Ca 2+ ‐activated K + current is essential for maintaining excitability and gene transcription in early embryonic cardiomyocytes

Aim Activity of early embryonic cardiomyocytes relies on spontaneous Ca 2+ oscillations that are induced by interplay between sarcoplasmic reticulum ( SR ) – Ca 2+ release and ion currents of the plasma membrane. In a variety of cell types, Ca 2+ ‐activated K + current (I K(Ca) ) serves as a link between Ca 2+ signals and membrane voltage. This study aimed to determine the role of I K (Ca) in developing cardiomyocytes. Methods Ion currents and membrane voltage of embryonic (E9‐11) mouse cardiomyocytes were measured by patch clamp; [Ca 2+ ] i signals by confocal microscopy. Transcription of specific genes was measured with RT ‐ qPCR and Ca 2+ ‐dependent transcriptional activity using NFAT ‐luciferase assay. Myocyte structure was assessed with antibody labelling and confocal microscopy. Results E9‐11 cardiomyocytes express small conductance ( SK ) channel subunits SK 2 and SK 3 and have a functional apamin‐sensitive K + current, which is also sensitive to changes in cytosolic [Ca 2+ ] i . In spontaneously active cardiomyocytes, inhibition of I K (Ca) changed action and resting potentials, reduced SR Ca 2+ load and suppressed the amplitude and the frequency of spontaneously evoked Ca 2+ oscillations. Apamin caused dose‐dependent suppression of NFAT ‐luciferase reporter activity, induced downregulation of a pattern of genes vital for cardiomyocyte development and triggered changes in the myocyte morphology. Conclusion The results show that apamin‐sensitive I K (Ca) is required for maintaining excitability and activity of the developing cardiomyocytes as well as having a fundamental role in promoting Ca 2+ ‐ dependent gene expression.