Effects Of Neuronal Nitric Oxide Synthase Signaling On Myocyte Contractile Function PDF Download

Abstract: Exercise is beneficial to one's health, reduces the risk of cardiomyopathies, and is utilized as a therapeutic intervention after disease [2-7]. This is due, in part, due to the beneficial chronic adaptations that enhance contraction and accelerate relaxation [8]. These intrinsic exercise-induced adaptations are observed at the level of the cardiomyocyte [9]. That is, ventricular myocytes from exercised (Ex) mice exhibit increased Ca2+ cycling and contraction-relaxation rates [6, 9-12]. Additionally, cardiac growth (physiological hypertrophy) and an increase in aerobic fitness (VO2max) are hallmark cardiac adaptations due to exercise training. The molecular mechanisms that explain how the heart adapts are not fully understood and studies examining signaling pathways are limited. A signaling molecule with a potential role in cardiac adaptations to exercise is nitric oxide (NO). Nitric Oxide (NO) has been shown to be a key regulator of myocyte contractile function. NO, produced via the neuronal nitric oxide synthase (nNOS or NOS1), enhances basal contraction by increasing Ca2+ cycling through the sarcoplasmic reticulum (SR) [13-15]. Data suggest that NOS1 signaling increases Ca2+ uptake by targeting the SR Ca2+ ATPase (SERCA2a)/phospholamban (PLB) complex. NOS1 signaling also targets the SR Ca2+ release channel (ryanodine receptor - RYR2) to increase its open time probability [16]. Together, NOS1 signaling increases Ca2+ transient amplitudes, shortening amplitudes, and accelerates relaxation rates [14, 16-19]. These are similar effects to exercise adaptations, but the role of NOS1 signaling on the beneficial effects of exercise on cardiac myocyte function has not been thoroughly investigated. After an 8 week aerobic interval training program, Ex mice had a higher VO2max and a physiological hypertrophy compared to sedentary (Sed) wildtype (WT) mice. Exercise induced an increase in NOS1 expression and nitric oxide production. Isolated ventricular myocytes from the Ex mice exhibited larger contraction and faster relaxation rates compared to Sed myocytes. Acute NOS1 inhibition with S-methyl-L-thiocitrulline (SMLT) resulted in a greater reduction in Ca2+ transient amplitude, Ca2+ transient RT50, shortening amplitude, SR Ca2+ load, and SR Ca2+ fractional release in Ex versus Sed. In fact, acute NOS1 inhibition normalized the Ex induced increase in contraction and Ca2+ decline rates to Sed levels. The NOS1 mediated effect on contraction was due to a shift in the kinase/phosphatase balance to increase PLB Serine16 phosphorylation (the PKA site). Surprisingly, trained NOS1KO mice, did not exhibit any of the cardiac adaptations. That is, Ex-NOS1KO mice did not have increased VO2max or hypertrophy compared to Sed-NOS1KO mice. In fact, Ex-NOS1KO mice had depressed Ca2+ transient amplitude, SR Ca2+ load, and slowed Ca2+ transient RT50 compared to Sed-NOS1KO. Upon further investigation, this resulted from elevated reactive oxygen species levels that contributed to increase protein phosphatase activity and subsequently decrease PLB Serine16 phosphorylation to cause detrimental Ca2+ handling. Lastly, we observed a similar effect in an exercise-trained canine model. Specifically, NOS1 inhibition elicited a greater reduction in myocyte contraction in Ex versus Sed. These data strongly suggest a more universal role for exercise induced enhancement of NOS1 signaling in both large and small mammalian species In conclusion, NOS1 signaling contributes to the adaptive cardiac effects of exercise. Specifically, exercise increases ventricular myocyte NOS1 expression and NO bioavailability, which is essential for aerobic fitness, hypertrophy, and enhanced contraction/relaxation. Hence, it may be possible to mimic the beneficial effects of exercise to the heart by enhancing NOS1 signaling. This pathway may provide a novel therapeutic for cardiac patients that are unable/unwilling to exercise.