The use of lanthanum to estimate the numbers of extracellular cation‐exchanging sites in the guinea‐pig's taenia coli, and its effects on transmembrane monovalent ion movements.

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Abstract
Tissues [guinea pig tenia coli] were allowed to equilibrate in a Tris-buffered Krebs solution and were then exposed to similar solutions containing up to 5 mM La. La caused shrinkage and significant losses of tissue K, Na, Mg and Ca. The shrinkage was exactly accountable for by a reduction in the extracellular space (ecs) as measured by 14C-sucrose. No significant change was seen in tissue Cl content. Exposure for 1 h to 5 mM La led to a total cation loss of 24.3 .+-. 1.6 meq/kg or, correcting for the small change in Cl content, a loss of positive charge of 23.8 .+-. 2.2 meq/kg fresh weight. Using the radioisotope 140La the loss of cation was balanced by an uptake of La3+. Subtraction of the ions in the measured 14C-sucrose space from the total tissue ion contents led to estimates of the cellular ion contents. The effects of 1 h exposure to 5 mM La on these were a loss of 12.9 .+-. 2.4 meq/kg of cation and a gain of 10.0 .+-. 2.6 meq/kg of Cl. Similar changes in ion content were produced by La on Na-loaded and K-loaded tissues, these being tissues which by exposure to K-free or Na-free (high K) solutions had replaced all their K with Na or vice versa. The uptakes of 24Na and 36Cl by Na-loaded tissues were both describable as the sum of 2 exponential processes: a fast component (t1/2 [half-time] .simeq. 1/2 min), which was presumed to be extracellular and a slower, presumed transmembrane, component. La reduced the rapid component of uptake of 24Na by an amount greater than that predicted by the reduction in the ecs, the extra amount lost being some 10-15 meq/kg. La also reduced the amount of rapidly exchanging 36Cl, but this reduction was entirely accounted for by the change in the ecs. La reduced the rate constant of the slow component of 24Na uptake.