Potassium (K) depletion is a very common clinical entity often associated with adverse cardiac effects. K depletion is generally considered to reduce muscular Na,K-ATPase density and secondarily reduce K uptake capacity. In K depleted rats we evaluated myocardial Na,K-ATPase density, ion contents and myocardial K re-uptake. K depletion for 2 weeks 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) and increased Mg to 11.0 ± 0.4 vs. 9.6 ± 0.2 µmol/g wet wt. (p<0.05, n=6). A tendency to a rise in Na was seen. Myocardial Na,K-ATPase alpha 2 subunit abundance measured by immunoblotting and ³H-ouabain binding was significantly reduced by 28% (n=6) and 29% (n=6), respectively, whereas significant increases in alpha 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. K depleted rats tolerated significantly more KCl. KCl infusion until animals died increased myocardial K by 34% in K depelted, 18% in controls (p<0.05, n=6 for both), but did not induce different net K uptake rates between groups. However, KCl infusion, clamping plasma K at ~5.5 mM caused a higher net K uptake rate in K depleted rats (0.22 ± 0.04 vs. 0.10 ± 0.03 µmol/g wet wt./min, p<0.05, n=8). In conclusion, a minor K depletion-induced decrease in myocardial K was associated with increased Na,K-pump density and in vivo increases in K tolerance and net myocardial K uptake rate during K repletion. Thus, the heart is not only protected from major K losses, but can accumulate considerable amounts of K during exposure to high plasma K. This may be of major clinical interest, as a therapeutically induced rise in myocardial K may affect contractility, impulse generation-propagation as well as attenuate a rise in myocardial Na, the hallmark of heart failure.