[Role of cardiac sympathetic nervous activity in exercise--changes in catecholamine level on forced swimming rat]

Endurance exercise demands full adaptive mechanisms in various organs. Sympathetic nervous system is said to play an important role on supporting the cardiac function through norepinephrine (NE) release. The purpose of this study is to investigate the role of cardiac sympathetic nervous system in exercise by measuring the activity of cardiac tyrosine hydroxylase (TH), rate-limiting enzyme in catecholamine (CA) synthesis and concentration of NE and 3,4-Dihydroxyphenylethylene glycol (DHPG), major metabolite of CA.

14 week old Wistar rats were forced to swim in 37 degrees C water bath for 90, 150, 240, 330 minutes, respectively. Some of them couldn't swim until the fixed time (group D). After swimming, they were promptly decapitated ++ and then concentration of plasma, cardiac and adrenal CA, activity of cardiac TH and concentration of cardiac DHPG were measured. Myocardial contractility was also measured using removed papillary muscle.

(1) Plasma CA concentration didn't change between control and each swimming group, but elevated in group D. (2) Cardiac CA concentration decreased as swimming was prolonged and markedly reduced in group D. (3) Adrenal CA concentration in swimming group tended to elevate and decreased in group D. (4) There were no differences in activity of cardiac TH between swimming group and control. (5) Concentration of cardiac DHPG were decreased in group D compared with control. (6) There were no differences in myocardial contractility between control and group D. These results suggest; prolonged exercise seemed to be supported by cardiac sympathetic NE release and was no longer continued when the cardiac sympathetic support was reduced.