Exercise training improves lusitropy by isoproterenol in papillary muscles from aged rats

Aging is associated with a decreased cardiac responsiveness to β- adrenergic stimulation. We examined the effect of endurance exercise training of old Fischer 344 male rats on β-adrenergic stimulation of the function of isolated left ventricular papillary muscle. Three groups were examined: sedentary mature (SM; 12-mo old), sedentary old (SO; 23-24 mo old), and exercised old (EO; 23-24 mo old) that were treadmill trained for 4-8 wk. The isometric contractile properties were studied at 0.2 Hz and 0.75 mM calcium. Without β-adrenergic stimulation, there were no group differences for peak tension, maximum rate of tension development (+dP/dt), or maximum rate of tension dissipation (-dP/dt). The time to peak tension was longer (P < 0.05) for both EO and SO than for SM rats. Half relaxation time (RT(1/2)) was prolonged (P < 0.05) for SO compared with SM and EO (which did not differ). The three groups did not differ in the β-adrenergic stimulation by isoproterenol of peak tension. -dP/dt, time to peak tension, or contraction duration. The inotropic response (+dP/dt) of SM was greater (P < 0.05) than that in SO or EO rats (which did not differ); however, the lusitropic response (RT(1/2)) was lesser (P < 0.05) in SO than in SM or EO rats (which did not differ). Thus exercise training of old rats improved the lusitropic response to isoproterenol without altering the age-associated impairment in inotropic response.