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A kinetic model of phosphorus metabolism in growing goats1

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A kinetic model of phosphorus metabolism in growing goats1
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  L. A. Crompton and J. FranceD. M. Vitti, E. Kebreab, J. B. Lopes, A. L. Abdalla, F. F. De Carvalho, K. T. De Resende, A kinetic model of phosphorus metabolism in growing goats 2000, 78:2706-2712.  J ANIM SCI http://jas.fass.org/content/78/10/2706the World Wide Web at: The online version of this article, along with updated information and services, is located on www.asas.org  by guest on July 13, 2011 jas.fass.orgDownloaded from   A kinetic model of phosphorus metabolism in growing goats 1 D. M. S. S. Vitti*, E. Kebreab† ,2 , J. B. Lopes‡, A. L. Abdalla*, F. F. R. De Carvalho§,K. T. De Resende¶, L. A. Crompton†, and J. France† *Animal Nutrition Laboratory, Centro de Energia Nuclear na Agricultura, Caixa Postal 96, CEP 13400-970,Piracicaba, SP, Brazil; †The University of Reading, Department of Agriculture, Earley Gate, P.O. Box 236,Reading RG6 6AT, United Kingdom; ‡Universidade Federal do Piaui, Centro de Cieˆncias Agra´rias, CampusUniversita´rio de Socopo, Teresina, PI, Brazil; §Universidade Federal Rural de Pernambuco, Departamento deZootecnia, Recife, PE, Brazil; and ¶Faculdade de Cieˆncias Agra´rias e Veterina´rias de Jaboticabal, Departmentode Zootecnia de Ruminantes, Jaboticabal, SP, Brazil  ABSTRACT:  The effect of increasing phosphorus (P)intake on P utilization was investigated in balance ex-periments using 12 Saanen goats, 4 to 5 mo of age andweighing20to30kg.Thegoatsweregivensimilardietswith various concentrations of P, and  32 P was injectedto trace the movement of P in the body. A P metabolismmodel with four pools was developed to compute P ex-changes in the system. The results showed that P ab-sorption, bone resorption, and excretion of urinary PandendogenousandfecalPallplayapartinthehomeo-static control of P. Endogenous fecal output was posi-tively correlated to P intake (  P  <  .01). Bone resorptionof P was not influenced by intake of P, and P recycling Key Words: Goats, Metabolism, Mineral Absorption, Phosphorus, Simulation Models ©  2000 American Society of Animal Science. All rights reserved.  J. Anim. Sci. 2000. 78:2706–2712 Introduction Phosphorusisanessentialnutrientand,asphosphate,isinvolved inmost ofthemetabolic activitiesof thebodyaswellasinboneformation(SymondsandForbes,1993).However, there is a controversy over the exact mecha-nisms involved in P homeostasis (Challa et al., 1989).It is not clear whether homeostasis is regulated by Pabsorption,salivaryP,excretionofPinurine,oracombi-nation of two or all factors.Work toward improving the understanding of P me-tabolism in ruminants has been carried out for sometime. A number of workers have used isotope dilution 1 The authors would like to thank Fundac˛a˜o de Amparo a` Pesquisado Estado de Sa˜o Paulo (FAPESP Proc. 98/13406-3) and ConselhoNacional de Desenvolvimento Cientı´fico e Tecnolo´gico for financialsupport. 2 Correspondence: phone:  + 44 (0) 1189 318498; fax:  + 44 (0) 1189318297; E-mail: E.Kebreab@reading.ac.uk.Received November 29, 1999. Accepted April 25, 2000. 2706 from tissues to the blood pool was lesser for low P in-take. Endogenous P loss occurred even in animals fedan inadequate Pdiet, resulting in anegative P balance.The extrapolated minimum endogenous loss in feceswas .067 g of P/d. The minimum P intake for mainte-nance in Saanen goats was calculated to be .61 g of P/ d or .055 g of P/(kg  .75  d) at 25 kg BW. Model outputsindicate greater P flow from the blood pool to the gutand vice versa as P intake increased. Intake of P didnot significantly affect P flow from bone and soft tissueto blood. The kinetic model and regressions could beused to estimate P requirement and the fate of P ingoats and could also be extrapolated to both sheepand cattle.and balance methods to study the absorption of P insheep and cattle (Braithwaite, 1983; Schneider et al.,1985;SalvianoandVitti,1998).Radioisotopeshavebeenused to study the distribution of P by applying compart-mentalmodels(Grace,1981;Schneideretal.,1987),and the kinetics of   32 P following intravenous injection havebeen studied using a compartmental analysis computerprogram (Boston et al., 1981). However, the studies hadlimited application in calculating P requirement and ex-trapolating to other ruminants because only one level of Pconcentrationwasconsidered.Inaddition,theexisting published information related to P metabolism in thegoatisrelativelylittleandinconsistent(e.g.,Akinsoyinu,1986; Muschen et al., 1988).Little is known, for example, about the blood and softtissue P pools of the body and their rates of P inflowand outflow in goats and other ruminants. Quantitativeinformation on many indices of P metabolism, such asfecalendogenousPlossesandtrueabsorption,isneeded. Variation in P intake might affect P utilization and ho-meostasis,andlowPintakesmightresultinlowPlevelsin the blood and soft tissue P pools of the body. The  by guest on July 13, 2011 jas.fass.orgDownloaded from   Phosphorus metabolism in goats  2707 Figure 1 . Schematic representation of the model of phosphorus metabolism in goats: (a) unlabeled P and (b)labeled P; F ij  is the total flux of pool i from j, F i0  is anexternal flux into pool i, and F 0j  a flux from pool j out of the system. Specific activity of pool i is represented bys i , and circles denote fluxes measured experimentally. purpose of this research was to use data from balanceand kinetic studies to propose and solve a model of Pmetabolism in growing goats fed increasing levels of P.Thenutritionalandmetabolicimplicationsofincreasing P intake are evaluated. Materials and Methods  Model Development The proposed model of whole-body P metabolism isthe simplified scheme based on P flows shown in Figure1a. It contains four pools of P: 1) gut lumen, 2) blood, 3)bone, and 4) soft tissue. The fluxes of P between poolsand into and out of the system are shown as arrowedlines. The gut lumen, bone, and soft tissue pools inter-change in two directions with the blood pool, with fluxesF 21  and F 12 , F 23  and F 32 , and F 24  and F 42 , respectively.Phosphorus entry to the system is via intake, F 10 , andexit via feces, F 01 , and urine, F 02 . The scheme adoptedforthemovementoflabelisshowninFigure1b.Labeled 32 P was administered intravenously as a single dose, D,at time zero, and the size and specific activity of theblood, bone, and soft tissue pools measured after 8 d.The scheme assumes there is no reentry of label fromexternal sources.Conservationofmassprinciplescanbeappliedtoeachpool in Figure 1 to generate differential equations thatdescribe the dynamic behavior of the system. For unla-beledP,thesedifferentialequationsare(themathemati-cal notation is defined in Table 1):dQ 1  /dt  =  F 10  +  F 12  −  F 01  −  F 21  [1]dQ 2  /dt  =  F 21  +  F 23  +  F 24  −  F 02  −  F 12  −  F 32  −  F 42  [2]dQ 3  /dt  =  F 32  −  F 23  [3]dQ 4  /dt  =  F 42  −  F 24  [4]and for label:dq 1  /dt  =  s 2 F 12  −  s 1 (F 01  +  F 21 ) [5]dq 2  /dt  =  s 1 F 21  +  s 3 F 23  +  s 4 F 24  −  s 2 (F 02  +  F 12  [6] +  F 32  +  F 42 )dq 3  /dt  =  s 2 F 32  −  s 3 F 23  [7]dq 4  /dt  =  s 2 F 42  −  s 4 F 24  [8]Consider the differential coefficient of s 3  with respect totime and differentiating q 3 Q 3 − 1 as a product:ds 3  /dt  =  d(q 3 Q 3 − 1 )/dt  =  [dq 3  /dt [9] −  (q 3  /Q 3 )dQ 3  /dt]/Q 3 Rearranging givesdq 3  /dt  =  Q 3 ds 3  /dt  +  s 3 dQ 3  /dt [10]Using Eq. [3] and [7] to substitute for dQ 3  /dt and dq 3  / dt, respectively, and approximating ds 3  /dt by [s 3  −  s 3 (0)]/ [t  −  0], Eq. [10] becomess 3  /t  =  (s 2  −  s 3 )F 32  /Q 3  [11]as s 3 (0), the value of s 3  at time zero, is zero. Similarly,consideration of ds 4  /dt yieldss 4  /t  =  (s 2  −  s 4 )F 42  /Q 4  [12] After 8 d, we assumed that Pool 1 (gut lumen) was incomplete steady state (i.e., both dQ 1  /dt and dq 1  /dt arezero) and Pool 2 (blood) was in nonisotopic steady state(i.e., dQ 2  /dt is zero). Equations [1], [2], [5], [11], and [12]now become, respectivelyF 10  +  F 12  −  F 01  −  F 21  =  0 [13]  by guest on July 13, 2011 jas.fass.orgDownloaded from   Vitti et al. 2708 Table 1 . Principal symbols used in the model F ij  Total flux of P to pool i from j; F i0  denotes an external flux into pool i and F 0j a flux from pool j out of the system; a tilde indicates a flux that can bemeasured experimentally. Units are g/d.D Dose of   32 P administered to blood at time zero: dpmQ i  Total quantity of P in pool i: g q i  Quantity of   32 P in pool i: dpms i  Specific activity of pool i ( =  q i  /Q i ): dpm/g t Time: d F 21  +  F 23  +  F 24  −  F 02  −  F 12  −  F 32  −  F 42  =  0 [14]s 2 F 12  −  s 1 (F 01  +  F 21 )  =  0 [15](s 2  −  s 3 )F 32  /Q 3  =  s 3  /8 [16](s 2  −  s 4 )F 42  /Q 4  =  s 4  /8 [17] Algebraic manipulation of Eq. [13] to [17] givesF 12  =  s 1 F˜ 10  /(s 2  −  s 1 ) [18]F 21  =  F˜ 10  +  F 12  −  F˜ 01  [19]F 32  =  s 3 Q 3  /[8(s 2  −  s 3 )] [20]F 42  =  s 4 Q 4  /[8(s 2  −  s 4 )] [21] | F 23  +  F 24 | =  F˜ 02  + F 12  +  F 32  +  F 42  −  F 21  [22]where the tilde indicates an experimentally measuredflux. The combined flux  | F 23  +  F 24 | , which denotes thesum of outflow from Pool 3 and outflow from Pool 4, | F 23  +  F 24 | =  F 23  +  F 24  [23]can be partitioned by combining Pools 3 and 4. Let s*denote the specific activity of the combined pool. This iscalculated ass*  =  (s 3 Q 3  +  s 4 Q 4 )/(Q 3  +  Q 4 ) [24]The outflow of label from the combined pool is the sumof the outflow of label from Pool 3 and the outflow of label from Pool 4:s*  × | F 23  +  F 24 | =  s 3 F 23  +  s 4 F 24  [25] Algebraic manipulation of Eq. [23] and [25] givesF 24  =  (s*  −  s 3 )  × | F 23  +  F 24 |  /(s 4  −  s*) [26]F 23  = | F 23  +  F 24 | −  F 24  [27]The model is applied by using Eq. [18] to [22], [24],and [26] to [27] to compute the unknown fluxes. Experimental Procedure Twelve Saanen goats, 4 to 5 mo of age and weighing 20 to 30 kg, were housed indoors in metabolism cratesdesigned for isotope studies and handling of feces andurine at the Faculty of Animal Science and VeterinaryofJaboticabal(UNESP).After15d,thegoatsweretrans-ferred to the Center for Nuclear Energy in Agriculture(CENA), University of Sa˜o Paulo, for  32 P injection andcollection of data. The goats received a diet consisting of aconcentrate mixtureand  Brachiariadecumbens  hay(Table 2). The hay was offered ad libitum, and P supple-mentation was offered as dicalcium phosphate to give.42 (low P level,  L ), 1.36 (medium P level,  M ) and 3.63g P/d (high P level,  H ) (Agricultural and Food ResearchCouncil, 1991). For L, the diet was not supplementedwith any additional P. Feed was given twice a day forone28-d period.All animalsreceived vitaminsA,D, andE by intramuscular injection (Table 2). Table 2 . Feed ingredients and composition of the dietfed to goats containing low (L), medium (M),and high (H) P concentrations a Item L M HIngredient, g/kg DMHay 601 602 576Cassava meal 380 292 292Ground corn 0 86.1 85.8Dicalcium phosphate 0 2.39 15.3White salt 2.89 2.69 2.93Mineral mixture b 3.01 3.02 3.15Urea 13.1 9.90 12.1Calcium carbonate 0 2.02 12.76Chemical compositionDry matter, g/kg as is 90.6 90.7 90.9Crude protein, g/kg 91.6 91.8 92.4NDF, g/kg 650 632 610 ADF, g/kg 266 265 254Ca, g/kg 2.20 3.70 10.7P, g/kg .80 1.50 3.80ME, MJ/kg DM 2.20 2.21 2.16Ca:P 2.70 2.54 2.81DMI, g/d 562 919 999BW, kg 26.4 28.3 28.8 a The goats received 20,000 IU vitamin A, 5,000 IU vitamin D, and6 mg vitamin E. b The mineral mixture contained, in milligrams per kilogram DM,8.73 Fe, 7.64 Cu, 44.4 Mn, 58.7 Zn, .11 Co, .21 I, and .032 mg Se.  by guest on July 13, 2011 jas.fass.orgDownloaded from   Phosphorus metabolism in goats  2709  After 21 d, each animal was given, as a single dose via the right jugular vein, 200   Ci of   32 P in 1 mL of sterile isotonic saline (9 g/L NaCl). Blood samples (10mL) weretaken byVacutainer fromthe leftjugular veinafter isotope administration at 24-h intervals for 7 d.BecausemostofthemobilePinbloodisfoundinplasma,thebloodwascentrifugedandplasmaremovedforanaly-sis. Nine milliliters of trichloroacetic acid (100 g/L) wasadded to 1 mL of plasma for protein precipitation. Aftercentrifugation (1,100  ×  g ), inorganic P was determinedby colorimetric analyses (Fiske and Subbarow, 1925).Phosphorus intake and excretion in feces and urinewererecordeddailyfor7d,andsubsamples(10%oftotaloutputs) were stored for further analysis. Feces samples(1 g) were dried overnight (105 ° C) and ashed (500 ° C for8 h). The ash was dissolved in concentrated HCl, and Pcontent was determined by a colorimetric method (Sar-ruge and Haag, 1974). A similar procedure was used todetermine P content of food intake. Urine samples (30mL) were acidified during collection using 100 mL of 12  N  HCl, whichwere thendried (55 ° C)andashed (500 ° C). Ashed samples were diluted (3  N   HCl), and volume wasmade up to 10 mL (Morse et al., 1992). Inorganic P wasdetermined using vanadate-molybdate reagents (Sar-ruge and Haag, 1974).For radioactivity measurements, 1-mL plasma andurine samples were added to 19 mL of distilled waterin counting vials. Ashed fecal samples (1 g) were dis-solvedin18  N  H 2 SO 4 andplacedincountingvials.Radio-activityof  32 PwasmeasuredinaPackardLiquidScintil-lation Spectrometer (model 2450B, A. Canberra Com-pany) using Cerenkov radiation. Specific activities inplasmaandfecesweredeterminedaccordingtoLofgreenand Kleiber (1953). Aftertheendofcollectionperiod,thegoatswerekilledby intravenous injection of pentobarbital (200 mg/mL),and tissues (liver, heart, kidney, and muscles) and bonesamples(12thrib)werecollectedonce.Thematerialwascleaned,weighed,andautoclaved.Samplesweregroundand dissolved in 18  N   H 2 SO 4 . The extract was trans-ferred to vials for radioactivity determination. For min-eral determination, bone samples were dissolved in con-centrated HCl (Sarruge and Haag, 1974). Bone specificactivity in 1 g DM and  32 P incorporation in bone werecalculated according to Lofgreen and Kleiber (1953). Statistical Analyses Experimental measurements (model inputs) andmodel outputs were analyzed as a completely random-ized design. The data used in the analysis were fromnine animals, with three from each level of P intake duetoincompleteresultsfortheotheranimals.Acomparisonof means between each level of P intake was carried outusingtheGeneralLinearModelsprocedure(SAS,1990),with the sources of variation being P concentrations (de-grees of freedom of the error  =  6). The SEM was calcu-lated as S/  √ 3. Treatment means were assessed using the least significant difference method when overalltreatment effects were  P  <  .05. Regression analysis wascarriedoutusingthePROCREGprocedure(SAS,1990),and the SE of coefficients was reported. Results DailyintakeanddailyexcretionofPandspecificactiv-ities and P contents in bone, blood, and soft tissues aresummarized in Table 3. The values are the means of three goats grouped according to P intake level. Diet Lsupplied only .42 g P/d and it was largely deficient in P.This resulted in a negative P balance ( − .081 g/d) forgoats fed diet L. Diet M supplied 1.36 g P/d, which wasconsidered moderately deficient, but the H diet supplied3.63gP/d,whichwasadequateaccordingtoARC(1980).Total P excreted in feces (F 01 ) increased with P intake(F 10 ), and there was a highly significant linear relation-ship between these fluxes. Total P excreted in feces wasabout 74 and 44% higher for goats on Diet H comparedwithLand,onMcomparedwithL,respectively.UrinarylossofPwaslowforalltreatments,representing.95,.29,and.17% ofdietaryPfor L,M,andH diets,respectively.There were no significant differences in losses of P inurine between the three diets.Specific activities and P contents in blood and softtissues were affected by P intake. The values for specificactivitiesweregreater(  P < .05)andbloodandsofttissueP were lower (  P  <  .05) in goats on L diet.The endogenous fecal loss (F e01 , where F e01  =  F 12 F˜ 01  / (F 12  +  F˜ 10 )  =  s 1 F˜ 10  /s 2 ) was .15, .35, and .89 g P/d for DietsL, M, and H, respectively. The endogenous fecal loss of P increased significantly with increased P intake asgiven by the following equation:F e01  =  .067  +  .21F 10 (SE  =  .034 and .014 for intercept and slope,[28]respectively; n  =  9; r 2 =  .97)Total endogenous losses represented 36% of P intake forunsupplemented animals (L). In goats fed Diets M andH, endogenous P represented about 25% of P intake.Truly absorbed P, which is the amount of dietary P ab-sorbed by the goats (F d21 , where F d21  =  F 10  −  [F 01  −  F e01 ])was .04, .77, and 2.49 g of P/d in L, M, and H dietsrespectively. Whereas P was truly absorbed from the Ldiet at only 10% efficiency (true absorption/intake), trueabsorption from M and H diets was at 57 and 69% effi-ciency, respectively. There was a significant linear rela-tionship between the truly absorbed P and P intake,which was similar to reported values of .64 to .70 (Ag-ricultural and Food Research Council, 1998):F d21  = − .18  +  .70F 10 (SE  =  .14 and .062 for intercept and slope,[29]respectively; n  =  9; r 2 =  .95)The retention of P was negative on the deficient Pdiet ( − .11 g/d) when the absorption was low but became  by guest on July 13, 2011 jas.fass.orgDownloaded from 
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