The effects of two levels of vasomotor tone at physiologic suffusate PO2 on acetylcholine- and sodium nitroprusside-induced relaxation of cremaster third-order arterioles in 5-hour ischemia-reperfusion control rats.

Intravital microscopy was used for 1 h in the cremaster skeletal muscles of normotensive 4- to 5-week-old rats. The total duration for experimentation was 5 h in order to mimic the controls used previously for a 4-hour ischemia and 1-hour reperfusion model which was equilibrated with room air. Responsiveness of third-order (3A) arterioles with resting vasomotor tone (VT) was assessed to topically applied 10(-2) to 10(-6) M acetylcholine (ACh) or sodium nitroprusside (NP) using a suffusate PO2 of 25-30 mm Hg. ACh (10(-4) M) or NP (10(-6) M) were retested at this Po2 in 3A arterioles with norepinephrine (NE) (10(-6) M)-enhanced VT. Results were compared against those using room air to increase VT. No dose-response relationships were demonstrated for ACh or NP in resting conditions. Moreover, our current and former responses were maximal and of a lesser magnitude than those reported by others using room air. All doses except 10(-4) or 10(-6) M ACh, or 10(-6) M NP, also severely depressed systemic arterial blood pressure. Enhancement of VT by 29% resulted in a 3-fold greater dilation to 10(-4) M ACh or 10(-6) M NP. The relative increase in volumetric blood flow (Q) to ACh or NP was 5.3-5.7 times greater than with resting VT. However, there were no differences in the absolute maximal values attained for internal diameter (D) or Q between drugs. The times to peak response and recovery were accelerated for Q but not D in NE-preconstricted arterioles, and the relative increases in D were less than reported by others after equilibration with room air. At both levels of tone, topical administration of NE at the end of each experiment caused similar decreases in D and Q, while mean centerline cellular velocity, wall shear rate (WSR), and VT were found to increase. These results suggested that the relative increases in D and Q were due to NE-induced decreases in absolute predrug baselines, while both the depressed VT and peak responses in D were caused by a fall in vasoreactivity. Po2 did not appear to be a factor attenuating endothelium-dependent responses, since ACh was equipotent to NP in resting or NE-preconstricted arterioles at physiological suffusate Po2 or in cremaster flaps equilibrated with room air. Therefore, either NE at physiological suffusate Po2 or room air appears acceptable for elevating initial VT/WSR when examining the endothelium-dependent and endothelium-independent mechanisms regulating physiological (dilator) tone and perfusion in 3A arterioles.