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 2 different electrophysiological techniques. A graphical analysis plotting exp(EmF/RT) vs. [K]o after the pump was blocked by ouabain and 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 (n = 9) or close to 3 Na for 2 K. The 2nd technique used to measure the coupling ratio was to iontophoretically inject Na ions into the neuron. Neurons were penetrated with 3 microelectrodes, 2 of which were filled with 2 M Na-citrate; the 3rd electrode contained either 2 M K-citrate or 3 M KCl. By passing current between the Na salt-containing electrodes, Na was injected into the cell soma. The injection system was calibrated by injecting 24Na-citrate into counting vials from representative microelectrodes (calculated 24Na 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 (n = 19), which is not significantly different from the value obtained by the 1st 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, n = 4). The pump normally operates with a 3:2 ratio both in steady state and under Na load; probably 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.