Dynamics of O2, CO2, lactate, and acid exchange during contractions and recovery

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Abstract
The net exchange of O2, CO2, lactate and non-CO2 acid, measured as base excess, were calculated during 10 min of 4/s isometric twitch contractions and during 20 min of recovery. These determinations were made using the in situ gastrocnemius plantaris muscle of the anesthetized dog with controlled ventilation. The tension developed during the contractions was also measured. The O2 uptake (.ovrhdot.VO2) rose quickly during the contractions with a half-time of .apprx. 30 s and reached a plateau in .apprx. 2 min. .ovrhdot.VO2 declined quickly, then slowly during recovery with a half-time of about 30 s. The CO2 output, which was lower than .ovrhdot.VO2 during the early phase of the contractions, approached .ovrhdot.VO2 during the steady-level phase and exceeded .ovrhdot.VO2 during early recovery. As a result the respiratory exchange ratio was decreased early in the contraction period, reached values near 1.0 during the steady-level phase, and > 1.0 during much of recovery. Lactate output increased slowly during contractions for .apprx. 3.5 min with a half-time of .apprx. 85 s and then declined during the latter half of the contraction period. Lactate exchange was small during recovery. There was uptake of non-CO2 acid (H.ovrhdot.A) during the 1st min of contractions. During the remainder of the contraction period there was progressive acid output, which exceeded lactate output. The H.ovrhdot.A declined slowly during recovery. There is significant exchange of acid other than lactic acid by muscle during and after contractions. There appears to be a transient period at the start of the contractions of intracellular alkalosis during the 1st min of contractions that causes retention of CO2 and, perhaps, the increase in developed tension (positive staircase). After the 1st min of contractions there appears to be progressive intracellular acidosis that may be related to the decreasing developed tension and fatigue.

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