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In an attempt to determine the functional role of the component broad white and narrow red fibre types during activity, the pectroalis muscle of the adult pigeon (Columba livia) was electrically stimulated in situ after anaesthetizing the birds with sodium pentobarbital. For examination of the effect of short term exercise, the pectoral muscle of one side was stimulated through the innervating brachial plexus, using an electronic stimulator for 1 hr, and a biopsy sample was taken from the stimulated muscle. The stimulation was resumed for 17 hr more with additional doses of anaesthesia, and a muscle sample was taken after the decapitation of the birds in the long term experiment. Muscle samples from non-stimulated pigeons under identical conditions served as control. Fixed and frozen sections were made from the samples and were stained with Periodic Acid-Schiff's reagent for the demonstration of glycogen, while additional frozen muscle sections were stained with Fettrot 7B according to Pearse, A.G.E. (1960) for the demonstration of neutral lipids in the specimen. Thus histochemical examinations were made to study effects of short and long term exercises on energy stores in skeletal muscles. Short-term stimulation resulted in a selective depletion of intracellular glycogen stores from white fibre populations whereas prolonged long term stimulation resulted in the resynthesis of glycogen within these fibres and a concomitant reduction in the intracellular fat and glycogen reserves in most red fibres. It is postulated that during flight the white fibres indulge only in brief bursts of intense phasic activity such as in quick take-off, rapid accelerations or sudden manoeuvres and utilize glycogen as fuel whereas the red fibres perform sustained activity as in cruising flight and metabolize chiefly fat.

EFFECTS OF SHORT AND LONG TERM EXERCISE ON INTRACELLULAR GLYCOGEN AND FAT IN PIGEON PECTORALIS