Abstract
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.
- 15298938
- active,
- after
- animals,
- atpase,
- biological
- biological,
- cardiac,
- cells,
- computer
- cultured,
- energy
- fluorescence
- fluorescence,
- gov't,
- heart
- microscopy,
- models,
- multiphoton,
- myocytes,
- non-u.s.
- p.h.s.,
- photobleaching,
- rats,
- recovery
- research
- resonance
- simulation,
- sodium,
- support,
- transfer,
- transport,
- u.s.
- ventricles,
- {n}a$^{+}$-{k}$^{+}$-exchanging
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