Myocardial catecholamine responsiveness of spontaneously hypertensive rats as influenced by swimming training

Summary Alterations of myocardial mechanical catecholamine responsiveness by swimming training (2×90 min/day, 4 weeks) were examined in 13-week-old spontaneously hypertensive male rats (SHR). The relationships between myocardial mechanical catecholamine responsiveness and ventricular -adrenoceptors as well as myosin isoenzyme pattern were also examined. Compared with sedentary controls, trained rats showed a greater responsiveness to isoproterenol (10^–6 mol/l) on isometric tension (T) and its first derivative (dT/dt) (T:0.45±0.55 vs. –0.15±0.11 10^–2 N/mm², p<0.01, dT/dt: 17.1±10.1 vs. 8.3±3.6 10^–2 N/mm²·s, p<0.05). In sedentary SHR, dT/dt_max increased significantly, whereas developed tension decreased slightly, coupled with a decrease of time to peak tension by high dose (10^–6 mol/l) isoproterenol. Therefore, it can be stated that dT/dt is a better indicator for catecholamine sensitivity than isometric tension. -adrenoceptor density ([³H]-dihydroalprenolol binding) decreased significantly in trained rats (68.7±7.62 vs. 102.4±4.37 fmol/mg protein, p<0.01) with no significant difference in K_D values (4.61±2.26 vs. 6.11±1.94 nM, ns). In addition, myosin isoenzyme pattern revealed by pyrophosphate gel electrophoresis shifted towards VM-1 after swimming training. The increased catecholamine sensitivity of fast contracting myocardium is, in principle, compatible with the assumption of cAMP-dependent regulation of myofibrillar ATPase activity (21) or cross bridge kinetics (9), although other postreceptor processes should also be taken into consideration for the increased catecholamine sensitivity.Supported by the Deutsche Forschungsgemeinschaft