Figure 4.

Decreased Na⁺, K⁺-ATPase (NKA) activity during hypokalemia inhibits Ca²⁺ extrusion by t-tubular Na⁺-Ca²⁺ exchanger (NCX), elevating sarcoplasmic reticulum (SR) Ca²⁺ content. A, NKA activity was measured during hyperpolarizing voltage ramps and calculated as the K⁺-sensitive current (see Online Figure I for protocol and representative traces). Hypokalemia induced a modest and similar reduction in NKA current in ventricular myocytes and atrial cells, regardless of the presence of t-tubules (n_cells=10, 7, 9; n_hearts=4, 4, 5 in ventricular, tubulated atrial, untubulated atrial cells). B, SBFI experiments revealed no change in global cytosolic [Na⁺] during the protocol (n_cells=12, 8; n_hearts=4, 4 for ventricular, atrial cells). C, However, tubulated cells showed slowed Ca²⁺ removal by NCX during hypokalemia, as indicated by the declining phase of Ca²⁺ transients stimulated in the continuous presence of 10 mmol/L caffeine (change in tau values shown at right, n_cells=8, 16; n_hearts=3, 5, in ventricular, tubulated atrial cells, P=0.016, 0.011 vs K_o=5.0 by Wilcoxon signed-rank test). No change in Ca²⁺ removal rate was observed in untubulated atrial cells (n_cells=16; n_hearts=5; P=0.912). Transient magnitude during K_o=2.7+caffeine: F/F₀=1.46±0.07, 1.66±0.12, 1.57±0.08 in ventricular, tubulated, and atrial cells. D, Ventricular cells and tubulated atrial cells exhibited increased SR Ca²⁺ content during hypokalemia, as assessed by the magnitude of caffeine-induced Ca²⁺ release (n_cells=8, 7, 7; n_hearts=6, 6, 5 in ventricular, tubulated atrial, untubulated atrial cells; representative traces illustrated in Online Figure VI). Statistics: (A): 2-way repeated measures ANOVA with Bonferroni correction (see Online Table II for full results); (B): Kruskal-Wallis test (difference in medians: P=1.000, 1.000 in ventricular, atrial cells); (C): Kruskal-Wallis test with Dunn correction (difference in medians: P=0.023); (D): paired t-test. *P<0.05 vs K_o=5.0.