Twenty-one enzymes of different metabolic systems were measured in the rabbit fast-twitch tibialis anterior (TA) muscle after electrical stimulation (10 Hz, 24 h/day) for 1 day to 10 wk. Nine analytical methods are either new, (3-oxoacid CoA-transferase, branched-chain-amino-acid aminotransferase, carnitine acetyltransferase, thiolase), improved (glutamate dehydrogenase, glycogen synthase, adenylic acid deaminase), or specially adapted (hexokinase, phosphoglucomutase). The activities (based on protein) of 12 mitochondrial or partly mitochondrial enzymes were lower in control TA than in control (slow) soleus (30-84% of soleus level). After 2 wk, 11 of these had surpassed the control soleus level. Maximal increases (3- to 14-fold) occurred after 2-5 wk, and thereafter six of the enzymes declined, whereas the other five maintained or increased their levels. Five glycolytic and two high-energy phosphate transfer enzymes, originally much higher in control TA than in control soleus, decreased gradually to levels at 8-10 wk only 27-123% higher than in soleus. Noncollagen protein concentration dropped 46%, explained largely by a sixfold increase in extracellular (chloride) space and a modest increase in collagen. The data constitute strong evidence for coordinate regulation of (mainly cytosolic) enzymes of glycolysis, glycogenolysis, gluconeogenesis, and high-energy phosphate transfer. Changes in the (mainly mitochondrial) enzymes of oxidative metabolism were more divergent, partly because of a hitherto undescribed secondary phase in the metabolic response. This phase may reflect a lower energy consumption in muscles adapted to continuous activity.

Single fibers were analyzed from rabbit fast-twitch tibialis anterior muscles freeze-clamped during continuous stimulation at 10 Hz for up to 8 wk. ATP declined after 1 wk to a stable level approximately 30% below controls. Phosphocreatine decreased earlier and to a greater extent (approximately 50%). Glycogen varied considerably among stimulated fibers and decreased on average approximately 75% by 8 wk. Glucose, lactate, citrate, and malate had changed little in the first 30 h and then increased four-, two-, four-, and sevenfold, respectively, over the next 5 wk. Glucose 6-phosphate showed the most unexpected behavior: with an overall upward trend, it descended to extremely low values (10% of control) after approximately 1 wk of stimulation. As long as high- and low-oxidative fibers were present, the former showed slightly higher levels of ATP, lactate, and malate; other metabolites did not differ in a consistent way. These unexpected observations, which differ strikingly from data for acute stimulation, shed light on adaptations that enable a chronically stimulated muscle to sustain a continuous high level of ATP utilization.