EXERCISE TRAINING IMPROVES LUSITROPY BY ISOPROTERENOL IN PAPILLARY-MUSCLES FROM AGED RATS

Aging is associated with a decreased cardiac responsiveness to beta-ad renergic stimulation. We examined the effect of endurance exercise tra ining of old Fischer 344 male rats on beta-adrenergic stimulation of t he function of isolated left ventricular papillary muscle. Three group s 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 treadmil l trained for 4-8 wk. The isometric contractile properties were studie d at 0.2 Hz and 0.75 mM calcium. Without beta-adrenergic stimulation, there were no group differences for peak tension, maximum rate of tens ion 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 beta-adrenergic stimulation by isoprotere nol of peak tension, -dP/dt, time to peak tension, or contraction dura tion. The inotropic response (+dP/dt) of SM was greater (P < 0.05) tha n that in SO or EO rats (which did not differ); however, the lusitropi c 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 t he lusitropic response to isoproterenol without altering the age-assoc iated impairment in inotropic response.