Se. Anderson et al., EFFECTS OF NA-K-2CL COTRANSPORT INHIBITION ON MYOCARDIAL NA AND CA DURING ISCHEMIA AND REPERFUSION, American journal of physiology. Cell physiology, 39(2), 1996, pp. 608-618
In the context of the ''pump-leak'' hypothesis (37), changes in myocar
dial intracellular Na (Nail during ischemia and reperfusion have histo
rically been interpreted to be the result of changes in Na efflux via
the Na-K pump. We investigated the alternative hypothesis that changes
in Na during ischemia are the result of changes in the Na ''leak'' ra
ther than changes in the pump. More specifically, we hypothesize that
the increase in Nai during ischemia is in part the result of increased
Na uptake mediated by Na/H exchange. Furthermore, we present data con
sistent with the interpretation that the Na-K-2Cl cotransporter is act
ive (or, alternatively, displaced from equilibrium) during ischemia an
d may contribute an additional Na efflux pathway during reperfusion. T
hus inhibition of Na efflux via Na-K-2Cl cotransport during ischemia a
nd reperfusion could result in increased Na-i and therefore decreased
force driving Ca efflux via Na/Ca exchange and ultimately increased in
tracellular Ca concentration I[Ca](i)). Na-i (in meq/kg dry wt) and [C
a](i) (in nM) were measured in isolated Langendorff-perfused rabbit he
arts using nuclear magnetic resonance spectroscopy. Except during the
65 min of ischemia, hearts were perfused with N-2-hydroxyethylpiperazi
ne-N'-2-ethanesulfonic acid-buffered Krebs-Henseleit solution equilibr
ated with 100% O-2 at 23 degrees C and pH 7.4 +/- 0.05. During ischemi
a, Na-i rose from 16.6 +/- 0.3 to 62.9 +/- 5.1 (Delta Na-i similar or
equal to 46) meq/kg dry wt and decreased during subsequent reperfusion
(mean +/- SE, n = 3 hearts). To measure Na uptake (''leak'') in the a
bsence of efflux via the Na-K pump, in all of the protocols described
below the perfusate was nominally K-free solution containing 1 mM ouab
ain for 10 min before ischemia and during the 30-min reperfusion. Afte
r K-free perfusion, Na-i rose from 20.2 +/- 0.5 to 79.1 +/- 5.3 (Delta
Na-i = 59) meq/kg dry wt (n = 3) during ischemia and decreased during
K-free reperfusion. When amiloride (1 mM) was added to the K-free per
fusate to inhibit Na/H exchange, Nai rose from 16.3 +/- 0.9 to 44.7 +/
- 5.1 (Delta Na-i similar or equal to 28) meq/kg dry wt (n = 3) during
ischemia; i.e., amiloride decreased Na uptake. When bumetanide (20 mu
M) was added to the nominally K-free perfusate to inhibit Na-K-2Cl co
transport, Na-i rose from 22.5 +/- 3.9 to 83.8 +/- 13.9 (Delta Na-i si
milar or equal to 61) meq/kg dry wt (n = 3) during ischemia and did no
t decrease during reperfusion; i.e., bumetanide inhibited Na recovery
during reperfusion (P < 0.05 compared with bumetanide free). For the s
ame protocol, the presence of bumetanide resulted in increased [Ca](i)
during ischemia and reperfusion (P < 0.05); these increases in [Ca](i
) are interpreted to be the result of increased Na-i. Thus the results
are consistent with the hypotheses.