MYOSIN LIGHT-CHAIN PHOSPHORYLATION DURING STAIRCASE IN FATIGUED SKELETAL-MUSCLE

It has been reported that the peak of the staircase or the enhanced te nsion response during low frequency stimulation is delayed in fatigued fast muscle. Our purpose was to determine if the rate and extent of r egulatory myosin light chain (P-LC) phosphorylation, a molecular mecha nism associated with the positive staircase, are also altered by fatig ue. The staircase contractile response, muscle metabolites and phospha te incorporation by the P-LC were assessed at 0, 5, 10 or 20 s of 10-H z stimulation, in either non-fatigued (control) or fatigued (10 Hz for 5 min, followed by 20 min of recovery) rat gastrocnemius muscle in si tu. The concentration of adenosine triphosphate (ATP) in fatigued musc les, 21 +/- 0.9 mmol . kg(-1) (dry weight) was significantly lower (P < 0.05) than in the control muscles, 26.1 +/- 1.5 mmol . kg(-1). In bo th groups, ATP content was significantly lower after 20 s of 10 Hz sti mulation. The P-LC phosphate content (in mol phosphate . mol(-1) P-LC) was 0.10, 0.38, 0.60 and 0.72 after 0, 5, 10 or 20 s of 10 Hz stimula tion in control muscles, but only 0.03, 0.08, 0.11 and 0.19 at these t imes in fatigued muscles. Although the absolute magnitude of tension p otentiation was attenuated in proportion to the depressed twitch ampli tude, these surprisingly low levels of phosphorylation were associated with 0, 48, 79 and 86% potentiation of the developed tension at these times in contrast with 0, 71, 87 and 49% potentiation in control musc les. These data demonstrate that while the rate and extent of phosphat e incorporation is depressed in fatigued muscle, tension potentiation is still evident. The persistence of potentiation in the fatigued stat e indicates that either this condition results in greater potentiation for a given level of P-LC phosphorylation, or that factors in additio n to P-LC phosphorylation are responsible for the staircase response.