Na/K pump-induced [Na](i) gradients in rat ventricular myocytes measured with two-photon microscopy.
Via the Na/Ca and Na/H exchange, intracellular Na concentration ([Na](i)) is important in regulating cardiac Ca and contractility. Functional data suggest that [Na](i) might be heterogeneous in myocytes that are not in steady state, but little direct spatial information is available. Here we used two-photon microscopy of SBFI to spatially resolve [Na](i) in rat ventricular myocytes. In vivo calibration yielded an apparent K(d) of 27 +/- 2 mM Na. Similar resting [Na](i) was found using two-photon or single-photon ratiometric measurements with SBFI (10.8 +/- 0.7 vs. 11.1 +/- 0.7 mM). To assess longitudinal [Na](i) gradients, Na/K pumps were blocked at one end of the myocyte (locally pipette-applied K-free extracellular solution) and active in the rest of the cell. This led to a marked increase in [Na](i) at sites downstream of the pipette (where Na enters the myocyte and Na/K pumps are blocked). [Na](i) rise was smaller at upstream sites. This resulted in sustained [Na](i) gradients (up to approximately 17 mM/120 microm cell length). This implies that Na diffusion in cardiac myocytes is slow with respect to trans-sarcolemmal Na transport rates, although the mechanisms responsible are unclear. A simple diffusion model indicated that such gradients require a Na diffusion coefficient of 10-12 microm(2)/s, significantly lower than in aqueous solutions.
Despa
Sanda
S
Department of Physiology, Loyola University Chicago, Maywood, Illinois, USA.
Kockskämper
Jens
J
Blatter
Lothar A
LA
Bers
Donald M
DM
D000818:Animals; D001693:Biological Transport, Active / Q000502:physiology; D002478:Cells, Cultured; D003198:Computer Simulation; D036681:Fluorescence Recovery After Photobleaching / Q000379:methods; D031541:Fluorescence Resonance Energy Transfer / Q000379:methods; D006352:Heart Ventricles / Q000166:cytology / Q000378:metabolism; D036641:Microscopy, Fluorescence, Multiphoton / Q000379:methods*; D008954:Models, Biological; D032383:Myocytes, Cardiac / Q000166:cytology / Q000378:metabolism*; D051381:Rats; D012964:Sodium / Q000378:metabolism*; D000254:Sodium-Potassium-Exchanging ATPase / Q000378:metabolism*
D012964:Sodium; D000254:Sodium-Potassium-Exchanging ATPase