Autoregulation of blood flow within individual arterioles in the rat cremaster muscle.

Autoregulatory responses to alterations in arterial or venous perfusion pressure were determined for individual arterioles within the rat cremaster muscle. The cremaster muscle of pentobarbital anesthetized rats (50 mg/kg, ip) was surgically exposed and maintained in a controlled tissue bath for visualization by in vitro television microscopy. Cremaster bath PO2 was controlled at either a high (approximately 70 mm Hg) or low (approximately 19 mm Hg) level. Inside diameter and red blood cell velocity were measured for individual first (1A), second (2A), or third (3A) branching order arterioles, and instantaneous blood flows within each arteriole were calculated. To measure the autoregulatory responses, we decreased arterial perfusion pressure to the microvascular bed by gradually occluding the sacral aorta. Significant autoregulation was observed in all orders of vessels, but, in general, autoregulation was more pronounced at all vessel levels when bath PO2 was low, and the autoregulatory gain was greater for the smaller vessels compared to the larger vessels. Elevation of venous pressure within the vascular bed by gradual occlusion of the inferior vena cava led to a significant vasoconstriction of the smaller vessels, suggesting that a significant myogenic component was present. The vasoconstriction response to elevated venous pressure was more pronounced when bath PO2 was high. Our data are not consistent with a purely myogenic or purely metabolic mechanism, but suggest that both mechanisms are simultaneously contributing to the local vascular regulation.