Figure 1.

Hypokalemia promotes a steady-state increase in Ca²⁺ transients and Ca²⁺ waves in ventricular myocytes and a subpopulation of atrial myocytes. A, In field-stimulated ventricular cells, rapidly lowering [K⁺]_o from 5.0 to 2.7 mmol/L produced an initial depression of Ca²⁺ transient magnitude. A secondary rising phase followed which ultimately yielded larger Ca²⁺ transients compared with control conditions (right, n=15 cells, 8 hearts). A similar biphasic response to hypokalemia was observed in some atrial cardiomyocytes (13 of 31 cells, 10 hearts), with associated over-activity (arrow). Other atrial cells exhibited only a monophasic reduction in Ca²⁺ transient amplitude. B, Ca²⁺ waves were assessed during pauses in the electrical stimulus. Ventricular cells and those atrial cells which exhibited a biphasic response demonstrated an increased frequency of Ca²⁺ waves during hypokalemia. For Ca²⁺ wave measurements, n_cells=17, 7, 12; n_hearts=10, 5, 11 in ventricular, biphasic atrial, and monophasic atrial populations. Statistics: Wilcoxon signed-rank test.