It is possible that the intracellular calcium
concentration of loin chops may not have become elevated
above control values as a result of feeding vitamin
D3 to pigs as has been shown for beef (Swanek et al.,
1999). Our hypothesis was that the greater extracellular
calcium concentration (Table 1) would have increased
the intracellular muscle calcium concentrations
sufficiently to result in greater calpain activity,
greater proteolysis, and thus greater tenderness (HuffLonergan
et al., 1996). Another possibility is that calpain
activity in loin chops is nonlimiting regarding tenderization
during postmortem aging of loin chops. Furthermore,
there may need to be a greater plasma calcium
increase in pigs to be able to observe an increase in
intracellular muscle calcium during postmortem aging
and a subsequent improvement in proteolysis and postmortem
tenderization. This possibility cannot be addressed
by the current study, because intracellular
muscle calcium concentrations were not determined. Perhaps feeding the 500,000 IU of vitamin D3 for
longer times before slaughter or feeding greater daily
dosages of vitamin D3 would have resulted in an improvement
in loin-chop tenderness. Feeding greater
dosages of vitamin D3 for less than 3 d before slaughter
may be most practical to avoid possible decreases in
feed intake because of the negative impact of dietary
vitamin D3 on rate of gain (Enright et al., 1998).