Interactions of calcium with sodium and potassium in membrane potentials of the lobster giant axon

Experiments were performed on the lobster giant axon to determine the relation between intracellular spike amplitude and external calcium ion concentration. Action potential decline in low external calcium is greatly accelerated by simultaneous removal of external sodium ion. Correlation of the time course of spike decline in low calcium-low sodium solution with the time courses of spike decline in low calcium alone and in low sodium alone indicates that the effect of simultaneous removal of both ions is significantly greater than the sum of the individual effects. For a given time of treatment, spike amplitude was a function of external calcium concentration. While spike height is proportional to the log of the external calcium concentration over the range 2.5 to 50 millimolar, the proportionality constant is dependent upon the sodium concentration. Under the conditions of low external sodium (50 per cent reduction) the slope of the linear relationship between the spike height and the log of the external calcium concentration is about 5 times greater than in normal external sodium. Decreasing external calcium concentration and simultaneously increasing external potassium concentration produce a greater spike reduction than the arithmetic sum of spike reductions in low calcium alone and in high potassium alone. It is suggested that calcium interacts strongly with sodium and potassium in the spike-generating mechanism. A theoretical basis for these results is discussed.

Potassium accumulation and sodium efflux by Porphyra perforata tissues in lithium and magnesium sea water

Cells of Porphyra perforata, a red marine alga, accumulate K in the absence of concomitant Na or Li extrusion while immersed in Li- or Mg-sea waters lacking Na. This suggests that the coupling observed between K and Na transport is facultative. No evidence is obtained for net extrusion of Li. Na efflux, with the concentration gradient, is facilitated by K and is proportional to the cellular Na content. Either Na efflux does not involve an ion carrier or the number of Na sites is large. Because K accumulation has been observed in the absence of Na extrusion, but not vice versa, it seems that K uptake is the primary secretory event, with Na extrusion a secondary process dependent upon K accumulation.

Movement of sodium across the mucosal surface of the isolated toad bladder and its modification by vasopressin

Studies have been made on the isolated urinary bladder of the toad, Bufo marinus, in an attempt to evaluate gradients of chemical activity across the mucosal surfaces of the epithelial cells which would serve to maintain a net movement of sodium from the mucosal medium into the cells. The likelihood of such chemical gradients has been established by the demonstration of lower contents of sodium within the tissue, expressed as microequivalents per gram of tissue water, than of concentrations of sodium in the mucosal medium at all levels of the latter examined. The transepithelial transport of sodium and the sodium content of the tissue were found to increase rapidly with rise in concentration of sodium in the mucosal medium up to values of 30 to 60 meq per liter. Further increase in concentration of the medium above this value failed to induce further stimulation of sodium transport or increase in the sodium content of the tissue. Vasopressin increased the rate of transport of sodium at every concentration of sodium in the mucosal medium without altering this relationship. Although entry of sodium across the mucosal surface of the epithelial cells may be passive it is not by free diffusion but involves some considerable interaction with the mucosal surface of the bladder and constitutes the major determinant of the rate of transepithelial transport of sodium. Vasopressin acts to enhance this initial step in the transport of sodium.

Studies on the carrier function of phosphatidic acid in sodium transport. I. The turnover of phosphatidic acid and phosphoinositide in the avian salt gland on stimulation of secretion

Incubation of slices of the salt gland of the albatross with acetylcholine, which is the physiological secretogogue for this tissue, led to a 13-fold increase in the rate of incorporation of P(32) into phosphatidic acid and a 3-fold increase in the incorporation of P(32) and inositol-2-H(3) into phosphoinositide. The incorporation of P(32) into phosphatidyl choline and phosphatidyl ethanolamine was increased relatively slightly or not at all. Respiration was doubled. The "phospholipid effect" occurred in the microsome fraction, which is known to contain fragments of the endoplasmic reticulum. The enzymes, diglyceride kinase and phosphatidic acid phosphatase, which catalyze the stimulated turnover of phosphatidic acid in brain cortex, were also found in highest concentration in the microsome fraction. The phosphatides which respond to acetylcholine are bound to protein in the membrane. On the basis of these findings it appears that phosphatidic acid and possibly phosphoinositide participate in sodium transport. A scheme, termed the phosphatidic acid cycle, is presented as a working hypothesis, in which the turnover of phosphatidic acid in the membrane, catalyzed by diglyceride kinase and phosphatidic acid phosphatase, functions as a sodium pump.