Uptake and Increased Binding by Smooth Muscle in Half Isotonic Sucrose and Its Relationship to Contractility

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Summary Contractility and content of Ca, Mg, Na and K were measured in strips of toad stomach muscle in 0–12 M sucrose (half istotonic). When sucrose was completely freed from contaminant C a, contractility was rapidly lost and the muscle swelled considerbly. Addition of Ca then in the low concentration of 0·05 mM supported spontaneous contractility and responses to ACh for nanny hours and greatly reduced the swelling. Strips which had lost contractility in deionized sucrose contained more Ca than non‐contractilc strips in Ca‐free NaCl or K2SO4 Ringer. Prior exposure to Ca‐containing sucrose prolonged the period of contractility in ion‐free sucrose and the Ca content when contractility failed was greater. Enhanced binding of Ca in sucrose and the measured contaminant Ca in analytical grade chemicals could explain previous reports of contractility in “non‐ionic” solutions. Compared with strips in NaCl or K2SO4‐Ringer, the uptake of Ca required to restore contractility was greater in muscle depleted in ion‐free sucrose. This enhanced uptake was presumably associated with the maintenance of cell volume as well as contractility. Attempts to determine separately the uptake involved in excitation‐contraction coupling by the use of isotonic sucrose or of Mg to prevent swelling were unsuccessful, due to prolonged contracture in hypertonic sucrose and to an incompletely reversible inhibition of contraction by Mg.