Early events in the response of fast skeletal muscle to chronic low-frequency stimulation. Polyamine biosynthesis and protein phosphorylation.

The concentrations of putrescine, spermidine and spermine and the activities of ornithine decarboxylase (ODC) and S-adenosyl-L-methionine decarboxylase (SAM-D) were investigated in fast muscle subjected to chronic low-frequency electrical stimulation. Both ODC and SAM-D activities increased markedly between 18 and 48 h of stimulation. Changes in enzyme activities were followed by phasic elevations in the concentrations of putrescine, spermidine and spermine. Peak levels were reached first by putrescine at 3-4 days, followed by spermidine at about 9 days and then by spermine at about 11 days. A possible relationship was sought between these events and changes produced in vitro in the phosphorylation pattern of cytoplasmic proteins and the total activity of cyclic AMP-dependent protein kinase. However, during the early stages of stimulation, no prominent changes were seen either in the phosphorylation pattern or in the activity of cyclic AMP-dependent protein kinase. These characteristics changed significantly at a later stage (by 12 days of stimulation) and became indistinguishable from those of slow muscle by 3 to 4 weeks of stimulation.