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

Alterations of myocardial mechanical catecholamine responsiveness by swimming training (2 X 90 min/day, 4 weeks) were examined in 13-week-old spontaneously hypertensive males rats (SHR). The relationships between myocardial mechanical catecholamine responsiveness and ventricular beta-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) (delta T: 0.45 +/- 0.55 vs. -0.15 +/- 0.11 10(-2) N/mm2, p less than 0.01, delta dT/dt: 17.1 +/- 10.1 vs. 8.3 +/- 3.6 10(-2) N/mm2 X s, p less than 0.05). In sedentary SHR, dT/dtmax 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. beta-adrenoceptor density [( 3H]-dihydroalprenolol binding) decreased significantly in trained rats (68.7 +/- 7.62 vs. 102.4 +/- 4.37 fmol/mg protein, p less than 0.01) with no significant difference in KD 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.