Post mortem changes in Ca2+ transporting proteins of sarcoplasmic reticulum in dependence on malignant hyperthermia status in pigs

Meat quality of pigs is dependent on biochemical and biophysical processes in the time course post mortem (p.m.) and is associated with the intracellu lar Ca2+ homeostasis. However, there is little known about changes in the C a2+ transporting proteins controlling the Ca2+ uptake of sarcoplasmic retic ulum (SR) in the time course p.m. In this study changes in the Ca2+ transpo rting proteins were investigated in homogenates of longissimus muscles of 4 malignant hyperthermia susceptible (MHS) and 6 malignant hyperthermia resi stant (MHR) Pietrain pigs. Muscle samples were obtained at different time i ntervals: biopsy 2 h prior slaughtering and from the carcass immediately af ter exsanguination (0 h), 45 min, 4 h, and 22 h p.m. The SR Ca2+ uptake rat e was measured immediately after homogenization with closed calcium release channel (CRC), with opened CRC and without manipulation of CRC. Additional ly the SR Ca2+ ATPase activity was determined. The results show: (i) The ability of SR to sequester Ca2+ declined to about 60% in the first 45 min p.m. in MHS samples irrespective of CRC state, whe reas in MHR samples this decline was about 5%; (ii) Ca2+ uptake and Ca2+ AT Pase activity were not different between the biopsy and 0 h samples, i.e. t he stress of slaughter was of no immediate influence; (iii) The Ca2+ ATPase activity of the SR declined at about the same rate as the Ca2+ uptake in b oth MHS and MHR pig samples in the course of time p.m.; (iv) In samples, ta ken immediately after exsanguination, the Ca2+ ATPase activity of MHS pigs was higher than that of MHR pigs. However, in samples taken 4 h p.m. Ca2+ A TPase activity of MHS pigs has declined to about 30% of the value at 0 h; ( v) The CRC can be closed and opened in all samples up to 22 h p.m. and seem s to be fully functional at all sampling times; (vi) The CRC of MHS pigs is almost fully open, whereas the CRC of MHR pigs is only partially open at a ll sampling times; (vii) The permeability of the SR membrane to Ca2+ (deter mined as the ratio of SR Ca2+ ATPase with and without ionophore A23187) is the same in both MHS and MHR and did not change with ongoing time; (viii) N o uncoupling of uptake from ATP hydrolysis occurred up to 4 h p.m., but the coupling differed between MHS and MHR for all time intervals with lower va lues for MHS pigs. The results suggest that the decreasing Ca2+ uptake rate of homogenates, sampled at different times p.m., is essentially caused by changes in the Ca2+ pump and not by changes in the CRC or an increased phos pholipid membrane permeability to Ca2+.