COMPARISON OF NA+ K+-ATPASE PUMP CURRENTS ACTIVATED BY ATP CONCENTRATION OR VOLTAGE JUMPS/

Using the giant patch technique, we combined two last relaxation metho ds on excised patches from guinea pig cardiomyocytes to compare the ra te constants of the involved reaction steps. Experiments were done in the absence oi intra- or extracellular K+. Fast ATP concentration jump s. were generated by photolysis of caged ATP at pH 6.3 with laser flas h irradiation at a wavelength of 308 nm and 10 ns duration, as describ ed previously. Transient outward currents with a last rising phase, fo llowed by a slower decay and a small stationary current, were obtained . Voltage pulses were applied to the same patch in the presence or abs ence oi intracellular ATP. Subtraction of the voltage jump-induced cur rents in the absence of ATP from those taken in the presence of ATP yi elded monoexponential transient current signals, which were dependent on external Na+ but did not differ between intracellular pH (pH(i)) va lues 6.3 or 7.4. Rate constants showed a characteristic voltage depend ence, i.e., saturating a positive potentials (similar to 200 s(-1), 24 degrees C) and exponentially rising with increasing negative potentia ls. Rate constants of the last component from transient currents obtai ned after an ATP concentration jump agree well with rate constants fro m currents obtained after a voltage jump to zero or positive potential s (pH(i) 6.3), and the two exhibit the same activation energy of simil ar to 80 kJ.mol(-1). For a given membrane patch, the amount of charge that is moved across the plasma membrane is roughly he same for each o f the two relaxation techniques.