Decreased vasopressin responsiveness in vasodilatory septic shock-like conditions.

Objective: To determine the effect of vasodilatory septic shock-like conditions on vasoconstricting responses to vasopressin and norepinephrine in isolated resistance arteries.

Design: Prospective, randomized animal study.

Setting: University research laboratory.

Subjects: Male adult Sprague-Dawley rats.

Interventions: Small mesenteric arteries (outside diameter, 50-150 microm) were cannulated and studied in vitro under physiologic conditions. A vasodilatory septic shock-like state was produced by treatment with the nitric oxide (NO) donor, S-nitroso-N-acetylpenicillamine (SNAP), and the phosphodiesterase inhibitor, 3-isobutyl-1-methylxanthine (IBMX). Vasoconstricting concentration-response relationships were determined for norepinephrine and vasopressin before and after application of SNAP or SNAP+ IBMX. Synergism between low-dose vasopressin and norepinephrine and between low-dose norepinephrine and vasopressin was determined before and after SNAP or SNAP+IBMX.

Main Results: Norepinephrine and vasopressin produced concentration-dependent contractions (half-maximal effective concentration [EC(50)] = 2.5 microM and 3.9 nM, respectively) that were significantly inhibited by 1 microM SNAP (EC(50) = 3.6 microM and 8.1 nM, respectively) or 100 microM SNAP + 10 microM IBMX (EC(50) = 10 microM and 8.2 nM, respectively). Low-dose vasopressin significantly increased the responsiveness to norepinephrine (EC50 = 0.5 microM) just as a low-dose norepinephrine significantly enhanced the vasopressin response (EC(50) = 2.3 nM). The synergistic effects of low-dose vasopressin and norepinephrine, or low-dose norepinephrine and vasopressin, were also significantly inhibited by 1 microM SNAP (EC(50) = 2.5 microM and 4.2 nM, respectively) or 100 microM SNAP + 10 microM IBMX (EC(50) = 9 microM and 8.4 nM, respectively).

Conclusions: Vasoconstriction produced by vasopressin or norepinephrine, and the synergistic vasoconstriction produced by the combinations, was inhibited in vasodilatory septic shock-like conditions. Thus, in addition to the well-described vasopressin deficiency in vasodilatory septic shock, these studies indicate that decreased vasopressin responsiveness further contributes to a state of relative vasopressin insufficiency in this condition.