In contrast to the putative regulatory role of GPAT, the low GPAT activities in the overfed cows did not always coincide with high 3-hydroxybutyrate levels. The higher serum concentrations of 3-hy droxybutyrate in the test cows could point to prefer ential /3-oxidation but could also simply reflect a greater hepatic uptake of fatty acids. After calving, group mean concentrations of hepatic total phospholipids were slightly, but consistently, lower in the test than in the control group, possibly pointing to a lower production of phospholipids in the liver. Because phospholipids are essential, struc tural components of VLDL (Raphael et al. 1973), a low phospholipid production would impair the secre tion of triacylglycerols from the liver and contribute to accumulation of triacylglycerols in the liver. After calving, microsomal activities of CPCT rose and cytosolic activities fell, indicating for translocation and consequent activation of the enzyme (Pelech et al. 1983) and thus a raised production of phospholipids. Microsomal CPCT activities were not significantly dif ferent between the two dietary groups, which seems to be at variance with the observed group difference in liver total phospholipid concentrations. However, CPCT only produces phosphatidylcholine, the other phospholipids being synthesized through other path ways. Apart from phospholipids, cholesterol is another important component of VLDL (Raphael et al. 1973). It is likely that, after calving, hepatic cholesterol pro duction in the overfed group did not limit VLDL pro duction because postpartum hepatic cholesterol levels were similar to those in the control group. Downloaded from by guest on December 18, 2012

84 VAN DEN TOP ET AL. 250 200 í 100 partus free access restricted 24« TIME (WEEKS) FIGURE 6 Ante- and postpartum serum concentrations of total (A) and free cholesterol (B), triacylglycerols (C) and phospholipids (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: triacylglycerols, P = 0.038 (0.5-12 wk postpartum). The lower serum triacylglycerol concentrations seen after calving in the test group, in combination with the greater rise in hepatic triacylglycerol concentra tions, would be compatible with an impaired VLDL secretion from the liver. Although the commencement of VLDL catabolism by the mammary gland may be responsible for the low serum triacylglycerol concen trations after calving, this process was probably not involved in the group difference because milk pro duction was similar for both experimental groups. 2 3 4 8 « ••rumtotal cholesterol (mmol/L) free access restricted FIGURE 7 Relationship between individual serum total cholesterol and phospholipid concentrations in cows with either restricted or free access to feed during the dry period (y = 0.463X + 0.278; adjusted r2 = 0.964; P < 0.001). However, we have not determined milk fat so that a greater drain of serum triacylglycerols via the udder of the test cows cannot be excluded. The lower post partum hepatic phospholipid and serum triacylglycerol concentrations in the test group were associated with no group difference in serum concentrations of total and free cholesterol and of phospholipids. The periparturient time courses of serum total cholesterol and phospholipids were nearly identical, and there was a direct correlation (adjusted r2 = 0.964; P < 0.001) be tween serum total cholesterol and phospholipid con centrations (Fig. 7), which was similar to that reported earlier for goats (Van den Top et al. 1994). In conclusion, this study indicates that postpartum fatty liver development in cows with free access to feed during the dry period can be explained by an ex cessively high influx of fatty acids into the liver. Ad ditionally, VLDL secretion might not keep pace with triacylglycerol formation. The postpartum activities of the triacylglycerol synthesizing enzymes, GPAT, PAP and DGAT, may not be upregulated as a result of overfeeding during the dry period but may be con trolled by substrate availability only. Hepatic mitochondrial GPAT activity after parturition was de pressed in the overfed cows, the function possibly being to divert fatty acids from esterification to /3-oxidation so that further fatty liver development, and perhaps also the associated liver damage, are dimin ished. Downloaded from by guest on December 18, 2012

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