The effect of hypoxia on shortening contractions in rat diaphragm muscle

Hypoxia is known to reduce isometric contractile properties of isolated rat diaphragm bundles. Its effect on isotonic contractile properties (i.e. for ce-velocity relationship and power output) has not been studied. We hypothe sized that hypoxia reduces velocity of shortening and consequently power ou tput of the unfatigued muscle, and shortens endurance time during isotonic contractions, Force-velocity relationship, power output, and fatigue resist ance of rat diaphragm muscle bundles were measured during hypoxia (PO2: 6.6 +/-0.2 kPa) and compared with hyperoxia (PO2: 91.8 +/-0.7 kPa). Force was clamped from 1 to 100% of maximal tetanic force (P-o), Fatigue during isoto nic contractions was induced by repeated stimulation every 2 s at a clamp l evel of 33% of P-o. Hypoxia did not affect isometric force generation compa red with hyperoxia, nor contraction or relaxation time, In contrast, maximu m shortening velocity decreased significantly (hypoxia: 4.2 +/-0.3, hyperox ia: 6.0 +/-0.2 L-o/s, P<0.05). The force-velocity curve shifted downwards ( P<0.05). Hypoxia lowered power output at each load compared with hyperoxia (P<0.05). The isotonic endurance time was shorter during hypoxia compared w ith hyperoxia (80<plus/minus>2 vs. 130 +/-3 s, P<0.05). These data show tha t hypoxia depresses isotonic contractile properties and power output, and r educes endurance time during repeated isotonic contractions.