Energy sources and mechanical efficiency of anaerobic work in dog gastrocnemius

Thermally isolated dog gastrocnemii were stimulated to exhaustion by rhythmic isotonic tetani of 0.2s duration (30 min-1) in complete occlusion of blood flow. Total enthalpy change, H = heat + work, work output, w, (kJ . kg-1) and average mechanical efficiency over the working period, n = w/H, were determined from deep muscle temperature increase and amount of shortening, and lactate produced, La, (mol . kg-1) from washout curves, respectively. Under these conditions H is the sum of the enthalpies due to a) alactic sources (Hal), i.e., net approximately P depletion and muscle O2 stores utilization, and b) formation: H = Hal + "delta HLa" . La. Thus, as "delta HLa", the molar enthalpy of La formation, amounts to 76 kJ . mol-1, Hal could be calculated. It was observed that: i) n ranged from 0.2-0.5 and was higher the lower H, this inverse relationship being essentially due to the dependence of n on Hal as described by: Hal = Ho . 10-kn + a (where Ho = 9.3, k = 3.7 and a = 0.15), and ii) the shortening speed of the first few tetani (vi, mm . s-1), was directly related to Hal as described by: vi = 63.3 + 30.7 Hal. Hal is essentially proportional to the approximately P content of resting muscle. These findings suggest therefore that the efficiency of contraction and the shortening speed are both affected by the approximately P concentration, the efficiency increasing, and the speed decreasing with decreasing approximately P.