Figure 3.

The NKA (Na⁺, K⁺-ATPase) and Na⁺-Ca²⁺ exchanger (NCX) are robustly expressed within the t-tubules of both ventricular and atrial cardiomyocytes. A, Immunolabeling in intact tissue and isolated ventricular cells showed a robust expression of the α₁ NKA isoform in both the surface membrane and t-tubules, while the α₂ isoform was preferentially expressed in t-tubules. B, In atrial cells, NKA α₁ was expressed in both the surface membrane and, when present, the t-tubules. NKA α₂ expression was low in atrial cells, never present in t-tubules, and appeared limited to the intercalated disks. NCX (right) was robustly expressed in both the surface membrane and t-tubules of ventricular and atrial cells. C, In ventricular cells, preferential blockade of NKA α₂ with 0.3 µmol/L ouabain¹⁹ inhibited the biphasic response of Ca²⁺ transients to hypokalemia (5 of 5 cells). In atrial cells, both biphasic (7 cells) and monophasic responses (3 cells) continued to be observed in the presence of low-dose ouabain, in agreement with limited α₂ expression in these cells. Scale bars in (A) and (B)=10 µm in zoomed-out images, 2 µm in enlargements.