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The Metabolism of Volatile Fatty Acids by Portal-Drained Viscera and Liver of Goats Fed Diets with Different Forage to Concentrate Ratio.

Byline: Zhuang Su, Zhang Teng, Chang Guang-Jun, Yan Rui, Muhammad Naeem, Nasir Rajput and Shen Xiang-Zhen

Abstract.- Six non-lactating Chinese goats (average weight 402 kg) fed low forage (LF) or high forage (HF) diets were used in a randomized crossover trial to determine the net flux of VFA across portal-drained viscera and liver for 8 h from initial feeding. Chronic catheters were established in mesenteric, portal, and hepatic veins, and femoral artery to measure the plasma flow by using p-aminogippurric acid. The concentrations of acetate, propionate, butyrate, isobutyrate, isovalerate and TVFA were higher (P less than 0.05) and a ratio of acetate to propionate lower (P less than 0.05) in the portal, hepatic vein and artery in the LF diet compared with HF diets, but not for butyrate in portal vein and artery. The plasma flux of acetate, propionate, butyrate and TVFA in the portal, acetate and TVFA in artery were not affected by diets. The hepatic VFAs fluxes were higher (P less than 0.05) in LF than HF diet. Net acetate was produced and net propionate, butyrate, isobutyrate and isovalerate were absorbed by the liver in LF or HF diet. Net gut plus hepatic output of acetate, propionate and TVFA were greater (P less than 0.05) in LF than HF diet. VFAs concentrations reached at peak after feeding and then decreased in the portal, hepatic vein and artery in both diets. Net portal appearance and net gut plus hepatic output of VFAs reached peak at 2 to 6 h post-feeding and the maximum hepatic absorption of propionate, butyrate and output of acetate were observed at 2 to 6 h post-feeding. The results suggested that the plasma concentration and net fluxes of VFAs were increased in LF diet.

Key words: Chinese goats, forage to concentrate ratio, liver, volatile fatty acids, metabolism.

INTRODUCTION

Ruminants depend on volatile fatty acids (VFAs) for up to 80% of their maintenance energy requirements (Tagang et al., 2011). Ruminal absorption of VFAs is quantitatively the most important nutrient flux in ruminants (Strom et al.,2012). In addition to their involvement as the major source of energy, the VFAs also serve as building blCks for milk synthesis; acetate is an essential component in the formation of milk fat, while propionate is used for glucose production, which is needed for synthesis of lactose (Tagang et al.,2011). VFAs, produced by microbial fermentation of organic matter in rumen, are absorbed mainly through the rumen wall into the portal blood and are an important source of energy for ruminants (Masson and Phillipson, 1951). The relative proportion and quantity of VFAs produced in rumen are influenced by a number of factors, including substrate composition, availability and rate ofdepolymerization and presence of microbial species (Cantalapiedra-Hijar et al., 2009; Liu et al., 2012; Wu et al., 2011; Yang et al., 2009).VFAs represented 69% of net energyabsorption by portal-drained viscera (PDV) (Baird et al., 1975) and acetate and propionate are the predominant VFAs absorbed; each represents 30% of net PDV energy absorption in lactating Holstein cows (Reynolds and Huntington, 1988). Although the tissues of the gastrointestinal tract (GIT) and liver represent only 6 to 10% of body weight (Burrin et al., 1991), liver is an important organ in VFA metabolism. VFAs derived from the gut are metabolized and removed by the liver. About 93% of propionate absorbed by PDV is removed by hepatic tissues in the lactating dairy cows (Lomax and Baird, 1983), and acetate that is an important precursor for lipid synthesis is metabolized in the liver. Previous studies have reported the net metabolism of VFAs by PDV and liver (Reynolds and Huntington, 1988; Reynolds et al., 1988a), and the prediction of net portal appearance of VFAs and glucose in ruminants (Loncke et al., 2009). However, in recent years few researches has been conducted on the influence of the diet containingdifferent proportion of forage and concentrate and its effects on the metabolism of VFAs in splanchnic tissue in goat. The objective of this study was to evaluate the effects of forage to concentrate ratio, on net metabolism of VFAs by PDV and liver in nonlactating goats.

MATERIALS AND METHODS

Animals and dietSix nonlactating and nonpregnant three years old Guanzhong (Chinese breed) dairy goats (average body weight 402 kg) were used in a randomized crossover trial designed to determine the effects of different proportion of forage and concentrate, on the metabolism of VFA by PDV and liver. Animals were housed in individual metabolic pens in an environmentally controlled intensive housing system. They were fed low (40% hay and 60% concentrate, LF) or high (60% hay and 40% concentrate, HF) forage diets (Table I), to meet the nutrients requirements of Chinese dairy goat. The equal amount of diet (0.39 kg DM/goat) was fed twice daily (at 08:00 am morning and 08:00 pm evening), and water was available ad libitum. After15 days of adaptation period, the blood sampleswere collected on three consecutive days.Catheterization and blood collectionCatheters were installed surgically in themesenteric, portal, and hepatic veins and a femoral artery to measure blood flow and net flow across the PDV, liver and splanchnic tissues (PDV + liver). After surgery, the goats were allowed for two weeks to recover before returning to normal feed consumption. The Institutional Animal Care and Use Committee of Nanjing Agricultural University (Nanjing, Peoples Republic of China) approved all of the prCedures (surgical prCedures and care of goats).The goats were infused with a sterile aqueous solution (pH 7.4) of para- aminohippurric acid (PAH, 1%, wt/vol, Alfa Aesar, CAS 94-16-6, from Alfa Aesar China (Tianjin) Co., Ltd) into mesenteric vein by using the syringe pump((SN-50F6, Sino Medical-Device Technology Co., Ltd., Shenzhen, China). The initial rate was at 3 ml/min for 5 min followed by a regular rate of 0.8 ml/min for at least30 min before collecting blood and the same rate was used until the end of sampling. The blood samples were obtained simultaneously from the portal, hepatic and arterial catheters at 0 h (before feeding), and at 2, 4, 6 and 8 h (after feeding). After blood collection, immediately all the samples were transferred from the syringe to heparinized (3.6 U heparin/ml blood) tubes placed on ice and transported to the laboratory. The blood samples were centrifuged at 1,469A-g for 20 min at 4C to separate the plasma. The PAH was immediately analyzed and the remaining plasma was stored at -20C for analyses of VFAs.

Table I.- Chemical composition and nutrient level of diets.

Ingredient###% of diet (air dry mater)

###Low forage###High forage

###(LF)###(HF)

Chinese wildrye hay###32.00###48.00

Alfalfa hay###8.00###12.00

Corn###43.17###28.78

Soybean meal###12.68###8.45

Limestone###1.15###0.77

Calcium phosphate dibasic###1.65###1.10

Salt###0.60###0.40

Premix a###0.75###0.50

Nutrient level

Dry mater, %###88.60###88.90

Net energy, MJ/kg###5.89###5.40

CP, %###13.45###12.24

NDF, %###27.69###36.55

ADF, %###17.54###24.04

Laboratory analysesThe plasma samples were deproteinized by anaddition of 0.5 mol/l trichloroacetic acid, then centrifuged at 2,296A-g for 20 min at 4C. The supernatant was analyzed for PAH concentration according to the prCedures described by Katz and Bergman (1969a). A portion of plasma was deproteinized with 5% 5-Sulfosalicylic acid dehydrate and kept at -20C for 12 h, then centrifuged at 20,664A-g for 30 min and the supernatant aliquots were analyzed for VFAs.Plasma acetate, propionate, butyrate, isobutyrate and isovalerate concentrations were determined by using gas chromatography (Agilent Technologies 7890A, GC system) with flam ionization detector, based on the method of (Bjorkman and Forslund, 1986; Reynold et al., 1986) with some modifications. Briefly, the gas chromatography was performed using a 30 mA-0.25 mm i.d. fused silica capillary column (Catalog No: 24107, Supelco) with a 0.25m film. Nitrogen was used as the carrier gas andhydrogen as a fuel gas at the flow rate of 30 ml/min, respectively. The pressure was 260 kPa and air was a combustion-supporting gas. The temperature of the column, the flame ionization detector and the injector were 125C, 210C and 220C, respectively. The split ratio was 40:1 and 1 l was injected. Portal and hepatic plasma flow were calculated from the plasma p-aminohippureate concentrations as described by Katz and Bergman (1969b). The portal appearance and net hepatic release of blood metabolites were calculated as reported by Wieghart et al. (1986).

Statistical analysisAll statistical computations were conducted in MIXED prCedure of SAS (version 9.0; SAS Institute Inc., Cary, NC). Goats were used as the random factor, diet and time were used as main effect factor, and dietATime was used as interaction. Means were compared by least squares. Treatment means within diets at different time were compared using a Tukeys adjustment multiple range test. Significance was declared at P less than 0.05.

RESULTS

The volatile fatty acid (VFA) concentrationThe plasma VFA concentrations of portal, hepatic veins and femoral artery are given in the Table II. The concentrations of acetate, propionate, butyrate, isobutyrate, isovalerate and total volatile fatty acids (TVFA) were markedly higher (P less than 0.05) (except butyrate concentration for portal vein and artery), while the ratio of acetate to propionate lower (P less than 0.01) in the portal, hepatic vein and artery in goats fed LF diets as compared with HF diets. Acetate, propionate, butyrate, isobutyrate, isovalerate and TVFA concentration were increased by 11.9%, 40.1%, 19.5%, 83.3%, 33.3% and 17.0% in portal, 9.5%, 101.4%, 45.3%, 328.6%, 117.6%, and 14.6% in hepatic vein, and 12.0%, 90.6%,16.4%, 187.5%, 55.6%, and 15.8% in artery, respectively, in LF diets than that of HF diets. The interaction between diet and time was not observed.

The VFA fluxThere were no differences (P greater than 0.05) in portal and hepatic plasma flow between diets (Table III). Although the portal vein fluxes of isobutyrate and isovalerate were significantly (P less than 0.05) higher in the goats fed with LF diet, while the acetate, propionate, butyrate and TVFA were not statistical different (P greater than 0.05) between both diets, but the high values were obtained in goats fed with LF diets. Results revealed highly significant increase (P less than 0.05) in hepatic and arterial fluxes of acetate, propionate, butyrate, isobutyrate, isovalerate and TVFA in LF diets compared with HF diets, except acetate and TVFA in arterial flux. There was no interaction of diet and time (P greater than 0.05) between both diets.

The venous-arterial concentration differences ofVFAsThe results for venous-arterial concentration differences of VFAs are summarized in Table IV. The portal-arterial (P-A) concentration, the hepatic- portal (H-P) concentration and the hepatic-arterial (H-A) concentration differences of acetate, butyrate and isobutyrate were not significantly different (P greater than 0.05) between diets, while P-A, H-P and H-A concentrations of propionate, isovalerate and TVFA were higher (P less than 0.05) in goats fed LF diets than HF diets, except TVFA in H-P. There was no interaction between diet and time (P greater than 0.05) in both diets.

The net flux of VFA across splanchnic tissuesNet portal appearances and net hepatic releaseof VFAs were not significantly different (P greater than 0.05) between the diets (Table V). On a net basis, hepatic tissues produced acetate and removed other VFAs for both LF and HF diets. Net hepatic removal accounted for 86.3%, 52.4%, 59.6%, 55.8%, 18.8% and 92.4%, 83.8%, 106.1%, 117.8%, 19.3% of net PDV appearance of propionate, butyrate, isobutyrate, isovalerate and TVFA for LF and HF

Table II.-###Mean portal, hepatic and arterial VFA concentrations in goats fed LF and HF diets.

###P-value

###LF###HF###SEM

###Diet###Time###DAT

Portal concentration (mmol/L)

Acetate###3.09###2.76###0.32###0.04###less than 0.001###0.66

Propionate###0.69###0.49###0.10###less than 0.001###less than 0.001###0.56

Butyrate###0.19###0.16###0.02###0.22###less than 0.001###0.28

Isobutyrate###0.05###0.03###0.01###0.01###0.19###0.27

Isovalerate###0.06###0.04###0.00###0.00###0.01###0.06

TVFA###4.07###3.48###0.43###0.00###less than 0.001###0.59

A:P###4.51###5.98###0.22###0.01###less than 0.001###0.57

Hepatic concentration (mmol/L)

Acetate###2.70###2.46###0.27###0.03###less than 0.001###0.60

Propionate###0.15###0.07###0.01###less than 0.001###0.04###0.45

Butyrate###0.11###0.07###0.08###less than 0.001###0.06###0.54

Isobutyrate###0.03###0.01###0.01###0.01###0.56###0.18

Isovalerate###0.04###0.02###0.00###less than 0.001###0.59###0.16

TVFA###3.03###2.64###0.27###0.00###less than 0.001###0.49

A:P###20.41###33.59###1.12###less than 0.001###0.18###0.99

Arterial concentration (mmol/L)

Acetate###1.78###1.59###0.11###0.00###0.00###0.39

Propionate###0.10###0.05###0.01###0.01###0.14###0.44

Butyrate###0.08###0.07###0.01###0.22###0.31###0.54

Isobutyrate###0.02###0.01###0.00###less than 0.001###0.31###0.54

Isovalerate###0.03###0.02###0.00###0.01###0.40###0.05

TVFA###2.01###1.72###0.11###0.00###0.00###0.49

A:P###20.55###30.17###0.85###less than 0.001###0.00###0.12

diets, respectively. Net gut plus hepatic output of acetate, propionate and TVFA were higher (P less than 0.05) in LF diets than in HF diets. The negative values of the net gut plus hepatic output of isobutyrate (-0.001 mmol/h/kg of LW) indicated that there was a little net removed by total viscera in goats fed HF diets.

The concentrations of VFA across splanchnic tissues in relation to feeding timeThe concentration of acetate was significantly affected (P less than 0.05) by feeding time in portal, hepatic and arterial in goats fed LF or HF diets, except arterial acetate concentration in LF diets, the peak of acetate was recorded at 2 h post feeding, and then declined gradually to the pre-feeding levels. The portal propionate concentration was significantly (P less than 0.05) effected for both diets, highest was obtained after 2 h post feeding and then progressively decreased in time dependent manner, while hepatic and arterial propionate concentrations were also non significantly increased after feeding, then decreased gradually as the time progressed towards the initial feeding. The portal concentration of butyrate reached a plateau at 2 h (P less than 0.05) for LF diets (Table VI). However it was not significant in case of hepatic and arterial for both diets.

The net flux of VFAs across splanchnic tissues in relation to feeding timeNet portal appearance for acetate was athighest levels at 4 h (P less than 0.05) post feeding in goats fed LF diet and at 2 h (P greater than 0.05) in HF diet, while for propionate it was higher (P less than 0.05) at 6 h for LF diet and (P greater than 0.05) at 2 h post feeding for HF diet. The net portal appearance for butyrate was at peak after 4 h (P less than 0.05) in LF diet and at 2 h in HF diet, respectively. There was a net hepatic absorption for propionate and butyrate, and a net hepatic output for

Table III.-###Mean portal, hepatic and arterial VFA flux in goats fed LF and HF diets.

###P-value

###LF###HF###SEM

###Diet###Time###DAT

Plasma flow (l/h)

Portal###57.62###59.17###2.28###0.38###0.01###0.18

Hepatic###93.87###92.18###2.56###0.94###less than 0.001###0.61

Portal flux (mmol/h/kg of LW)

Acetate###4.57###4.16###0.39###0.16###less than 0.001###0.22

Propionate###0.99###0.74###0.17###0.11###less than 0.001###0.04

Butyrate###0.28###0.24###0.03###0.33###less than 0.001###0.16

Isobutyrate###0.08###0.04###0.10###0.03###0.03###0.08

Isovalerate###0.08###0.06###0.01###0.00###0.00###0.00

TVFA###6.01###5.25###0.57###0.11###less than 0.001###0.11

Hepatic flux (mmol/h/kg of LW)

Acetate###6.53###5.81###0.60###0.05###less than 0.001###0.53

Propionate###0.36###0.17###0.04###0.00###0.01###0.36

Butyrate###0.27###0.18###0.03###0.00###0.01###0.49

Isobutyrate###0.07###0.02###0.02###0.03###0.37###0.09

Isovalerate###0.09###0.04###0.10###0.00###0.16###0.09

TVFA###7.32###6.22###0.62###0.03###less than 0.001###0.42

Arterial flux (mmol/h/kg of LW)

Acetate###1.67###1.36###0.15###0.11###less than 0.001###0.70

Propionate###0.09###0.05###0.02###0.03###0.02###0.49

Butyrate###0.07###0.06###0.01###0.01###0.01###0.22

Isobutyrate###0.02###0.01###0.00###less than 0.001###0.10###0.06

Isovalerate###0.03###0.02###0.00###0.00###0.02###0.01

TVFA###1.89###1.48###0.16###0.09###less than 0.001###0.71

acetate during pre and post feeding time in both diets. The maximum propionate absorption was 1.08 mmol/h/kg of LW at 6 h (P less than 0.05) post feeding in LF diet and 0.94 mmol/h/kg of LW at 2 h (P less than 0.05) post feeding in HF diet, respectively. Net gut plus hepatic output of acetate increased after feeding and reached to the highest level at 4 h (P less than 0.05) in LF and at 2 h (P greater than 0.05) in HF diet. The maximum total splanchnic output was at 2 h post feeding in both diets for propionate, at 4 h in LF diet and at 2 h in HF diet for butyrate, respectively (Table VII).

DISCUSSION

Volatile fatty acids, mainly acetate, propionate and butyrate, are mainly absorbed through the rumen wall into the portal blood (Masson and Phillipson, 1951). The type of digested carbohydrate is a decisive factor in determining the ratio and quantity of the resultant rumen VFAs (Cantalapiedra-Hijar et al., 2009; Huntington et al.,2006). The net production of total VFAs were increased in cows fed low roughage diets (Liu et al.,2012; Sutton et al., 2003; Wu et al., 2011).Similarly, Annison et al. (1974) found the increased portal concentration and molar proportion of propionic acid in the rumen of adult dairy cows fed high concentrate diet. The similar results were obtained in non-lactating cows (Wiltrout and Satter,1972). The results of the present study showed that VFA concentrations were higher in goats fed LF diets than HF diets, furthermore, acetate, propionate, and butyrate concentration in portal plasma accounted for 75.8%, 16.8% and 4.7% of total VFA in LF diet, and 79.3%, 14.1% and 4.6% of total VFA in HF diet, respectively. The ratio of acetate to

Table IV.-###The venous-arterial concentration differences of VFA in goats fed LF and HF diets.

###P-value

###LF###HF###SEM

###Diet###Time###DAT

Portal-arterial difference (P-A) (mmol/L)

Acetate###1.30###1.17###0.21###0.20###0.00###0.49

Propionate###0.58###0.44###0.10###0.01###less than 0.001###0.67

Butyrate###0.11###0.09###0.02###0.46###less than 0.001###0.39

Isobutyrate###0.03###0.02###0.01###0.13###0.13###0.22

Isovalerate###0.03###0.02###0.00###0.03###0.05###0.00

TVFA###2.06###1.74###0.33###0.01###less than 0.001###0.49

Hepatic-portal difference (H-P) (mmol/L)

Acetate###-0.39###-0.30###0.08###0.44###0.01###0.82

Propionate###-0.54###-0.42###0.10###0.02###less than 0.001###0.68

Butyrate###-0.08###-0.08###0.02###0.82###0.08###0.95

Isobutyrate###-0.03###-0.02###0.00###0.06###0.03###0.38

Isovalerate###-0.02###-0.03###0.00###0.01###0.64###0.62

TVFA###-1.05###-0.84###0.17###0.19###less than 0.001###0.95

Hepatic-arterial difference (H-A) (mmol/L)

Acetate###0.92###0.87###0.17###0.48###0.01###0.42

Propionate###0.05###0.02###0.00###less than 0.001###0.00###0.39

Butyrate###0.03###0.01###0.01###0.05###0.25###0.68

Isobutyrate###0.01###0.00###0.00###0.22###0.61###0.08

Isovalerate###0.01###-0.00###0.00###0.01###0.49###0.51

TVFA###1.01###0.90###0.17###0.02###0.01###0.38

propionate was lower (P less than 0.05) and TVFA higher (P less than 0.05) in portal, hepatic vein and artery in LF than in HF diet. This might be due to the fact that low forage diet provided more net energy (+9.07%) and less NDF (-24.2%) than did high forage diets for animals, and which increased more digestible matter to produce more VFAs in rumen and finally increased portal VFAs quantity although goats fed the same feed intake (0.88 kg/d). Results of the present study were in accordance to the previous studies reported by various researchers (Lomax and Baird, 1983; Nasrullah et al., 2013; Reynolds et al.,1988b).There were no significant difference for mean portal and hepatic plasma flows between both diets (Table III), which was in agreement with previous results (Burrin et al., 1991). The positive values of the net hepatic release of acetate indicated there was a net output of acetate in the present study in both diets, consistent with previous in vivo measurements dairy cows (Snoswell et al., 1978; Lomax and Baird, 1983). The liver takes up the majority of VFAs absorbed into the portal vein. However, acetate was simultaneously produced and utilized by all tissues (Bergman and Wolff, 1971; Pethick et al.,1981) while, the endogenous acetate was produced mainly by liver.Propionate and n-butyrate are almost completely removed from portal blood by the hepatic tissue as found in studies on sheep(Bergman and Wolff, 1971) and dairy cattle (Lomaxand Baird, 1983). Within the liver, propionate serves as a major substrate for gluconeogenesis, which is absolutely critical to the ruminant because almost no glucose reaches the small intestine for absorption.Propionate is not only an important glucose precursor in ruminant hepatic tissue (DanfAr et al.,1995), but also may enter the tricarboxylic acid cycle, be oxidized, or be incorporated into amino acids, and conversion into glucose accounts for 95%of propionate metabolism in sheep liver (Reynoldset al., 1988a). In the present study, the plasma propionate concentration was high, which indicated that more propionate was absorbed by PDV tissues

Table V.-###Net flux of VFA across splanchnic tissues of goats fed LF and HF diets

###P-value

###LF###HF###SEM

###Diet###Time###DAT

Net portal appearance (mmol/h/kg of LW)

Acetate###1.95###1.74###0.23###0.26###0.00###0.47

Propionate###0.85###0.66###0.16###0.21###less than 0.001###0.10

Butyrate###0.16###0.14###0.03###0.60###less than 0.001###0.15

Isobutyrate###0.05###0.03###0.01###0.23###0.04###0.04

Isovalerate###0.04###0.03###0.00###0.53###0.86###0.76

TVFA###3.05###2.62###0.41###0.19###less than 0.001###0.46

Net hepatic release (mmol/h/kg of LW)

Acetate###0.29###0.29###0.11###0.95###0.51###0.53

Propionate###-0.73###-0.61###0.15###0.38###less than 0.001###0.04

Butyrate###-0.09###-0.12###0.02###0.06###0.45###0.90

Isobutyrate###-0.03###-0.04###0.07###0.45###0.08###0.45

Isovalerate###-0.02###-0.03###0.02###0.14###0.49###0.54

TVFA###-0.57###-0.51###0.12###0.68###0.49###0.31

Net gut plus hepatic output (mmol/h/kg of LW)

Acetate###2.28###2.04###0.36###0.02###less than 0.001###0.58

Propionate###0.12###0.05###0.01###less than 0.001###0.04###0.56

Butyrate###0.08###0.00###0.00###0.09###0.22###0.63

Isobutyrate###0.02###0.00###0.01###0.24###0.46###0.05

Isovalerate###0.02###0.00###0.00###0.28###0.58###0.46

TVFA###2.47###2.11###0.38###0.01###less than 0.001###0.47

in goats fed LF diets, it might be due to the increase in net splanchnic flux of propionate with increased propionate absorption to the portal vein (Berthelot et al., 2002; Majdoub et al., 2003). Net hepatic releases of propionate showed a negative value in both diets, which suggested that much propionate absorbed from gastrointestinal tract was removed. In the present study, the liver received 86.3% and92.4% of the net PDV appearance of propionate in goats fed LF diets and HF diets, respectively. These results are in accordance with previous in vive study in dairy cows (Kristensent, 2005). The propionate absorbed by liver would be used to synthesize glucose in liver tissues (Table V) so that net hepatic releases of glucose were positive (35.50 vs 30.49 mmol/h for LF diets and HF diets, respectively, unpublished), while the net portal appearances of glucose were negative (-16.18 vs -8.99 mmol/h, unpublished) for LF diets vs HF diets, respectively. Besides propionate, there was a net hepatic uptake of butyrate, isobutyrate and isovalerate in both diets, findings similar to those observed by others researchers (Lomax and Baird, 1983; Reynolds et al., 1988b; DanfAr, 1994). In ruminant tissues, particularly in liver, much butyrate is removed to AY- hydroxybutyrate (BOHB) (Katz and Bergman,1969b), which was confirmed by our other experiment (unpublished) which showed that net hepatic release of BOHB was higher compared with net portal appearance (9.99 vs 5.05, and 14.18 vs5.51 mmol/h, for LF diets and HF diets, respectively).The net absorption of VFAs increased aftermorning feeding (Huntington and Reynolds, 1983). There was a positive relationship between metabolizable energy intake and the rates of net portal appearance of VFAs, hydroxybutyrate and lactate in fed and fasted cows and sheep. Fasting caused a rapid decrease but re-feeding rapid increases in the blood concentration of the VFA (Lomax and Baird, 1983). In present study, the portal concentration and net splanchnic flux of

Table VI.-###VFA concentrations in goats fed different diets in relation to feeding time.

###Post feeding time (h)

###0###2###4###6###8###SEM

Acetate concentration (mmol/L)

###Portal###LF###2.39b###3.96a###3.59ab###3.03ab###2.46b###0.39

###HF###1.99b###3.48a###3.00ab###2.80ab###2.56ab###0.39

###Hepatic###LF###2.20b###3.52a###2.99ab###2.71ab###2.08b###0.33

###HF###1.88b###3.05a###2.65ab###2.49ab###2.25ab###0.33

###Arterial###LF###1.76###2.19###1.73###1.76###1.48###0.17

###HF###1.25b###2.04a###1.71ab###1.47ab###1.49ab###0.17

Propionate concentration (mmol/L)

###Portal###LF###0.39c###0.94a###0.72abc###0.83ab###0.55bc###0.12

###HF###0.27b###0.66a###0.54ab###0.53ab###0.45ab###0.12

###Hepatic###LF###0.12###0.20###0.18###0.16###0.09###0.02

###HF###0.06###0.09###0.08###0.08###0.06###0.02

###Arterial###LF###0.09###0.11###0.14###0.11###0.06###0.02

###HF###0.05###0.06###0.06###0.05###0.04###0.02

Butyrate concentration (mmol/L)

###Portal###LF###0.14bc###0.26a###0.24ab###0.19ab###0.12c###0.03

###HF###0.11###0.21###0.16###0.16###0.15###0.03

###Hepatic###LF###0.09###0.15###0.13###0.11###0.07###0.02

###HF###0.06###0.10###0.72###0.07###0.07###0.02

###Arterial###LF###0.08###0.10###0.08###0.07###0.06###0.01

###HF###0.07###0.07###0.07###0.06###0.06###0.01

VFAs reached to maximum values at 2, 4 and 6 h post-feeding, and then gradually declined to the pre- feeding level in both diets. Similarly, Evans et al. (1975) reported that the values obtained between 1.5 to 5.5 h post-feeding were greater than 0.5 h pre- feeding and 7.5 h post-feeding values. It was also observed that the peaks of net portal appearance of VFAs delayed for LF diet as compared with HF diets. The reason may be that the LF diet supplied more digestible matter, increased VFAs concentration and also kept high level for long time in rumen, which resulted in high concentration of VFAs in portal vein (Tables II and VI). On the other hand, the portal plasma flow reached to peak at 2 h in HF diet and 6 h in LF diets, therefore which caused net portal appearance delay in LF diets. The mean net gut plus hepatic output of TVFA was higher (P less than 0.05) in LF diet as compared to HF diets, it might be due to production of the more VFAs in rumen when goats fed LF diets. CONCLUSIONS

Based on present results, it was concluded that the plasma VFA concentration and flux were increased, whereas ratio of acetate to propionate was decreased in portal, hepatic vein and femoral artery of goats fed low forage diets compared with high forage diets. Net acetate output was produced and net propionate, butyrate, isobutyrate and isovalerate were removed by goat liver. Net gut plus hepatic outputs of acetate, propionate and TVFA were greater in LF diet than HF diet. The concentration and net flux of VFAs were increased in after feeding then gradually declined and close to pre-feeding levels.

ACKNOWLEDGEMENTS

This study was supported by a grant from the National Key Basic Research Program, Ministry of Science and Technology, P. R. China (N0.2011CB100802).

Table VII.- Net flux of VFA across splanchnic tissues in relation to feeding time.

###Post feeding time (h)

###0###2###4###6###8###SEM

Net portal appearance (mmol/h/kg of LW)

###Acetate###LF###0.83b###2.27ab###3.20a###2.18ab###1.26ab###0.46

###HF###0.89###2.47###2.13###1.72###1.50###0.46

###Propionate###LF###0.41b###1.07ac###0.98abc###1.21a###0.57bc###0.19

###HF###0.27###1.02###0.79###0.61###0.61###0.19

###Butyrate###LF###0.08b###0.19ab###0.27a###0.21ab###0.07b###0.04

###HF###0.04b###0.27a###0.15ab###0.13ab###0.12ab###0.04

Net hepatic release (mmol/h/kg of LW)

###Acetate###LF###0.10###0.74###0.34###0.27###0.01###0.25

###HF###0.38###0.18###0.36###0.59###0.06###0.25

###Propionate###LF###-0.33c###-0.85ab###-0.88ab###-1.08a###-0.51bc###0.17

###HF###-0.24b###0.94a###-0.74ab###-0.55ab###-0.55ab###0.17

###Butyrate###LF###-0.04###-0.09###-0.13###-0.11###-0.06###0.05

###HF###-0.06###-0.18###-0.15###-0.09###-0.11###0.05

Net gut plus hepatic output (mmol/h/kg of LW)

###Acetate###LF###0.93c###3.01abc###3.54a###2.45abc###1.25bc###0.55

###HF###1.28###2.65###2.49###2.22###1.57###0.55

###Propionate###LF###0.08###0.21###0.10###0.12###0.06###0.03

###HF###0.03###0.08###0.05###0.06###0.04###0.03

###Butyrate###LF###0.04###0.11###0.13###0.09###0.01###0.00

###HF###-0.03###0.09###0.00###0.04###0.01###0.00

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Author:Zhuang Su; Zhang Teng; Chang Guang-Jun; Yan Rui; Naeem, Muhammad; Nasir Rajput; Shen Xiang-Zhen
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