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This study has shown that porcine serum HDL-C and LDL-C concentrations, as determined by the precipitation method and the modified Friedewald formu la respectively, agree well with concentrations determined by sequential ultracentrifugation. With the exception of the fasting HDL-C concentration, the agreement was better when the ultracentrifuged concentrations were corrected for cholesterol recovery. Conversely, for both the fasting and postprandial samples, there was poor agreement between the VLD L trig and the VLDL fuge cholesterol concentrations. This occurred despite the fact that the Friedewald formula was modified to reflect the higher triglyceride: cholesterol ratio in the VLDL of pigs compared to human beings. While only small amounts of VLDL (30-80 mg/dL) are to be expected in pig serum (Knipping et aI, 1 975) any loss of VLDL d uring tube-slicing would lower the concentration of VLDLfuge even further. Repeating fractionation with larger samples, which should allow for better recovery, might reveal the significance of such losses. However, since the quantity of cholesterol contained within the VLDL fraction is <5% of the total cholesterol concentration, any error inherent in either of the methods used to

determine the VLDL-C concentration had l ittle bearing on the agreement betwee n LDLfriede and LDLfuge.

Since neither of the methods for VLDL-C determination used in this study takes into account postprandial chylomicronaemia, the VLDL-C concentration after feeding is effectively the cholesterol concentration i n both the VLDL and chylomicron fractions. The mean VLDL fuge doubled but the VLDL trig remained unchanged after feeding. Luhman et a/ (1 992) found that the cholesterol concentration of a VLDUchylomicron fraction in pigs tended to be greater four hours after feeding tha n after a 1 2-hour fast, which is consistent with the observation of a significant i ncrease in the VLD Lfuge following feeding in the current study. Conversely. L uhman et a/ ( 1 992) found that compared to fasting sam ples, the plasma triglyceride concentration was significantly lower four hours after feeding but there was no difference in the plasma triglyceride concentration one hour after feeding. Given that in the current study, the VLD L trig _{is determined from the serum triglyceride concentration, the results of} Luhman et a/ ( 1 992) may explain why the VLDL trig was not affected by feeding. Consequently, the use of the serum triglyceride concentration to determine VLD Llch ylomicron cholesterol concentrations should be avoided on

postprandial samples.

The raw and corrected sequ ential u ltracentrifugation data have been presented because both sets of values are inherently subject to error. The quantity -of cholesterol recovered from the various lipoprotei n fractions fol lowing sequential u ltracentrifugation was approximately 1 5% less than the q uantity of cholesterol determined by assay of the original sample. This 'loss' of cholesterol is recogn ised as a shortcoming of lipoprotein p reparation following extensive u ltracentrifugation (Janado et ai, 1 966; Cohn et aI, 1 988) and consequently, the raw data may underestimate the actual cholesterol concentration in each of the lipoprotein fractions. By correcting for recovery however, it is assumed that the cholesterol loss following sequential ultracentrifugation is the same in each lipoprotein fraction. This may not be the case and for some classes of

lipoprotein, correction may have led to overestimation of the actual cholesterol concentration.

The density range of porcine LDL2 has been shown to overlap the lower density l im it of porcine H DL (Janado et ai, 1 966). This has resu lted i n the employment of differential flotation to separate LDL2 from H D L (Janado et aI, 1 966). In the current study, contamination of the LDL layer with HDL (and vice versa) was not detected by electrophoresis. The second layer that was harvested fol lowing ultracentrifugation at a density of 1 .063 kg/L m ight have contained the LDL2 subfraction, thereby preventing H D L contamination.

Feeding significantly reduced the serum total cholesterol and LDL-C concentrations but increased the HDL-C concentration. This is i n contrast to the findings of Luhman et a/ ( 1 992) who found that feeding had no effect on these parameters i n pigs. In h uman bei ngs, the postprandial H DL cho lesteryl ester concentration appears to be inversely related to the postprandial rise in plasma triglyceride concentrations (Tall, 1 986). Despite an increase in LCAT activity within H DL postprandially, the presence of increased levels of substrate for C ETP resu lts in a net transfer of cholesteryl ester from H D L to triglyceride­ rich l ipoproteins (Rose & Juliano, 1 979). I n contrast to human beings, pigs lack C ETP activity (Chapman, 1 986; Knipping et aI, 1 987), which may explain why an increase in the postprandial H D L-C concentration was observed. Cohn et a/ ( 1 988) fou nd that LDL-C concentrations i n h uman beings were significantly lower 3 and 6 hours after eating a fat-rich m eal. This was associated with a decrease in the plasma concentration of apolipoprotein B and the a uthors hypothesised that the postprandial rate of catabolism of LDL exceeded its synthesis. Although apolipoprotein concentrations were not measured, a s i mi lar mechanism may explain the decrease in LDL-C concentrations postprandially in the pigs i n this study.

I n summary, this study has demonstrated that a precipitation method for determining the HDL-C concentration, and a modification of the Friedewald

formu la for estimating the LDL-C concentration, closely approximate values obtained from sequential u ltracentrifug ation i n porcine serum samples. This methodo logy was applied in the next study in order to evaluate the effect of dietary lipids on porcine LDL-C and H DL-C concentrations. Conversely, the VLDL-C concentration, determined by m ultiplying the triglyceride concentration by 0.25, did not agree with concentrations determin ed by sequential ultracentrifugation. I n order to address this issue, the same study s hould be repeated on large-volume or pooled blood samples. This study has also demonstrated that like human beings, porcine LDL-C concentrations decrease postprandially, but unlike h u man beings H DL-C concentrations increase postprandially. This latter phenomenon may be due to the lack of CETP activity in porcine blood.