CARDIAC NA-VOLTAGE RELATIONSHIPS AT VARIOUS TRANSMEMBRANE GRADIENTS OF THE PUMPED CATIONS( PUMP CURRENT)

Thermodynamic considerations predict changes of the Na+ pump current ( I-p)-voltage (V) relationship of animal cells upon variations of the e lectrochemical gradients against which cations must be pumped. Experim ental data in support of the predictions are sparse. Therefore, the ef fect on the I-p-V relationship of various electrochemical gradients fo r pumped Na+ and Cs+ was studied at constant Delta G(ATP) (approximate to -39 kJ/mol) in cardioballs from sheep Purkinje fibres. Control of the subsarcolemmal ionic concentrations during whole-cell recording wa s ensured by activation of I-p below its half maximal activity or by m easuring the initial I-p following reactivation of the Na+/K+ pump. Wi th gradients close to physiological conditions I-p was outward over th e entire voltage range and the I-p-V relationship showed a maximum nea r zero potential. Steepening the ionic gradients diminished the I-p am plitude and outward pump current was no longer detectable between -65 mV and -110 mV. Flattened ionic gradients increased the I-p amplitude and shifted apparently the reversal potential E(rev) to more negative values. These changes are in line with theoretical considerations. The measured I-p-V relationships were fitted by curves computed on the ba sis of a simplified Post-Albers scheme of Na+/Cs+ pumping. The increas ed I-p amplitude at flat ionic gradients was due to a decrease of [Cs](o) for half maximal I-p activation. The maximal I-p amplitude remain ed unaffected.