CHANGES IN SUBSARCOLEMMAL SODIUM CONCENTRATION MEASURED BY NA-CA EXCHANGER ACTIVITY DURING NA-PUMP INHIBITION AND BETA-ADRENERGIC STIMULATION IN GUINEA-PIG VENTRICULAR MYOCYTES

Measurement of Na-Ca exchange activity was used to examine subsarcolem mal sodium levels ([Na+](s)) in single, voltage-clamped guinea-pig car diac myocytes while Na-K pump activity was modulated pharmacologically . Changes in Na-s were evaluated from phase-plane analysis of the chan ges in intracellular calcium, measured using the fluorescent indicator s Fura-red and Fluo-3. Activation of beta-adrenoceptors with 1 mu M is oprenaline resulted in activation of the cAMP-dependent chloride curre nt, but had no effect on the calcium transient mediated via the Na-Ca exchanger, regardless of whether the Na-K pump was active or inhibited (with strophanthidin). The ability of Na-Ca exchange activity to repo rt [Na+](s) was demonstrated by the effect of changing the extracellul ar rubidium concentration from 1 to 5.4 mM to modulate Na-K pump activ ity. We suggest that beta-adrenergic stimulation does not directly aff ect either the Na-K pump or the Na-Ca exchanger and that the Na-Ca exc hanger can be used as a sensitive indicator of changes in [Na+](s) and Na-K pump activity.