Age dependency of vasopressin pulmonary vasodilatory effect in rats

Abstract

Background: Vasopressin is a systemic vasoconstrictor. Its pulmonary vasodilatory effect is controversial, and limited data are available on its use in neonates with pulmonary hypertension. Hypothesizing that the vasopressin-induced pulmonary vasodilation is developmentally regulated, we evaluated its pulmonary and systemic arterial response in newborn and adult rats.

Methods: Vessels were mounted on a wire myograph, and the vasopressin-induced changes in vasomotor tone measured. The vessel- and age-dependent differences in vasopressin V1a and V2 receptors' expression were evaluated by western blotting.

Results: Vasopressin induced a dose-dependent increase in mesenteric arterial tone at both ages, but of greater magnitude in adult vessels (P < 0.01). At lower concentrations, vasopressin induced pulmonary vasodilation in adult vessels and vasoconstriction in newborn arteries. The adult vasopressin-induced pulmonary vasodilation was inhibited by ibuprofen, suggesting that the response is prostaglandin mediated. Pulmonary tissue V1a receptor protein expression was higher in adult, when compared with newborn arteries (P < 0.01). The adult vessels V1a expression predominated in the pulmonary arteries, and V2 was only detected in mesenteric arteries.

Conclusion: The vasopressin-induced pulmonary vasodilation is absent in newborn rats likely due to the lower tissue V1a expression early in life. These animal data challenge the therapeutic use of vasopressin in neonatal pulmonary hypertension.

Figure 1

Vasopressin effect in adult pulmonary and mesenteric arteries without agonist-induced tone. Non precontracted adult pulmonary (N=4) and mesenteric arteries (N=4) response to vasopressin. Open circles = Pulmonary Artery; Solid Circles = Mesenteric Artery. *P<0.05 by One-way ANOVA with repeated measures and Fisher’s least-square-difference multiple-comparison test.

Figure 2

Vasopressin induced vasomotor tone changes in newborn and adult pulmonary and mesenteric arteries. Vasopressin dose-response in thromboxane A₂ analogue (U46619) precontracted (EC₇₅ concentration) newborn and adult mesenteric (Panel A. N=4 each) and pulmonary arteries (Panel B. Newborn N=6; Adult N=8). The vasopressin-induced changes are expressed as percentage of precontraction force values. Positive values indicate vasopressin-induced contraction and negative values indicate vasorelaxation. Open circles = Newborn; Solid circles = Adult. ** P< 0.01 versus respective initial 10⁻¹² M values by two-way ANOVA and Tukey-Kramer multiple comparison testing.

Figure 3

Vasopressin single dose effect on newborn and adult pulmonary and mesenteric arterial tone. Single concentration (6×10⁻⁷ M) vasopressin-induced force in Newborn and adult mesenteric (Panel A. Newborn N=3; Adult N=4) and pulmonary arteries (Panel B. Newborn N=4; Adult N=4) precontracted with thromboxane A₂ analogue (U46619 (EC₇₅ concentration). Vasopressin-induced response is expressed as % change from U46619-induced force and negative values indicate relaxation. ** P< 0.01 versus newborn values by unpaired Student t-test.

Figure 4

Vasopressin pulmonary vasodilatory effect and cyclooxygenase/NOS inhibition. Vasopressin dose-response in Thromboxane A₂ analogue (U46619) precontracted (EC₇₅ concentration) adult pulmonary arteries in the absence (Control; N=8) and presence of either the cyclooxygenase inhibitor ibuprofen (10⁻⁴ M; N=4) or the nitric oxide synthase blocker L-NAME (10⁻⁴ M; N=4). Positive values indicate vasopressin-induced contraction and negative values indicate vasorelaxation. Open circles = Control; Solid circles = L-NAME; Solid triangles = Ibuprofen. ** P< 0.01 relates to ibuprofen data significantly different when compared with control vessels at the same vasopressin concentrations by two-way ANOVA and Tukey-Kramer multiple comparison testing.

Figure 5

Pulmonary and mesenteric tissue, endothelial and smooth muscle cell vasopressin V_1a receptor protein expression. Newborn and adult rat 3^rd-4^th generation pulmonary (Panel A; N=4 each) and mesenteric arterial tissue (Panel B; N=4 each), as well as their respective primary endothelial (Panels C and D; N=4 each) and smooth muscle cells (Panels E and F; N=3 each). Vasopressin V_1a receptor was determined by Western blotting and normalized to the tissue and cell GAPDH content. ** P< 0.01 versus adult values by unpaired Student t-test. Representative Western blots are illustrated.

Figure 6

Pulmonary and mesenteric arteries vasopressin V₂ receptor protein expression. Newborn and adult rat 3^rd–4^th generation pulmonary (Panel A; N=3 each) and mesenteric arterial tissue (Panel B; N=3 each). Vasopressin V₂ receptor was determined by Western blotting and normalized to the tissue and cell GAPDH content. Adult rat kidney tissue was used as a positive control * P<0.05 versus newborn values by unpaired Student t-test. Representative Western blots are illustrated.