Isoproterenol does not enhance Ca-dependent Na/Ca exchange current in intact rabbit ventricular myocytes.
Cardiac Na/Ca exchange (NCX, NCX1.1) is critical in cardiac myocyte Ca regulation, and its altered function contributes to inotropic state, systolic dysfunction in heart failure and arrhythmogenesis. Regulation of NCX is multifaceted, but protein kinase A (PKA) effects on NCX function are controversial. Here, we use three different and complementary approaches to compare NCX function +/-1 microM isoproterenol (ISO) in intact rabbit cardiac myocytes (in paired comparisons). First, in field-stimulated intact cells we inferred the cytosolic [Ca] ([Ca](i)) dependence of NCX function from the decay rate of caffeine-induced [Ca](i) transients. Second, we measured caffeine-induced [Ca](i) and inward I(NCX) simultaneously (perforated patch voltage clamp), to measure directly the [Ca](i) dependence of NCX rate. Third, using whole cell ruptured patch with [Ca](i) heavily buffered to 100 nM, [Na](i)=10 mM, and I(Ca), SR Ca release and Na/K pump all blocked, we recorded I(NCX) ramps at 37 degrees C. We find that NCX function is not altered by PKA activation under any of these three protocols, where intracellular conditions ranged from near-physiological to highly controlled. This does not rule out NCX modulation by PKA under all conditions, or in species other than rabbit. However, such effects are likely to be either minor (vs. other PKA actions on myocyte Ca handling) or indirect, such as secondary effects dependent on altered local [Ca](i) and [Na](i).
Ginsburg
Kenneth S
KS
Department of Physiology, Loyola University Chicago, Stritch School of Medicine, 2160 South First Avenue, Maywood, IL 60153, USA.
Bers
Donald M
DM
D000818:Animals; D001145:Arrhythmias, Cardiac / Q000378:metabolism; D002110:Caffeine / Q000494:pharmacology; D002316:Cardiotonic Agents / Q000494:pharmacology*; D002478:Cells, Cultured; D006333:Heart Failure / Q000378:metabolism; D006352:Heart Ventricles / Q000166:cytology / Q000201:enzymology; D017136:Ion Transport / Q000187:drug effects; D007545:Isoproterenol / Q000494:pharmacology*; D032383:Myocytes, Cardiac / Q000166:cytology / Q000201:enzymology*; D010726:Phosphodiesterase Inhibitors / Q000494:pharmacology; D011188:Potassium / Q000378:metabolism*; D011817:Rabbits; D012964:Sodium / Q000378:metabolism*; D000254:Sodium-Potassium-Exchanging ATPase
D002316:Cardiotonic Agents; D010726:Phosphodiesterase Inhibitors; D002110:Caffeine; D012964:Sodium; D000254:Sodium-Potassium-Exchanging ATPase; D007545:Isoproterenol; D011188:Potassium