Myosin heavy chain isoforms influence myofibrillar ATPase activity under simulated postmortem pH, calcium, and temperature conditions

The pH and Ca2+ sensitivity of myofibrillar ATPase activity plays an integral role in regulating postmortem muscle ATP utilization and likely paces postmortem glycolysis. The objective of this study was to determine the influence of pH and Ca2+ concentration on the ATPase activity of myofibrils from red semitendinosus (RST) and white semitendinosus (WST) porcine muscles. Myofibrillar ATPase was measured at 39 °C over a pH range 5–7.5 and a [Ca2+] range pCa 4–9 (10−4–10−9M). At maximum Ca2+-dependent activation (pCa 4), RST myofibrils had lower (p<0.0001) ATPase activity than WST myofibrils. This maximum activity of myofibrils from both muscle regions was not influenced from pH 7.5 to 6.5, declined between pH 6.5 and 5.75 (Hill coefficient, nH=2.7–3.4; pH at half maximum activity, pH50=5.97) and was near zero at pH 5.5. At pH 7, pCa-activity relationships showed that RST required less Ca2+ for half-maximum activation (higher pCa50; 6.50) than WST myofibrils (pCa50=6.35) but had no difference in nH. At pH 7, both RST and WST myofibrils had maximum Ca2+-dependent, actin-activated ATPase activity at pCa ⩽6 and Ca2+-independent myosin ATPase activity at pCa ⩾6.75. pCa-activity relationships at different pH levels indicated that pCa50 decreased with pH from pH 6.5 to 6.125 in both RST and WST myofibrils. At pH <5.75, [Ca2+] did not influence ATPase activity in RST or WST myofibrils. These data show that myofibrils with predominantly fast MyHC (WST) have a higher actin-activated myosin ATPase activity than myofibrils with primarily slow MyHC isoforms (RST) at Ca2+ concentrations and pH values characteristic of postmortem muscle.