Thermal instability of rat muscle sarcoplasmic reticulum Ca2+-ATPase function

• Published in: American journal of physiology: endocrinology and metabolism 2002-10, Vol.46 (4), p.E722-E728
• Main Authors: SCHERTZER, J. D, GREEN, H. J, TUPLING, A. R
• Format: Article
• Language: English
• Subjects: Biological and medical sciences; Calcium; Enzymes; Fundamental and applied biological sciences. Psychology; Muscular system; Oxidation; Rodents; Striated muscle. Tendons; Temperature; Vertebrates: osteoarticular system, musculoskeletal system
• ISSN: 0193-1849; 1522-1555

To examine the thermal instability and the role of sulfhydryl (SH) oxidation on sarcoplasmic reticulum (SR) Ca2+-ATPase function, crude homogenates were prepared from the white portion of the gastrocnemius (WG) adult rat muscles (n = 9) and incubated in vitro for 60 min either at a normal resting body temperature (37 deg C) or at a temperature indicative of exercise-induced hyperthermia (41 deg C) with DTT and without DTT (CON). In general, treatment with DTT resulted in higher Ca2+-ATPase and Ca2+ uptake values (nmol dot mg protein minus 1 dot min minus 1, P less than 0.05), an effect that was not specific to time of incubation. Incubations at 41 deg C resulted in lower (P less than 0.05) Ca2+ uptake rates (156 plus or minus 18 and 35.9 plus or minus 3.3) compared with 37 deg C (570 plus or minus 54 and 364 plus or minus 26) at 30 and 60 min, respectively. At 37 deg C, ryanodine (300 Mu), which was used to block Ca2+ release from the calcium release channel, prevented the time-dependent decrease in Ca2+ uptake. A general inactivation (P less than 0.05) of maximal Ca2+-ATPase activity (Vmax) in CON was observed with incubation time (0 greater than 30 greater than 60 min), with the effect being more pronounced (P less than 0.05) at 41 deg C compared with 37 deg C. The Hill slope, a measure of co-operativity, and the pCa50, the cytosolic Ca2+ concentration required for half-maximal activation of Ca2+-ATPase activity, decreased (P less than 0.05) at 41 deg C only. Treatment with DTT attenuated the alterations in enzyme kinetics. The increase in Vmax with the Ca2+ ionophore A-23187 was less pronounced at 41 deg C compared with 37 deg C. It is concluded that exposure of homogenates to a temperature typically experienced in exercise results in a reduction in the coupling ratio, which is mediated primarily by lower Ca2+ uptake and occurs as a result of increases in membrane permeability to Ca2+. Moreover, the decreases in Ca2+-ATPase kinetics in WG with sustained heat stress result from SH oxidation.