Lack of direct endotoxin-induced vasoactive effects on isolated skeletal muscle arterioles

Septic shock continues to be a major cause of mortality in the intensive care unit. This study was conducted to determine if endotoxin exerts a direct effect on the major determinant of peripheral vascular resistance, skeletal muscle arterioles. First order cremasteric arterioles were isolated from male Sprague-Dawley rats, cannulated with glass micropipettes, superfused in physiologic saline solution, and allowed to achieve spontaneous basal tone in the absence of intraluminal flow. Phenylephrine responsiveness was assessed before and after exposure to 2.5 micrograms/mL Salmonella enteritidis endotoxin (ET) for 1-2 h. There was no change in either basal diameter (91 +/- 5 microns before ET and 98 +/- 5 microns after ET) or phenylephrine responsiveness. In vivo exposure to ET (15 mg/kg) resulted in no change in basal tone at 1 h, however increased tone was observed in arterioles harvested after 4 h of systemic ET (43 +/- 4% without ET and 57 +/- 3% with, p < .05). To determine if upstream conduit vessels released factors responsible for the vasodilation of skeletal muscle arterioles, isolated cannulated cremasteric arterioles were connected in series to a 1 cm segment of aorta and superfused without and with 2.5 micrograms/mL ET. An increase in basal diameter was observed in arterioles from 94 +/- 14 microns before ET to 120 +/- 17 microns after ET (p < .05). These studies demonstrate that ET has no direct effect on isolated cannulated skeletal muscle arterioles, and that a vasodilating substance not consistent with nitric oxide is released from the upstream arterial bed.