Effects of Hypoxia on Isometric Force, Intracellular Ca 2+ , pH, and Energetics in Porcine Coronary Artery

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Abstract
—When exposed to hypoxic conditions, coronary arteries dilate, which is an important protective response. Although vessel sensitivity to oxygen is well documented, the mechanisms are not known with certainty. To further characterize the mechanisms of oxygen sensing in the coronary artery, we tested the major classes of hypotheses by measuring the effects of hypoxia on energetics, [Ca 2+ ] i , K + channel function, and pH i . Hypoxia relaxes porcine coronary arteries stimulated with either KCl or U46619. The extent of relaxation is dependent on both the degree and kind of stimulation. [Ca 2+ ] i was measured in endothelium-denuded arteries using fura 2-AM and ratiometric fluorescent techniques. At lower stimulus levels, hypoxia decreased both force and [Ca 2+ ] i . Inhibitor studies suggest that K Ca and K ATP channels are not involved in the hypoxic relaxation, whereas K V channels may play a minor role, if any. Despite the hypoxia-mediated decrease in force, [Ca 2+ ] i was unchanged or increased at high levels of stimulation. Despite a marked increase in lactate content, pH i (measured with the ratiometric fluorescent dye BCECF) was also little affected by hypoxia. Measurement of the phosphagen and metabolite profile of freeze-clamped arteries with analytical isotachophoresis indicated that hypoxia increased lactate content by 4-fold and decreased phosphocreatine to 60% of control. However, neither ATP nor P i was affected by hypoxia. Interestingly, additional stimulation under hypoxia increased force but not ATP utilization, as estimated from measurements of anaerobic lactate production. Thus, surprisingly, the economy of force maintenance is increased under hypoxia. In porcine coronary artery, both Ca 2+ -dependent and, importantly, Ca 2+ -independent mechanisms are involved in hypoxic vasodilatation. For the latter, mechanisms involving either ATP, [Ca 2+ ] i , pH i , or P i cannot be invoked. This novel oxygen sensing mechanism involves a decreased Ca 2+ sensitivity.