Changes in subsarcolemmal sodium concentration measured by Na-Ca exchanger activity during Na-pump inhibition and β-adrenergic stimulation in guinea-pig ventricular myocytes

Abstract

Measurement of Na-Ca exchange activity was used to examine subsarcolemmal sodium levels ([Na+]s) in single, voltage-clamped guinea-pig cardiac myocytes while Na-K pump activity was modulated pharmacologically. Changes in Nas were evaluated from phase-plane analysis of the changes in intracellular calcium, measured using the fluorescent indicators Fura-red and Fluo-3. Activation of beta-adrenoceptors with 1 microM isoprenaline resulted in activation of the cAMP-dependent chloride current, 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 report [Na+]s was demonstrated by the effect of changing the extracellular rubidium concentration from 1 to 5.4 mM to modulate Na-K pump activity. We suggest that beta-adrenergic stimulation does not directly affect either the Na-K pump or the Na-Ca exchanger and that the Na-Ca exchanger can be used as a sensitive indicator of changes in [Na+]s and Na-K pump activity.

β-Adrenergic stimulation does not activate Na+/Ca2+ exchange current in guinea pig, mouse, and rat ventricular myocytes

“…However, this augmentation was most likely due to the activation of I CFTR-Cl , which is also sensitive to Ni 2ϩ . Our results support previous studies (2,5,13,20) and a recent report of studies of rabbit ventricular cells by Ginsburg and Bers (11). Using guinea pig ventricular cells, Perchenet et al (25) and Zhang et al (33) reported the enhancement of I NCX by ␤-adrenergic stimulation.…”

“…(30) also reported that PKA phosphorylated the Na ϩ /Ca 2ϩ exchange protein and increased I NCX by ϳ500% in control and by ϳ100% in failing ventricular cells from pig heart, suggesting that cardiac Na ϩ / Ca 2ϩ exchange is hyperphosphorylated in heart failure. To the contrary, no stimulatory effect of ␤-adrenergic receptor stimulation or PKA on exchange activity was detected in giant membrane patches excised from blebs of guinea pig ventricular cells (5), vesicle preparations of rat hearts (2), guinea pig ventricular cells (20), or BHK cells expressing the dog cardiac NCX1 (13).…”

Full-length cardiac Na+/Ca2+ exchanger 1 protein is not phosphorylated by protein kinase A

“…While some have observed increased NCX current after treatment with PKA activators (26,33,44), others have not observed any change (9,20,21). We were unable to detect PKA-dependent phosphorylation of the full-length NCX1 protein and suggest that the discrepancy between previous functional studies likely results from experimental differences in ionic homeostasis.…”

Pharmacology of Brain Na+/Ca2+Exchanger: From Molecular Biology to Therapeutic Perspectives

“…Inorganic Cations (Colvin et al, 2000), and Ni 2ϩ (Kimura et al, 1987) , and Cd 2ϩ , may also function as substrates for the exchanger (Iwamoto and Shigekawa, 1998b). Among these divalent cations, Ni 2ϩ is the element most commonly used for blocking NCX activity during electrophysiological measurements (Fujioka et al, 1998;Main et al, 1997). The use of Ni 2ϩ as a NCX inhibitor is limited because its blocking concentrations are in the order of 2 to 5 mM, levels at which Ni 2ϩ is also able to inhibit other membrane currents (Iwamoto and Shigekawa, 1998b).…”