Studies on the Cation Channel in Sarcoplasmic Reticulum Vesicles. I. Characterization of Ca2+-Dependent Cation Transport by Using a Light Scattering Method1

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Abstract
The characterization of the cation channel in sarcoplasmic reticulum (SR) vesic'es was performed by measuring the choline influx. The choline influx in SR vesicles was measured by following the change in light scattering intensity using a stopped flow apparatus. From the analysis of the initial rate of choline influx, the following results were obtained. (1) The choline influx was activated by extravesicular Ca2+ with an apparent dissociation constant of 8.3×10−7 M. and was inhibited through two steps with inhibition constants of 2.6×10−5 M and 7.4×10−4 M. (2) The dependence of choline influx on the ion concentration followed the Michaelis-Menten kinetics with a half-saturation constant of 30 mM. (3) The choline influx was inhibited by lowering the pH. (4) The activation energy of choline influx was 3.5 kcal/mol. (5) The choline influx was strongly blocked by Cs+ with an inhibition constant of 10 mM. These properties of the choline transporting system in SR vesicles are similar to those of the cation channel reported by Miller (J. Membrane Biol. (1978) 40, 1–23). Lastly, the effect of extravesicular (Ca2+ on the choline transport can be explained by an allosteric model.