Endotoxin releases a substance from the aorta that dilates an isolated arteriole by up-regulating INOS.
BACKGROUND
Loss of vascular tone in resistance arterioles has been implicated as the cause of hypotension in septic shock. It is believed that the overproduction of nitric oxide (NO) by the inducible isoform of nitric oxide synthase (iNOS) results in the vasodilatation seen in septic shock. However, we have shown that endotoxin has no effect on vascular tone of an isolated resistance vessel unless the endotoxin flows over a segment of aorta or vena cava upstream in the superfusion line. The aim of this study was to determine if the subsequent vasodilation was due to the release of a direct vasodilator or production of NO in the arteriole and if its source was iNOS by using its selective inhibitor, aminoguanidine.


METHODS
First-order rat cremaster arterioles (n = 36) were isolated and cannulated onto micropipettes, superfused with physiological buffer at 34 degrees C, pressurized to 70 mm Hg, and allowed to gain spontaneous tone over 90 min. A segment of abdominal aorta was then placed in series with the arteriole so that the superfusate passed over the aorta and then into the tissue bath containing the isolated arteriole. The vessels were allowed to equilibrate over 60 min. During this interval, the arteriole was exposed to l-NAME (100 mum), aminoguanidine (100 mum), or buffer. The aorta and arteriole were then superfused with endotoxin (Salmonella enteritidis 2.5 mug/ml). Internal diameters of cannulated arterioles were measured and recorded with videomicroscopy and videocalipers at a resolution of +/-1 mum every 15 min for 1 h. Six groups were created with n = 6 for each group: Group 1, endotoxin; Group 2, control; Group 3, l-NAME and endotoxin; Group 4, l-NAME; Group 5, aminoguanidine and endotoxin; and Group 6, aminoguanidine.


RESULTS
After the 60-min equilibration period, there was no significant difference in resting tone among the six groups. At t = 120, the percentage of tone in the control group was 42.7 +/- 0.4% (mean +/- SEM) and this was not changed by treatment with aminoguanidine (42.2 +/- 0.7%). However, exposure to l-NAME alone resulted in vasoconstriction with a gain in tone to 49.5 +/- 1.6% (P > 0.05). Endotoxin alone caused arteriolar tone to fall to 33.5 +/- 1.2% (P < 0.05). Arterioles treated with aminoguanidine did not lose tone (42.6 +/- 1.7%) when exposed to endotoxin and arterioles treated with l-NAME retained their elevated tone (46.0 +/- 2.2%) after treatment with endotoxin.


CONCLUSIONS
This study demonstrates that the aorta exposed to endotoxin releases a substance that vasodilates resistance arterioles through the up-regulation of iNOS. Aminoguanidine prevented the fall in tone following exposure to endotoxin, while use of the nonselective NOS inhibitor, l-NAME, not only blocked the fall due to endotoxin but increased basal tone by blocking the constitutively active eNOS.
Viol
Anthony W
AW
Eastern Virginia Medical School, Department of Surgery, 825 Fairfax Avenue, Norfolk, Virginia 23507-1912, USA.
Prewitt
Russell L
RL
Doviak
Michael
M
Britt
L D
LD
D000818:Animals; D001011:Aorta / Q000187:drug effects* / Q000378:metabolism*; D001160:Arterioles / Q000187:drug effects / Q000378:metabolism / Q000502:physiology*; D004731:Endotoxins / Q000494:pharmacology*; D004791:Enzyme Inhibitors / Q000494:pharmacology; D006146:Guanidines / Q000494:pharmacology; D008297:Male; D018482:Muscle, Skeletal / Q000098:blood supply; D019331:NG-Nitroarginine Methyl Ester / Q000494:pharmacology; D019001:Nitric Oxide Synthase / Q000037:antagonists & inhibitors / Q000378:metabolism*; D052247:Nitric Oxide Synthase Type II; D051381:Rats; D017207:Rats, Sprague-Dawley; D015854:Up-Regulation; D014664:Vasodilation; D014665:Vasodilator Agents / Q000378:metabolism*; D014666:Vasomotor System / Q000187:drug effects
D004731:Endotoxins; D004791:Enzyme Inhibitors; D006146:Guanidines; D014665:Vasodilator Agents; D019001:Nitric Oxide Synthase; D052247:Nitric Oxide Synthase Type II; C496318:Nos2 protein, rat; C004479:pimagedine; D019331:NG-Nitroarginine Methyl Ester