Na+,K(+)-ATPase pump currents in giant excised patches activated by an ATP concentration jump

Na+,K(+)-ATPase pump currents activated by an ATP concentration jump. Comparison of studies with purified membrane fragments and giant excised patches

The lipid bilayer and the giant patch technique were used to study the Na+,K(+)-ATPase. In excised patches from ventricular myocytes, Na+,K(+)-pump currents show a saturable ATP dependence with a Km of approximately 150 microM at 24 degrees C. Partial reactions in the transport cycle were investigated by generating ATP concentration jumps through photolytic release of ATP from caged ATP at pH 7.4 and 6.3. Transient outward currents were obtained at pH 6.3 with a fast rising phase followed by a slow decay to a stationary current. Experiments with purified pig kidney Na+,K(+)-ATPase attached to a planar lipid bilayer resulted in similar pump current signals and the same outcome. It was concluded that the fast rate constant of approximately 200 s-1 at 24 degrees C reflects a step rate limiting the electrogenic Na+ release.

Na/K pump current in guinea pig cardiac myocytes and the effect of Na leak

INTRODUCTION

Steady-state Na/K pump current (Ip) in adult guinea pig ventricular myocytes was studied to determine the effect on the Na/K pump of transmembrane Na leak, membrane potential, and pipette Na concentration.

METHODS AND RESULTS

Using conventional whole cell, patch clamp techniques, Ip was identified as either Ko-sensitive or ouabain-sensitive current when most other membrane currents were inhibited. Control experiments showed that there were no Ko-sensitive currents other than Ip under the conditions of our experiments. Ip was found to be similar to that reported by others being voltage dependent between -130 and 0 mV and having a half maximal activation by Nai of 28 mM. Ouabain sensitivity was also measured, and it was found that there were two binding sites with the high affinity site comprising 5% to 10% of the total and having an apparent affinity 1000-fold higher than the low affinity site. Apparent affinity of both sites was shifted about 10-fold (higher affinity) by increasing Nai from 10 to 85 mM. When internally perfused with 0 Na solution, Na leak through the membrane was found to be linearly related to Na/K pump activity. In contrast to prior suggestions, Ip was not correlated with series resistance when there was a large transmembrane Na gradient.

CONCLUSION

These data suggest that, under conditions of high transmembrane Na gradient, Na leak through the membrane plays a significant role in determining Na/K pump activity.

Comparison of Na+/K(+)-ATPase pump currents activated by ATP concentration or voltage jumps

Using the giant patch technique, we combined two fast relaxation methods on excised patches from guinea pig cardiomyocytes to compare the rate constants of the involved reaction steps. Experiments were done in the absence of intra- or extracellular K+. Fast ATP concentration jumps were generated by photolysis of caged ATP at pH 6.3 with laser flash irradiation at a wavelength of 308 nm and 10 ns duration, as described previously. Transient outward currents with a fast rising phase, followed by a slower decay and a small stationary current, were obtained. Voltage pulses were applied to the same patch in the presence or absence of intracellular ATP. Subtraction of the voltage jump-induced currents in the absence of ATP from those taken in the presence of ATP yielded monoexponential transient current signals, which were dependent on external Na+ but did not differ between intracellular pH (pHi) values 6.3 or 7.4. Rate constants showed a characteristic voltage dependence, i.e., saturating at positive potentials (approximately 200 s-1, 24 degrees C) and exponentially rising with increasing negative potentials. Rate constants of the fast component from transient currents obtained after an ATP concentration jump agree well with rate constants from currents obtained after a voltage jump to zero or positive potentials (pHi 6.3), and the two exhibit the same activation energy of approximately 80 kJ.mol-1. For a given membrane patch, the amount of charge that is moved across the plasma membrane is roughly the same for each of the two relaxation techniques.