Glucose metabolism and adrenal function in goats bred for fibre production (Angora goat) or meat production (Boer goat)

Abstract

It has been proposed that the abortions, cold-stress fatalities, and slow growth rates typical of the South African type of Angora goat can be explained by congenital adrenal hypofunction incident to genetic selection for hair production. The aim of this experiment was to compare glucose metabolism and adrenal function in Angora goats vs. Boer goats under a range of dietary conditions. Glucose plasma concentration was 7% lower in Angoras (P < 0.01). Glucose flux rate was increased by dietary level (P < 0.01), and there was an interaction between breed and diet (P < 0.05). The rate of increase in glucose flux rate with increasing dietary energy level was slower in Angoras. The amount of glucose in the metabolic pool was 8% more in Angoras (P < 0.05), and this was distributed throughout a 17% larger volume of body fluid (P < 0.01). Both pool size and volume of distribution increased (P < 0.05) with level of feed. Glucose clearance rate did not differ between breeds (P > 0.05), and there was no evidence of impaired adrenal function in Angora goats following glucose loading. Acetate clearance rate was 20% slower (P < 0.01) in Angora goats. There was a positive linear (P < 0.05) relationship between acetate clearance rate and diet, but no interaction (P > 0.05) between breed and diet. Although much of the data reported is consistent with impaired gluconeogenic capability, no evidence of adrenal hypofunction could be found. It was concluded that a more likely hypothesis would be that selection for hair production has resulted in a shift in the partitioning of amino acids away from gluconeogenesis towards hair-protein synthesis to the extent that the Angora goat may be unable to mobilize sufficient endogenous protein reserves rapidly enough for gluconeogenesis during times of sudden demand.

Daar is voorgestel dat die aborsies, kouestresvrektes en stadige groeitempo's wat kenmerkend van die Suid-Afrikaansetipe Angorabok is, aan 'n toestand van adrenaal-ontoereikendheid toegeskryf kan word as gevolg van volgehoue genetiese seleksie vir haargroei. Die doel van hierdie proef was om die glukosemetabolisme en adrenaalfunksie van die Angorabok met dié van die Boerbok onder 'n reeks voedingstoestande te vergelyk. Glukoseplasmakonsentrasie was 7% laer in die Angorabok (P < 0.01). Glukose-omsettempo is deur dieetvlak (P < 0.01) versnel, en daar was 'n interaksie tussen ras en dieet (P < 0.05). Die mate waartoe glukose-omsettempo deur dieet verhoog is, was laer in die geval van die Angorabok. Die hoeveelheid glukose teenwoordig in die metaboliese poel was 8% meer in die Angora (P < 0.05), en dit was deur 'n 17% groter volume van vloeistof versprei (P < 0.01). Beide poelgrootte en verspreidingsvolume van glukose is deur toenemende dieetvlak verhoog (P < 0.05). Glukoseverwyderingstempo het nie tussen rasse verskil nie (P > 0.05), en daar was geen tekens van verswakte adrenaalfunksie ná glukose-lading in Angorabokke nie. Asetaatverwyderingstempo was egter 20% stadiger in die Angorabok (P < 0.01). Daar was 'n positiewe lineêre verwantskap (P < 0.05) tussen asetaatverwyderingstempo en dieet, maar geen interaksie tussen ras en dieet nie (P > 0.05). Alhoewel baie van die data wel met verswakte glukoneogeniese vermoë samehangend is, kon geen getuienis van adrenaalontoereikendheid gevind word nie. 'n Meer waarskynlike hipotese is dat seleksie vir haarproduksie tot 'n verskuiwing in die verdeling van aminosure weg van glukoneogenese na haarproteïensintese gelei het, tot so 'n mate dat die Angorabok nie die vermoë besit om proteïenreserwes vinnig te mobiliseer tydens periodes van verhoogde behoefte soos kouestres en dragtigheid nie.

Keywords: Acetate, adrenal, glucose, goat, metabolism, nutrition, ruminant, selection.