Oxygen saturation and pH changes in cremaster microvessels of the rat

Recent studies have shown a considerable extent of diffusive O-2 transfer from precapillary vessels, but the occurrence of O-2 influx into postcapillary vessels (diffusive shunting) is still controversial. In this study, we investigated diffusive shunting between pre- and postcapillary vessels in muscle tissue and pH changes in the microvessels in an attempt to determine the physiological significance of the parallel arrangement of arterial and venous vessels in tissue oxygenation. We used rat cremaster muscles and controlled the breathing of anesthetized rats using urethane with FIO2 set at 0.3 and 0.12. We employed a noninvasive spectrophotometric method of measuring pH, using a pH-sensitive dye (1-hydroxypyrene-3,6,8-trisulfonic acid) and the O-2 saturation level of hemoglobin (So(2)) in microvessels from small arteries down to arterioles and from venules up to small veins. At FIO2 0.30, pH significantly decreased from 7.39 +/- 0.02 to 7.26 +/- 0.05 (SD) in precapillary vessels and remained close to constant during passage through postcapillary vessels. At FIO2 0.12, pH decreased from 7.36 +/- 0.03 to 7.01 +/- 0.06 in precapillary vessels and then increased in postcapillary vessels from 6.75 +/- 0.19 to 7.27 +/- 0.08. At FIO2 0.30, So(2) decreased from 98.6 +/- 5.4 to 64.2 +/- 4.5% in precapillary vessels and increased from 62.5 +/- 6.5 to 89.4 +/- 4.1% in postcapillary vessels. At FIO2 0.12, So(2) decreased from 47.0 +/- 5.5 to 25.1 +/- 3.8% in precapillary vessels and remained close to constant in postcapillary vessels. In conclusion, during hyperoxia, diffusive shunting of O-2 was enhanced, and during hypoxia, acid was accumulated in peripheral vessels. It is speculated that acid accumulated due to back-diffusion of CO2 from venous to arterial microvessels, liberating O-2 bound to hemoglobin via the Bohr effect. The diffusive shunting of O-2 and counterdiffusion and accumulation of CO2 may contribute to the homeostasis of tissue oxygenation levels.