Contribution of capillary recruitment to regulation of tissue oxygenation in rat cremaster muscle.

Microvascular regulation of tissue oxygenation can be thought of as being accomplished through the interaction of two basic mechanisms: (1) control of tissue blood flow rate (rate of O2 convection) and (2) control of oxygen diffusion distance by alterations in intercapillary distance (exchange control or capillary recruitment). The purpose of this study was to investigate the contribution of the capillary recruitment mechanism to regulation of tissue oxygenation in response to alterations in the tissue perfusion pressure under varying prevailing local PO2 conditions. The cremaster muscle of anesthetized rats (Nembutal, 50 mg/kg, ip) was surgically exposed and maintained in a controlled bath environment for in vivo television microscopy. Intercapillary distances (ICD) between flowing capillaries within the cremaster were measured directly on the face of a TV monitor. The effects of alterations in tissue oxygenation on the ICD were determined by controlling the PO2 of the cremaster bath solution at different levels: high PO2 (approx. 73 mm Hg), intermediate PO2 (approx. 21 mm Hg) or low PO2 (approx. 8 mm Hg). The ICD responses to alterations in perfusion pressure were determined with both the low and the high bath PO2 levels by reducing the cremaster perfusion pressure using a servo-controlled occluder placed around the sacral aorta. Reductions of bath PO2 significantly reduced the mean ICD and resulted in significant alterations in the shape of the ICD distribution, leading to a more homogeneous form. The mean ICD was also significantly reduced in response to reduced perfusion pressure, and the relative ICD reduction was more pronounced when the prevailing bath PO2 was high. These results support the concept of a shifting locus of vasoregulation with changing tissue metabolism, with control shifting toward the terminal precapillary portions of the microvascular network when metabolic stresses are reduced.