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Department of Kinesiology, University of Waterloo, Waterloo, Ontario, Canada N2L 3G1
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°C) or at a temperature indicative
of exercise-induced hyperthermia (41°C) with DTT and without DTT
(CON). In general, treatment with DTT resulted in higher
Ca2+-ATPase and Ca2+ uptake values
(nmol · mg protein
1 · min
1,
P < 0.05), an effect that was not specific to time of
incubation. Incubations at 41°C resulted in lower (P < 0.05) Ca2+ uptake rates (156 ± 18 and 35.9 ± 3.3) compared with 37°C (570 ± 54 and 364 ± 26) at 30 and
60 min, respectively. At 37°C, ryanodine (300 µM), which was used
to block Ca2+ release from the calcium release channel,
prevented the time-dependent decrease in Ca2+ uptake. A
general inactivation (P < 0.05) of maximal
Ca2+-ATPase activity (Vmax) in CON
was observed with incubation time (0 > 30 > 60 min), with
the effect being more pronounced (P < 0.05) at 41°C
compared with 37°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 < 0.05) at 41°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°C compared with 37°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.
muscle; Ca2+ regulation; temperature; Ca2+ uptake; Ca2+-ATPase
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