Responses observed in individual arterioles and venules of rat skeletal muscle during systemic hypoxia.

1. In rats anaesthetized with Saffan, responses induced in individual arterial and venous vessels of the spinotrapezius muscle by systemic hypoxia (breathing 12 or 6% O2 for 3 min) were directly observed by in vivo microscopy. 2. Both 12 and 6% O2 induced gradual tachycardia and a fall in arterial pressure. Concommitantly, in each section of the vascular tree, some vessels showed a gradual increase in diameter, others, a gradual decrease. 3. During 12% O2, mean diameter changes were graded from mean increases of approximately 2% in main arteries (resting diameter 40-90 microns) to approximately 20% in terminal arterioles (7-13 microns) and ranged from mean increases of 5-8% in collecting and secondary venules (9-18 microns, 18-30 microns), to a decrease of approximately 2% in main veins (65-130 microns). 4. During 6% O2, constrictor responses were more common in arterial vessels. Thus, mean changes amounted to diameter decreases of less than 5% in main arteries and secondary arterioles (13-18 microns), and increases of approximately 5% in primary arterioles (22-50 microns) and terminal arterioles. By contrast, diameter increases predominated in venous vessels being graded from approximately 20% in collecting venules to approximately 2% in main veins. 5. In seventeen rats, 6% O2 was administered for eight 3 min periods separated by 30 min control periods. The changes evoked in arterial pressure and heart rate were consistent throughout. Diameter changes evoked in individual arterial and venous vessels were consistent in the first two hypoxic periods. However, diameter changes in the third and successive periods were significantly different from those recorded in the first period: increases in diameter became more common and pronounced. 6. These changes in vessel diameter, especially their variability, are considered in relation to recordings made previously of changes in gross blood flow and vascular conductance of limb muscle during systemic hypoxia.