Modification of alpha-adrenergic responses of small arteries by altered PCO2 and pH

Closed-circuit television microscopy was used to measure in vivo small artery (75--140 microns) and vein (105--230 microns) diameters to determine if changes in tissue PCO2 and/or pH would alter the microvascular responses to norepinephrine. Sprague-Dawley rats were anesthetized with a combination of urethane (800 mg/kg) and alpha-chloralose (60 mg/kg). The cremaster muscle with intact circulation and innervation was suspended by sutures in a 60-ml bath which contained a modified Krebs solution (31 degrees C) that was buffered by Tris of bicarbonate. There were four groups of animals with different combinations of bath PCO2 and pH: (1) PCO2 less than 10 mm Hg and pH = 7.2, (2) PCO2 less than 10 mm Hg and pH = 6.9, (3) PCO2 = 60--70 mm Hg and pH = 7.2, and (4) PCO2 = 60--70 MM Hg and pH = 6.9. The maximal responses of the small artery and vein to norepinephrine were similar for the four groups. The artery sensitivity to norepinephrine was significantly lower for group 4 when compared to groups 1, 2 and 3, but there was no effect on small vein sensitivity. Thus, the combination of decreased pH and increased PCO2 reduces small artery sensitivity to norepinephrine in the cremaster muscle of the rat.