Neurally-mediated increase in calcineurin activity regulates cardiac contractile function in absence of hypertrophy

Objective: The calcineurin pathway has been involved in the development of cardiac hypertrophy, yet it remains unknown whether calcineurin activity can be regulated in myocardium independently from hypertrophy and cardiac load. Methods: To test that hypothesis, we measured calcineurin activity in a rat model of infrarenal aortic constriction (IR), which affects neurohormonal pathways without increasing cardiac afterload. Results: In this model, there was no change in arterial pressure over the 4-week experimental period, and the left ventricle/body weight ratio did not increase. At 2 weeks after IR, calcineurin activity was increased 1.8-fold (P<0.05) and remained elevated at 4 weeks (1.7-fold, P<0.05). Similarly, the cardiac activity of calcium calmodulin kinase II (CaMKII) was increased significantly after IR, which confirms a regulation of Ca2+-dependent enzymes in this model. In cardiac myocytes, the increased activity of calcineurin was accompanied by a significant decrease in L-type Ca2+ channel activity (I-Ca) and contraction velocity (-dL/dt). Cardiac denervation prevented the activation of calcineurin after IR, which demonstrates that a neurohormonal mechanism is responsible for the changes in enzymatic activity. In addition, cardiac denervation suppressed the effects of IR on I-Ca and -dL/dt, which shows that calcineurin activation is related to altered contractility. However, action potential duration, the densities of inward rectifier K currents (I-K1), and outward K+ currents (I-to and I-k) were not altered in IR myocytes. Conclusions: Calcineurin can be activated in the heart through a neural stimulus, which induces alterations in Ca2+ currents and contractility. These effects occur in the absence of myocyte hypertrophy, electrophysiological changes in action potential, and K+ channel currents. (C) 2003 European Society of Cardiology. Published by Elsevier B.V. All rights reserved.