title: Rapid inhibition of the Na+-K+ pump affects Na+-Ca2+ exchanger-mediated relaxation in rabbit ventricular myocytes.
abstract: The direct influence of Na+-K+ pump activity on the ability of the Na+-Ca2+ exchanger to remove Ca2+ was investigated in isolated adult rabbit ventricular myocytes. Cell shortening was measured using an edge-detection system. Cytoplasmic [Ca2+] was monitored using the fluorescent indicator indo-1. Electrophysiological parameters were recorded using high-resistance microelectrodes. The Na+-K+ pump was rapidly inhibited by removal of extracellular K+ and measurements were taken almost immediately to minimise effects on other cellular compartments. Activity of the Na+-Ca2+ exchanger was monitored during release of Ca2+ from the sarcoplasmic reticulum (SR) elicited by rapid application of 15 mM caffeine. When Na+-K+ pump activity was affected by K+ removal, cell relaxation and indo-1 fluorescence decline were slowed by approximately 40 %. The charge calculated by integrating the caffeine-induced transient inward current was unchanged, suggesting that there was no difference in the SR Ca2+ content in the two conditions. However Ca2+ flux via the Na+-Ca2+ exchanger was slower when the Na+-K+ pump was inhibited. Similar experiments were performed by inhibiting the Na+-K+ pump using 0.5 mM strophanthidin. In this condition similar results to the ones observed by K+ removal were obtained, suggesting a specific role of the Na+-K+ pump in the phenomenon observed. This study suggests that the activity of the Na+-K+ pump influences Na+-Ca2+ exchanger function in the absence of changes in SR Ca2+ content. This can be explained by a slower removal of Na+ from the subsarcolemmal space. The source of the increase in subsarcolemmal [Na+] requires further investigation. However, calculations derived from modelling suggest that the Na+-Ca2+ exchanger itself could be involved.
authors.affiliation: Imperial College School of Medicine, National Heart and Lung Institute, London, UK. c.terracciano@ic.ac.uk
authors.forename: C M
authors.initials: CM
authors.lastname: Terracciano
mesh_terms: D000818:Animals; D002110:Caffeine / Q000494:pharmacology; D005456:Fluorescent Dyes / Q000494:pharmacology; D006352:Heart Ventricles / Q000166:cytology; D066298:In Vitro Techniques; D007211:Indoles / Q000494:pharmacology; D008297:Male; D008564:Membrane Potentials / Q000187:drug effects / Q000502:physiology; D018485:Muscle Fibers, Skeletal / Q000502:physiology*; D009200:Myocardial Contraction / Q000502:physiology*; D009206:Myocardium / Q000166:cytology*; D010726:Phosphodiesterase Inhibitors / Q000494:pharmacology; D011188:Potassium / Q000493:pharmacokinetics; D011817:Rabbits; D012519:Sarcoplasmic Reticulum / Q000378:metabolism; D012964:Sodium / Q000378:metabolism; D019831:Sodium-Calcium Exchanger / Q000378:metabolism*; D000254:Sodium-Potassium-Exchanging ATPase / Q000378:metabolism*
chemical_list: D005456:Fluorescent Dyes; D007211:Indoles; D010726:Phosphodiesterase Inhibitors; D019831:Sodium-Calcium Exchanger; D002110:Caffeine; D012964:Sodium; D000254:Sodium-Potassium-Exchanging ATPase; C048960:indo-1; D011188:Potassium