While cardiac and skeletal sarcoplasmic reticulum share similar phosphorylation mechanisms, they exhibit differences in the reaction pathways or rates of phosphoenzyme breakdown.
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Animal cardiac SR phosphoenzyme kinetics differ from skeletal; should not change practice and leaves open tissue-specific mechanisms for human study.
The kinetics of the reactions of the Ca2+-ATPase of sarcoplasmic reticulum (SR) from canine and rabbit cardiac muscle were investigated at 21°C by means of a rapid mixing acid-quench technique and compared with data obtained from fast skeletal muscle preparations.Over a range of Ca2+ (2 to 14 p ~) and ATP (10 to 100 p ~) concentrations, membrane vesicles from cardiac SR were phosphorylated by ATP with half-times for approach to maximal phosphoenzyme levels similar to those found in skeletal SR.The time course of phosphoenzyme formation displayed a transient overshoot, which was more pronounced in canine and rabbit cardiac SR than in rabbit skeletal SR.Rapidly and slowly hydrolyzing phosphoenzymes were detected in both types of SR following phosphorylation by ATP and dephosphorylation in the presence of EGTA.The initial decay of the phosphoenzyme shows monophasic behavior and occurs at similar rates in canine and rabbit cardiac SR, whereas in rabbit skeletal SR the phosphoenzyme disappears more rapidly and shows an early biphasic pattern.These observations, while favoring similar phosphorylation mechanisms for cardiac and skeletal SR, indicate possible differences in the reaction pathways or rates of reactions involved in breakdown of the phosphoenzymes.
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Sumida et al. (1980) studied this question.
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