EARLY METABOLIC ADAPTATIONS OF RABBIT FAST-TWITCH MUSCLE TO CHRONIC LOW-FREQUENCY STIMULATION

To investigate early adaptive responses to chronic low-frequency stimu lation (CLFS), rabbit tibialis anterior (TA) muscles were continuously stimulated at 10 Hz for 8 days, allowed to rest for 1 h, and then sub jected to a 15-min fatigue test at 10-Hz stimulation. The contralatera l TA muscles which had not been exposed to CLFS, served as controls du ring the fatigue test. Compared to the controls, the initial tension o utput of the 8-day prestimulated muscles was reduced by 25%. However, these muscles maintained higher tensions during the fatigue test than the controls. Citrate synthase activity, an indicator of aerobic-oxida tive capacity, was only slightly elevated (40%) in the X-day stimulate d muscles. Unlike the controls, the prestimulated muscles failed to pr oduce potentiation during the fatigue test. Control muscles responded to the fatigue test with pronounced reductions in contents of adenosin e 5'-triphosphate (ATP), phosphocreatine (PCr), and glycogen, as well as with large increases in contents of inosine monophosphate (IMP), in organic phosphate (P-i), creatine (Cr), and lactate. Under the same co nditions contents of ATP, PCr Cr, glycogen, lactate, P-i, and IMP were unaltered in the 8-day prestimulated muscles. These findings demonstr ated that CLFS for 8 days elicited pronounced alterations in energy me tabolism and contractile properties. These adaptive changes occurred p rior to fibre type transitions and substantial increases in aerobic-ox idative potential.