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MEMBRANE TRANSPORTERS, ION CHANNELS, AND PUMPS

Potassium depletion improves myocardial potassium uptake in vivo

Medical Department B 2142, Heart Centre, Rigshospitalet, National University Hospital, University of Copenhagen, 2100 Copenhagen, Denmark

Submitted 23 December 2003 ; accepted in final form 27 February 2004

Potassium depletion (KD) is a very common clinical entity often associated with adverse cardiac effects. KD is generally considered to reduce muscular Na-K-ATPase density and secondarily reduce K uptake capacity. In KD rats we evaluated myocardial Na-K-ATPase density, ion content, and myocardial K reuptake. KD for 2 wk reduced plasma K to 1.8 ± 0.1 vs. 3.5 ± 0.2 mM in controls (P < 0.01, n = 7), myocardial K to 80 ± 1 vs. 86 ± 1 µmol/g wet wt (P < 0.05, n = 7), increased Mg, and induced a tendency to increased Na. Myocardial Na-K-ATPase ₂-subunit abundance was reduced by 30%, whereas increases in ₁- and K-dependent pNPPase activity of 24% (n = 6) and 13% (n = 6), respectively, were seen. This indicates an overall upregulation of the myocardial Na-K pump pool. KD rats tolerated a higher intravenous KCl dose. KCl infusion until animals died increased myocardial K by 34% in KD rats and 18% in controls (P < 0.05, n = 6 for both) but did not induce different net K uptake rates between groups. However, clamping plasma K at 5.5 mM by KCl infusion caused a higher net K uptake rate in KD rats (0.22 ± 0.04 vs. 0.10 ± 0.03 µmol·g wet wt^–1·min^–1; P < 0.05, n = 8). In conclusion, a minor KD-induced decrease in myocardial K increased Na-K pump density and in vivo increased K tolerance and net myocardial K uptake rate during K repletion. Thus the heart is protected from major K losses and accumulates considerable amounts of K during exposure to high plasma K. This is of clinical interest, because a therapeutically induced rise in myocardial K may affect contractility and impulse generation-propagation and may attenuate increased myocardial Na, the hallmark of heart failure.

Na-K-ATPase; ion homeostasis; heart failure; iatrogenic potassium depletion