Functional coupling of calcineurin and protein kinase A in mouse ventricular myocytes

We examined the role of the Ca2+‐regulated protein phosphatase calcineurin in controlling Ca2+ signalling in mouse ventricular myocytes. Membrane currents and voltage were measured in single myocytes using the patch‐clamp technique. Cytoplasmic Ca2+ concentration ([Ca2+]i) was measured in cells loaded with the fluorescent Ca2+ indicators fluo‐4 or fura‐2 using a confocal or epifluorescence microscope. Inhibition of calcineurin with cyclosporin A (CsA, 100 nm) or the calcineurin auto‐inhibitory peptide (CiP, 100 μM), increased the amplitude and rate of decay of the evoked [Ca2+]i transient and also prolonged the action potential (AP) of ventricular myocytes to a similar extent. The effects of CsA (100 nm) and 100 μM CiP on the [Ca2+]i transient and AP were not additive. Calcineurin inhibition did not modify the K+ currents responsible for repolarisation of the mouse ventricle. Instead, inhibition of calcineurin increased the amplitude of the Ca2+ current (ICa) and the evoked calcium transient normalized to the ICa. Calcium sparks, which underlie the [Ca2+]i transient, had a higher frequency and amplitude, suggesting an elevation of SR calcium load. Inhibition of protein kinase A (PKA) prevented the effects of calcineurin inhibition, indicating that calcineurin opposes the actions of PKA. Finally, immunofluorescence images suggest that calcineurin and PKA co‐localize near the T‐tubules of ventricular myocytes. We propose that calcineurin and PKA are co‐localized to control Ca2+ influx through calcium channels and calcium release through ryanodine receptors.