Studies of fragmented sarcoplasmic reticulum from human skeletal muscle

Sarcoplasmic reticulum (SR) vesicles were isolated from muscle biopsies of 4 normal volunteers, a patient with McArdle disease (before and during contracture), and a patient with normokalemic periodic paralysis. Fractions were analyzed for purity by electron microscopy and biochemical analysis of specific marker enzymes. Adenosine triphosphate (ATP)‐dependent calcium ion uptake was measured kinetically by the absorbence changes of murexide, a metallochromic indicator of ionized Ca++ concentrations, in the absence of oxalate or other calcium‐complexing anions. In these experiments, time resolution of the Ca++ transport rate was limited by the manual mixing of reagents, which occurred in 1 to 3 seconds. In 1 case the “true” initial velocity of Ca++ uptake was measured by rapid mixing of ATP in a stopped‐flow apparatus and by following the change in absorbence of murexide in a storage oscilloscope. In SR from normal human muscle the ATP‐dependent Ca++ uptake was 3.5 nmoles per second per milligram of protein for the first 5 seconds after ATP mixing. Fast kinetic experiments showed that Ca++ uptake proceeded linearly for the first 500 msec at a rate of 9 nmoles per second per milligram of protein (at 25°C) and then progressively declined to reach steady‐state levels in 40 to 50 seconds. No abnormality of Ca++ transport was found in SR vesicles from the patient with McArdle disease and the patient with normokalemic periodic paralysis.