Mechanism Of Calcitonin Gene Related Peptide In Exercise-induced Cardiac Remodeling And Cardioprotection

Objective: The profound heart changes including morphological enlargement, functional improvement, capacity elevation that occur in athletes are a normal adaptive response to chronic athletic training, which have been recognized as the athlete's heart. The notion has been accepted that the process of exercise-induced cardiac remodeling is not only the alternation of myocardium volume and responding ultrastructure by hemodynamic overload, but also the re-establishment of cardiac morphology, function and metabolism under the regulation of nerves and hormones. Calcitonin gene-related peptide (CGRP) is one of the most potent vasodilator substances identified to date. Biological effects of CGRP include increasing cardiac output, elevating coronary blood flow, and significant prevention against ischemia-reperfusion injury. Consequently, CGRP is thought to play a vital role in cardiac remodeling and cardioprotection. The fact that plasma and myocardial concentration of CGRP increased after systematic endurance training has indicated that CGRP is a candidate regulator of exercise-induced cardiac remodeling and cardioprotection. At present, the focus is mostly on the changes of plasma and myocardial content of CGRP during or/ and after exercise, no comprehensive and systematic data exist on the effect of exercise on CGRP, especially for its synthesis and gene expression. The purpose of the present study was to investigate the changes and possible mechanism of CGRP involved with exercise-induced cardiac remodeling and cardioprotection following the establishment of animal model of exercise-induced cardiac hypertrophy, to provide the newer theory and experiment foundation for optimal sports program, effective prevention of cardiac injury, heart health promotion, exercise prescription for cardiovascular disease.Methods: Male SD rats were randomly divided into two groups: sedentary control group (CG) and endurance training group (EG). After the animal model establishment of exercise-induced cardiac hypertrophy by graded treadmill training lasting 10 weeks (at the intensity of 75%VO2max), half rats of each group were subjected to consecutive, exhaustive treadmill running three times, to test the effect of endurance training and induce cardiac injury. Blood, heart and dorsal root ganglions were removed immediately after exhaustion. The level of CGRP and cardiac troponin I in serum were determined by methods of enzyme immunoassay and chemiluminescence, respectively. Histological examination of Cardiac muscle was determined by hematoxylin-eosin staining and special dyeing of ischemia and hypoxia. Concentration of CGRP in left ventricular myocardium was detected by radio immunoassay. CGRP-immunoreactivity in heart and dorsal root ganglion tissue were showed by immunohistochemistry method and quantitatively analyzed by the system of computerized image analysis. The expression of CGRP mRNA in dorsal root ganglions were determined by real-time fluorescence quantitative polymerase chain reaction. Spectrophotometry were used to demonstrate the content of lactic acid, nitric oxide and malondialdehyde and activity of superoxide dismutase in serum and left ventricular myocardium.Results: (1) The ratio of cardiac weight to body weight and distance of run to exhaustion was greater in EG than in CG after 10-week endurance treadmill training (P=0.01). Meanwhile, histological evidence of heart specimen from EG demonstrated myocyte hypertrophy to some degree. Concentration of lactic acid in blood, not in myocardium, was more significantly increased in EG than in CG after exhaustive exercises. The results showed that cardiac hypertrophy by endurance training is a physiological, not pathphysiological, adaptation to exercise. (2) In EG, CGRP concentration in serum, immunoreactivity in cardiac muscle and dorsal root ganglions and content in left ventricular increased significantly (P<0.05 or 0.01, respectively), but gene expression in dorsal root ganglia decreased significantly (P<0.01), by endurance training. It suggested that CGRP release, reserve and synthesis in exercise-induced hypertrophied heart increased, but gene expression decreased, which demonstrates the fact that endurance training may regulate cardiac CGRP at the post transcription level. (3) After exhaustive exercises, CGRP immunoreactivity in cardiac muscle and dorsal root ganglia was markedly weakened in CG and EG (P<0.05), and CGRP content in left ventricular myocardium of EG decreased (P<0.05). Meanwhile, the CGRP release in EG increased and gene expression of CGRP in EG down-regulated significantly (P<0.01), but results of serum CGRP in CG and CGRP mRNA in EG showed no changes. It was reasonable to conclude that exhaustive exercises do harm to CGRP gene expression, protein synthesis and local store, but pretraining can maintain the normal expression and release of CGRP to improve the tolerance to prolonged exercise. (4) Nitric oxide content in serum and cardiac muscle did not differ between CG and EG after 10-week endurance training. However, lower nitric oxide content in cardiac muscle from CG was found immediately after exhaustive exercises (P<0.01).It showed that exhaustive exercise attenuates the nitric oxide synthesis in cardiac muscle of untrained rats. (4) Serum cTnI concentration ratio of after exhaustive exercise to before was greater in CG than in EG, and evident changes demonstrated by special dyeing of ischemia and hypoxia was found in some rats, so minor cardiac injury by exhaustive exercises occurred. (5) Although cardiac activity of superoxide dismutase in EG increased, serum superoxide dismutase activity and malonaldehyde contents in serum and cardiac muscle did not so, in CG and EG after endurance training, which suggested that endurance training can elevate the activity of superoxide dismutase in heart to reduce lipid peroxidation. (6) In CG, exhaustive exercise could elevate malonaldehyde content in cardiac muscle and serum, superoxide dismutase activity in cardiac muscle, but did not affect superoxide dismutase activity in serum. On the other hand, exhaustive exercise could elevate malonaldehyde content in cardiac muscle, depress superoxide dismutase activity in cardiac muscle and serum, but did not affect malonaldehyde content in serum, and lipid peroxidation in blood and heart was increased by exhaustive exercise.Conclusion: (1) 10 week endurance treadmill training results in greater ratio of cardiac weight to body weight, hypertrophied myocyte, elevated cardiac function, and can be proposed as a reliable animal model of exercise-induced cardiac hypertrophy. (2) By endurance training, CGRP synthesis in dorsal root ganglia is enhanced, content in cardiac muscle raised, level in serum elevated, thereby cardiac hypertrophy is physiologically remodeled. (3) Exhaustive exercise stress impaires gene expression and synthesis of CGRP in dorsal root ganglia, decreases CGRP reserve in heart, and as the result, available release of CGRP reduces, minor cardiac injury occurs due to lowered ability of cardioprotection. (4) Cardiaoprotection by endurance training may be mediated by elevated release of CGRP that up-regulates the activity of superoxide dismutase to lower lipid peroxidation, and promotes nitric oxide synthesis to increase coronary blood flow.