Fasting-induced glycogen depletion in different fibre types of red and white pig muscles--relationship with ultimate pH

The aim of the present work was to study the effect of fasting times (0, 24, 48 and 72 h) on glycogen level in different fibre types of a predominantly white (Lonqissimus, LD) and a predominantly red (Semispinalis, SS) pig muscle, and on ultimate pH (pHu, 24 h post mortem). Thirty two individually housed Large White castrates eight pigs per treatment) were slaughtered individually under minimal stressful conditions. Muscle samples were taken during bleeding for enzymic determination of muscle metabolites and histological examination using computerised image analysis. Myofibres were classified by a combination of metabolic (W, white; R, red) and contractile (alpha, fast-twitch: beta, slow-twitch) characteristics. The SS showed higher glycogen depletion after 24 h fasting than the LD (-55 versus -26%), respectively). No further significant changes in glycogen level were recorded with enhanced fasting times (up to 72 h), although in the SS a marked trend towards glycogen depletion was observed. Fasting for 24 h resulted in significantly increased pHu in the SS whereas for the LD, a significant increase in pHu occurred only, after 48 h fasting. In response to fasting, significant decrease in histochemical staining intensity for glycogen occurred in the three fibre types of the SS. The R fibres of the SS showed an almost complete glycogen depletion after 24 h fasting. With increasing fasting time, glycogen was further depleted in fast-twitch fibres (alpha W and alpha R). In the LD, only fast-twitch fibres exhibited significant decrease in glycogen level after 24 h fasting. However, this was of a much lower magnitude than that observed in the SS and furthermore, no further variation was recorded with increasing fasting time, regardless of fibre type. The present work is the first to report a significant interaction between muscle type and fasting-induced glycogen depletion in pig. Furthermore. these results demonstrate that within each muscle, the extent of fasting-induced glycogen depletion is strongly fibre-type-dependent.