DECREASED PO2 AND RABBIT AORTIC SMOOTH-MUSCLE MECHANICS

Rabbit thoracic aorta was used to determine the effects of decreasing PO2 on the mechanical properties of contractions in response to norepi nephrine and KCl. Aortae were aerated with 45% O-2/5% CO2/50% N-2 and stimulated with 10 mu M NE or 50 mM KCI. At 5 min of stimulation, 5% C O2/95% N-2 aeration was introduced for 15 min, defined as hypoxia. Thi s time period was previously shown to produce similar decrease in [ATP ] in either stimulation condition. Force, stiffness and isotonic short ening velocity were monitored during the initial stimulation, during h ypoxia and during re-oxygenation. Hypoxia produced a substantial and r apid decrease in force and a concomitant decrease in stiffness during NE stimulation; delayed and smaller decreases in force and stiffness w ere observed during KCl stimulation. The force-stiffness relationship was steeper during KCl than NE stimulation, and hypoxia did not affect these relationships. Isotonic shortening velocity was significantly d epressed by hypoxia during both stimulations although the decrease dur ing KCl stimulation required a longer time. These data demonstrate tha t relaxation of an agonist-induced contraction in response to hypoxia results from a decrease in the number of activated crossbridges and no t formation of rigor bridges.