Does inadequate oxygen delivery trigger pressor response to muscle hypoperfusion during exercise?

In dogs running on a treadmill at 2 or 4 mph or 4 mph plus 10% incline, graded reductions in hindlimb perfusion reflexly elicited pressor responses. To test the idea that systemic arterial pressure (SAP) is raised by accumulation in muscle of a nerve-activating "pressor substance" release when O2 delivery becomes inadequate, arterial O2 content (CaO2) was reduced 29.1% by carbon monoxide (CO) inhalation before repeating exercise at 2 mph. We reasoned that the pressor substance, or related substances, should appear in femoral venous blood and be correlated to SAP. [K+] behaved inappropriately as a signal to raise SAP, i.e., when flow was reduced, SAP rose markedly with little or no change in [K+]. SAP was well correlated to pH and [lactate] over the three work loads. Compared with the same work load with normal CaO2, CO shifted the relation between SAP and terminal aortic flow rightward 0.30 l/min (34.5%) and the relation between SAP and PO2 leftward 7.7 mmHg. CO did not affect the relation of SAP to terminal aortic O2 delivery, hindlimb O2 uptake index, pH, or [lactate]. Thus pressor responses are apparently generated when O2 delivery falls below some critical level causing accumulation of a pressor substance the release of which is linked to a metabolic event that precipitates lactate accumulation.