Homogeneity of extracellular space measurement in smooth muscle

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Abstract
Several C14-labeled compounds were used to measure extracellular space (ml/100 g) in the isolated longitudinal smooth muscle layer from guinea-pig ileum. During incubation time intervals of 20-180 min. similar values were obtained at 25 C with sucrose (37.9 [plus or minus] 1.0), inulin 38.0 [plus or minus] 0.7), and mannitol 38.2 [plus or minus] 0.8), at 0 C with sucrose (37.0 [plus or minus] 1.1), and at 37 C with sucrose (37.7 [plus or minus] 1.3). Stretching the muscle decreased the values obtained with sucrose to 27.7 [plus or minus] 1.5, whereas addition of histamine or substitution of K ion for Na ion significantly increased smooth muscle tone and decreased the sucrose space. Washout curves indicated that virtually all of the sucrose, inulin, and to a lesser degree, mannitol were removed as one rapid component. These data can be contrasted with that obtained with C14-labeled mannose. The magnitude of the mannose space increases during the incubation interval of 20-180 min (48.8 [plus or minus] 2.3 to 61.5 [plus or minus] 1.7) and mannose is removed from the tissue as two distinct components. The more rapid washout component is estimated to occupy an average space of 38.2 mg/100 g. It is concluded that all four compounds measure the same extracellular compartment in this tissue.