Abstract
The Na$^+$/K$^+$-ATPase (NKA) is the main route for Na$^+$
extrusion from cardiac myocytes. Different NKA alpha-subunit isoforms
are present in the heart. NKA-alpha1 is predominant, although there
is a variable amount of NKA-alpha2 in adult ventricular myocytes
of most species. It has been proposed that NKA-alpha2 is localized
mainly in T-tubules (TT), where it could regulate local Na$^+$/Ca$^2+$
exchange and thus cardiac myocyte Ca$^2+$. However, there is
controversy as to where NKA-alpha1 vs. NKA-alpha2 are localized in
ventricular myocytes. Here, we assess the TT vs. external sarcolemma
(ESL) distribution functionally using formamide-induced detubulation
of rat ventricular myocytes, NKA current (I(Pump)) measurements and
the different ouabain sensitivity of NKA-alpha1 (low) and NKA-alpha2
(high) in rat heart. Ouabain-dependent I(Pump) inhibition in control
myocytes indicates a high-affinity NKA isoform (NKA-alpha2, K(1/2)
= 0.38 +/- 0.16 microM) that accounts for 29.5 +/- 1.3\% of I(Pump)
and a low-affinity isoform (NKA-alpha1, K(1/2) = 141 +/- 17 microM)
that accounts for 70.5\% of I(Pump). Detubulation decreased cell
capacitance from 164 +/- 6 to 120 +/- 8 pF and reduced I(Pump) density
from 1.24 +/- 0.05 to 1.02 +/- 0.05 pA/pF, indicating that the functional
density of NKA is significantly higher in TT vs. ESL. In detubulated
myocytes, NKA-alpha2 accounted for only 18.2 +/- 1.1\% of I(Pump).
Thus, approximately 63\% of I(Pump) generated by NKA-alpha2 is from
the TT (although TT are only 27\% of the total sarcolemma), and the
NKA-alpha2/NKA-alpha1 ratio in TT is significantly higher than in
the ESL. The functional density of NKA-alpha2 is approximately 4.5
times higher in the T-tubules vs. ESL, whereas NKA-alpha1 is almost
uniformly distributed between the TT and ESL.
- /&/
- animals;
- antagonists
- atpase,
- cardiac,
- cytology/enzymology;
- dose-response
- drug
- drug;
- effects/enzymology;
- enzyme
- factors
- formamides,
- heart
- inhibitors,
- inhibitors/metabolism;
- isoenzymes,
- membrane
- metabolism;
- myocytes,
- ouabain,
- pharmacology;
- potentials;
- rats;
- relationship,
- sarcolemma,
- sodium,
- sodium-potassium-exchanging
- time
- ventricles,
Users
Please
log in to take part in the discussion (add own reviews or comments).