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On the Physiology of Amoeboid Movement ON THE PHYSIOLOGY OF AMOEBOID MOVEMENT. IV.—THE ACTION OF MAGNESIUM. BY C. F. A. PANTIN. {The Marine Biological Laboratory, Plymouth?) (Received January 6th, 1926.) CONTENTS 1. Maintenance of the cell-surface . 297 6. The action of Ce'" in the pres- 2. The action of barium . 300 ence of Ca" .... 306 3. The action of cerium . 301 4. Action of Mg" in the presence 7. Discussion 306 of Ca" 302 8. Summary 310 5. Interaction of the ions of sea- 9. References 311 water 303 THE previous paper of this series (Pantin, 1926) described the action of certain ions upon a species of marine amoeba, and especially the relation of calcium to amoeboid movement. The same paper detailed the method of preparing isotonic salt solutions and of determining their effect on amoeboid movement. The essential feature is that the average velocity is taken as a measure of the effect of a solution upon the power of movement, apart from other effects produced upon the cell. Unless otherwise stated, the CH of the solutions was maintained at /H 7.0 to 7.2. 1. Maintenance of the cell-surface. It was shown previously that whereas cytolysis occurred rapidly in pure isotonic NaCl or KCl (at about />H 7), amoebae remained alive for a longer time in isotonic CaCl= and MgClj. Movement was inhibited in these solutions, but reversibly, for, provided immersion had been brief, recovery occurred on return to natural sea-water. In mixtures of two salts, it was found that movement only occurred if Ca were 297 C. F. A. Pantin present. On the other hand, the amoeba lived almost as well in mixtures of (NaCl +MgCl,) as in mixtures of (NaCl + CaCls), for although no movement took place in the Mg mixture yet within certain limits of concentration movement was ultimately resumed on transference of the amoeba to sea-water, even after some hours immersion. The same relation was found between solutions of (KC1 + MgClj) and solutions of (KC1 + CaCl,). But in mixtures containing only (NaCl+KCl) the permeability of the cell seemed to increase, and cytolysis occurred rapidly as in pure NaCl or KC1. These considerations lead to the conclusion that although Mg cannot replace Ca with respect to the mechanism of move- ment, yet either Mg or Ca can ensure stability of the cell- surface. Sr and Ba can also do this. This action seems to be connected with a reduction of the permeability of the cell which tends, as is well known, to be brought about not only by Ca but by Mg or indeed any divalent metal (Osterhout, 1922 ; Lillie, 1923). If mixtures of NaCl + MgCl, or of KCl+MgCls are made alkaline (J>H 8 to 8.5), a very feeble movement is occasionally seen for a short time, when the molecular ratio of alkali-metal to magnesium lies between 5 and 10. This might be expected, for although there is no Ca in the medium the Mg will prevent loss of Ca from the cell by lowering the permeability. The increased alkalinity will also tend to produce movement by preventing loss of Ca from the actual contractile mechanism itself, just as it does in solutions of (NaCl + CaCls) deficient in Ca (Pantin, 1926). It might be .suggested that if the Mg acts by reducing permeability an increase in alkalinity of the external medium could not affect the alkalinity of the cell-interior. But Mg alone cannot stabilise the cell-surface completely, and the fast OH-ion may be able to penetrate to some extent. A con- sideration of more importance is that owing to the continuous production of CO, in the cell there is a dynamic diffusion gradient of Ch across the cell-membrane. Raising the C0H of the external medium is thus bound to raise the COH just below the cell-membrane, in order to maintain the steepness of the 398 The Physiology of Amoeboid Movement gradient. Because of buffer action it is unlikely that there will be any effect on the protoplasm except immediately below the cell- membrane, at which point the gradient is presumably steepest; but it is just this region that appears to be most intimately concerned with the mechanism of amoeboid movement. If amoebae are placed in a series of solutions of (NaCl + MgCls), it is seen that, apart from the absence of movement, the behaviour in the Mg - deficient solutions resembles that which occurs in mixtures of (NaCl + CaClg) where calcium is deficient. It is found that in both cases the adverse changes which lead to death occur successively, com- mencing in the mixtures where the concentration of the divalent Mod.tTo.te FIG. I.—Amoebae in NaCl in the presence of various metals which maintain the cell surface, tut which will not support movement The distribution of the granules is represented diagrammatically. metal is least. Where the molecular ratio is greater than Na/Mg = 500, swelling and cytolysis occur as rapidly as in pure NaCl. From solutions containing Na/Mg =128 down- wards the increasing Mg concentration begins to have a definite stabilising influence on the cell-surface. The effect increases more and more rapidly as the Mg concentration is raised till Na/Mg = 10. The stability is indicated by the longer time the amoeba remains alive in the solution and the inhibition of the swelling which occurs in pure NaCl. Osterhout (1922) has shown parallel changes in the viability of Laminaria in the presence of increasing concentrations of divalent metals. In low concentrations of Mg the amoeba assumes an irregular spherical form : as the concentration increases the amoeba tends to assume a "proteus" form (fig. 1) and appears more normal. VOL. in.—NO. 4. 299 u C. F. A. Pantin The fact that these changes are closely similar to those occurring in mixtures of (NaCl + CaClf), although in the Ca mixtures they are accompanied by movement, supports the suggestion made- in the previous paper (Pantin, 1926) that the reduction of movement which occurs in Ca-deficiency is directly related to an increase in permeability, probably accompanied by loss of Ca from the cell: this inhibits movement because Ca is specifically necessary for the mechanism to function. The similarity of the action of Mg and Ca ceases when the molecular ratio falls below Na/Mg =10. In the case of Ca, an Na/Ca ratio below this value rapidly inhibits movement and appears to produce a characteristic gelation of the ectoplasm. But in solutions of NaCl + MgClj the "proteus" condition seen in lower Mg concentrations is maintained, to some extent, right up to (Na/Mg = 1.5) and even, for a short time, in pure MgCla. The viscosity of the protoplasm does not seem to increase enormously as it does in excess of Ca; but this is difficult to judge because of the absence of movement. It may be noted that in experiments conducted by Professor Chambers and the writer on these amoebae the cell- surface was torn in excess of Ca and of Mg and the medium freely admitted to the protoplasm : in both cases coagulation followed. But whereas the coagulum formed with Ca was exceedingly tough, that formed with Mg was much looser and "slimy." In high concentrations of Mg the "proteus" condition becomes enormously exaggerated. In such a solution an amoeba may become reduced to a small central mass of protoplasm with a dozen or more irregular pseudopodia radiating from it (fig. 1), in which condition it remains. The significance of this is not obvious, though it seems to indicate that the excitor mechanism becomes inco-ordinated. 2. The action of barium. Like Mg, Ba is able to maintain the stability of the cell- surface but cannot support amoeboid movement. The range over which stabilisation occurs is approximately the same as it is with Ca and Mg. Yet the action of Ba seems to be more closely allied to that of Ca and Sr than to Mg, for like Ca 300 The Physiology of Amoeboid Movement excess of Ba markedly increases the viscosity of the ectoplasm. In solutions containing a large proportion of Ba (Na/Ba = 10 or less) the amoeba assumes a form very like that which occurs in excess Ca and quite unlike the Mg " proteus " form (fig. i). Very occasionally feeble activity was seen for a short time in solutions of about the composition Na/Ba = 20, and in these cases the amoebae approached the unipodal limax type seen in Ca. It is possible that Ba not only stabilises the cell-surface but also acts, like Ca, directly on the contractile mechanism itself; but although it acts at the same site as Ca the resulting system does not sufficiently approach the normal conditions to allow movement. Although Ba is definitely toxic to amoeba, yet inhibition in Ba is for a short time reversible where Na/Ba>io. 3- The action of cerium. Mines (1912) suggested that the action of Mg on various contractile tissues was essentially connected with the fact that it is a divalent cation. He showed that, as one might expect, the far more powerful trivalent cations are able to perform the function of Mg when they were present in very low concentration. Osterhout (1922) and Gray (1916) have also shown that, like Mg, Ce can reduce the permeability of the cell and is effective in great dilutions. As in solutions of NaCl + MgClj, amoebae did not move in a solution of isotonic NaCl in the presence of any concentration of CeCl8. The solutions were rigidly buffered at pW 6.8 to 7.6 in different experiments, so this effect is not due to the acidity consequent on hydrolysis of CeCl8.
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