Sensitivity of small subcutaneous vessels to altered respiratory gases and local pH

Television microscopy was used to quantitate the responses of small arteries and veins, in the wings of unanesthetized bats, to alterations in the inspired concentrations of O2 and CO2. Mean arterial pressure, heart rate, and the diameters of small arteries (28-54 mum) and veins (50-128 mum) were measured during a 90-min protocol--30 min with an inspiratory gas mixture of 20% O2 and 80% N2 (control period); 30 min with a gas mixture containing 5% O2 (hypoxic period) or 12, 20, or 28% CO2 (hypercapnic period); and 30 min with the original control gas. The hypoxic responses were dilatation of arteries and no change in the veins in both innervated and surgically denervated wings. Hypercapnia resulted in artery dilatation in innervated wings. Hypercapnia resulted in artery dilatation in innervated wings and constriction in denervated wings. The veins constricted in both innervated and denervated wings during the hypercapnia period. In another series, topical application of Krebs solutions (pH ranging from 7.7 to 6.7) to exposed segments of small arteries and veins produced dilatation of both vessels with decreasing pH. Artery dilatation during hypoxia and vein constriction during hypercapnia involve non-neural mechanisms, while both a neural stimulus for dilatation and a non-neural stimulus for constriction are components in the response of innervated arteries to hypercapnia. The non-neural stimulus for artery and vein constriction during hypercapnia is not a local decrease in pH.