EFFECT OF CYANIDE ON OXYGEN TENSION-DEPENDENT MECHANICAL TENSION IN RABBIT AORTA

We determined relationships between oxygen uptake and mechanical tension in isolated strips of rabbit aorta treated with various agonists; organ bath Po2 was varied over the range 4-350 mm Hg, or cyanide (CN) was added to the organ bath in concentrations of from 6 x 10-5 to 3 x 10-3 M. Oxygen tension-dependent mechanical tension changes were similar during contractions caused by norepinephrine and angiotensin. During norepinephrine contractions, relaxations resulting from CN were much smaller than those resulting from decreases in organ bath Po2, which caused relaxation of mechanical tension. The threshold fall in oxygen uptake rate (Vo2) for a fall in mechanical tension was larger with graded CN than with graded hypoxia. Under conditions in which measurable oxygen uptakes by the strips were completely inhibited with CN or maximally inhibited with antimycin A, and tissue Po2 was expected to be equilibrated or nearly equilibrated with organ bath Po2, mechanical tension changes were seen with changes in Po2 over the range 4-80 mm Hg. These data are consistent with a postulate that an O2 sensor other than cytochrome a1a3 is involved in the mechanism of oxygen tension-dependent mechanical tension, although inhibition of the respiratory chain may explain some oxygen tension-dependent mechanical tension. During K+ contractions, hypoxic decreases in mechanical tension were much smaller than those seen during norepinephrine or angiotensin contractions. This finding does not give information about the oxygen sensor involved, but the K+ contraction preparation is useful in control experiments. The finding that there was no difference in mechanical tension-oxygen uptake plots with CN and with hypoxia during K+ contractions, and with CN in norepinephrine contractions, suggests that effects of CN on tension may result from inhibition of the respiratory chain, and strengthens the conclusion that hypoxic relaxations during norepinephrine contractions may be initiated by a mechanism independent of respiratory chain activity.