Coupling ratio of the Na-K pump in the lobster cardiac ganglion.

Abstract

The electrogenic Na-K pump coupling ratio in the large neurons of the lobster cardiac ganglion was determined by two different electrophysiological techniques . A graphical analysis plotting exp(E.F/RT) vs .[K]o after the pump was blocked by ouabain was used to determine values for [K]i, PNa/PK, and the pump coupling ratio. These measurements were made 4-8 h after the cells were penetrated with microelectrodes, and thus represent non-Na-loaded steady state values . The value obtained for the pump coupling ratio under these conditions was 1 .44 ± 0.06 (11 = 9) or close to 3 Na for 2 K. The second technique used to measure the coupling ratio was to iontophoretically inject Na ions into the neuron . Neurons were penetrated with three microelectrodes, two of which were filled with 2 M Na-citrate ; the third electrode contained either 2 M K-citrate or 3 M KCI. By passing current between the Na salt-containing electrodes, Na was injected into the cell soma . The injection system was calibrated by injecting z4 Na-citrate into counting vials from representative microelectrodes (calculated z4 Na transport = 0.92) . By knowing the Na load injected into the cells, and by measuring the time-current area produced by the Na activation of the Na-K pump, the coupling ratio was calculated to be 1 .54 ± 0.05 ()1 = 19), which is not significantly different from the value obtained by the first method . This value represents a Na-loaded experimental situation . When Na was removed from the external bathing solution, the coupling ratio shifted to 2 Na to 1 K (2 .0 ± 0.07, 11 = 4) . These results suggest that the pump normally operates with a 3 :2 ratio both in steady state and under Na load but that in the absence of external Na, it can operate with less than a full complement (2) of K on the external surface of the pump .

Determination of ionic permeability coefficients of the plasma membrane of Xenopus laevis oocytes under voltage clamp.

“…It has been suggested that this stoichiometry is not fixed, depending on internal sodium (Mullin & Brinley, 1969;Gorman & Marmor, 1974), on external potassium (Liberman, 1979) or on external sodium (Livengood, 1983). Lafaire & Schwarz (1986) (1977) it is possible to estimate a net pump flux of 2 7-5 pmol cm-2 s-' (assuming a cell diameter range from 1 to 14 mm), which compares well with our figure; this also falls within values reported in the literature for different systems (Eisner & Lederer, 1980;Cavieres, 1977;Daut & Riidel 1982).…”

“…Substituting the current values in eqn (7a) and dividing 'pNa and IpK by Ip yields a ratio of 3/1P8, which compares well with the ratio 3 Na+/2 K+ which has been attributed to the stoichiometry of the sodium pump in many types of cells. It has been suggested that this stoichiometry is not fixed, depending on internal sodium (Mullin & Brinley, 1969;Gorman & Marmor, 1974), on external potassium (Liberman, 1979) or on external sodium (Livengood, 1983). Lafaire & Schwarz (1986) used dihydroouabain to inhibit the sodium pump in full-P. F. COSTA AiND OTHERS grown oocytes of Xenopus.…”

Reliable, Responsive Pacemaking and Pattern Generation With Minimal Cell Numbers: the Crustacean Cardiac Ganglion

“…A refractory period for initiation of DPs is imposed by a combination of the inactivation of 7 Ca by intracellular accumulation of Ca 2+ as well as the increased K + conductance from the Ca 2 + -activation of 7 KCa . When Na + -mediated impulses are present (in non-TTX-treated preparations), a hyperpolarizing current from activation of electrogenic ion transport also contributes to the afterhyperpolarization (Livengood and Kusano, 1972;Livengood. 1983).…”

“…It is possible that differences in the prominence of pacemaker potentials in recordings from large cells, even in the same species on the same equipment in the same laboratory (e.g., Tazaki and Cooke, 1979a;Benson, 1980;Berlind, 1982), represent differences in techniques of ganglion isolation or of intracellular electrode penetration. During the hyperpolarization following bursts and the interburst interval, the gradual reduction in conductance to K ' or in hyperpolarizing electrogenic active transport (Livengood and Kusano, 1972;Livengood, 1983) can lead to net depolarization as the balance of currents unmasks an outward injury or "leak" current mediated by nonspecific ionic conductances. The greater prominence of pacemaker potentials in small cells, and thus their role in initiating bursting, may be related merely to their smaller size.…”

Inhibition of the Na,K pump by vanadate in high-Na solutions. Modification of the reaction mechanism by external Na acting at a high-affinity site.

“…There is, however, reason to believe that there are two sites for K at the outside of the pump, and that a transport cycle can occur when only one of the sites is combined with K (Sachs, 1977;Kropp and Sachs, 1977 ;Livengood, 1983). If such cycles are possible, one can propose a model in which pumps with both activating sites combined with K, pumps with one site bound to K and the other site empty, and pumps with K bound at one site and Na at the other are all capable oftransport, but only the two Na-free species can combine with vanadate .…”