The TRPM4 channel is functionally important for the beneficial cardiac remodeling induced by endurance training

Cardiac hypertrophy (CH) is an adaptive process that exists in two distinct forms and allows the heart to adequately respond to an organism's needs. The first form of CH is physiological, adaptive and reversible. The second is pathological, irreversible and associated with fibrosis and cardiomyocyte death. CH involves multiple molecular mechanisms that are still not completely defined but it is now accepted that physiological CH is associated more with the PI3-K/Akt pathway while the main signaling cascade activated in pathological CH involves the Calcineurin-NFAT pathway. It was recently demonstrated that the TRPM4 channel may act as a negative regulator of pathological CH by regulating calcium entry and thus the Cn-NFAT pathway. In this study, we examined if the TRPM4 channel is involved in the physiological CH process. We evaluated the effects of 4 weeks endurance training on the hearts of Trpm4 ^+/+ and Trpm4 ^-/- mice. We identified an elevated functional expression of the TRPM4 channel in cardiomyocytes after endurance training suggesting a potential role for the channel in physiological CH. We then observed that Trpm4 ^+/+ mice displayed left ventricular hypertrophy after endurance training associated with enhanced cardiac function. By contrast, Trpm4 ^-/- mice did not develop these adaptions. While Trpm4 ^-/- mice did not develop gross cardiac hypertrophy, the cardiomyocyte surface area was larger and associated with an increase of Tunel positive cells. Endurance training in Trpm4 ^+/+ mice did not increase DNA fragmentation in the heart. Endurance training in Trpm4 ^+/+ mice was associated with activation of the classical physiological CH Akt pathway while Trpm4 ^-/- favored the Calcineurin pathway. Calcium studies demonstrated that TRPM4 channel negatively regulates calcium entry providing support for activation of the Cn-NFAT pathway in Trpm4 ^-/- mice. In conclusion, we provide evidence for the functional expression of TRPM4 channel in response to endurance training. This expression may help to maintain the balance between physiological and pathological hypertrophy.