Potentiation of adrenaline vasoconstrictor response by sub-threshold concentrations of U-46619 in human umbilical vein: involvement of smooth muscle prostanoid TP(alpha) receptor isoform.

Considering the potential physiological, pharmacological and therapeutic relevance of synergistic interaction of thromboxane A(2) with adrenaline at postjunctional receptor sites, we examined whether sub-threshold concentrations of thromboxane A(2) mimetic U-46619 (9,11-dideoxy-9alpha, 11alpha-methanoepoxy prostaglandin F(2alpha)) could amplify adrenaline-induced contraction in human umbilical vein. The receptor involved in U-46619-induced potentiation of adrenaline contractility was also investigated. Umbilical cords (n=125) from healthy patients after full-term vaginal or caesarean deliveries were employed. The vein was dissected out of cords and rings used for isolated organ bath experiments or reverse transcription-polymerase chain reaction (RT-PCR) and Western blot. Presence of endothelium did not modify U-46619-induced contraction in human umbilical vein. Prostanoid TP-selective receptor antagonist, SQ-29548 (7-[3-[[2-[(phenylamino)carbonyl]hydrazino]methyl]-7-oxabicyclo[2.2.1]hept-2-yl]-[1S(1alpha,2alpha(Z),3alpha,4alpha)]-5-Heptenoic acid), inhibited U-46619-induced contraction (pA(2)=8.22+/-0.11). U-46619 sub-threshold concentrations (0.1-0.3 nM) potentiated adrenaline-vasoconstriction response in a concentration-dependent manner. SQ-29548 (0.1 microM) abolished this potentiation. Using RT-PCR, we found that human umbilical vein rings with or without endothelium express the prostanoid TP(alpha), but not the prostanoid TP(beta) receptor isoform. Western blot allowed the identification of proteins with an electrophoretic mobility (47- and 55-kDa) indistinguishable from human platelet prostanoid TP receptor, a rich source of prostanoid TP(alpha) receptor isoform. Collectively, present results demonstrate that prostanoid TP(alpha) is the major receptor isoform localized on smooth muscle cells which participate in both direct vasoconstriction and potentiating effects of U-46619 on adrenaline contractions in human umbilical vein. These results suggest that thromboxane A(2) may interact synergistically with adrenaline in pathophysiological situations that lead to an increase of its umbilical venous levels (e.g. preeclampsia associated with fetal distress) raising the possibility of vasoconstriction affecting fetal blood flow.

Oxygen delivery to the tissues and mitochondrial respiration.

In mammals, the O2 transport from the inspired air to the tissues is made by convective and diffusive mechanisms. The convective mechanisms are provided by the cardio-respiratory system and comprised by the basic variables of cardiac output and blood O2 content. Microcirculation in arterioles and capillaries is adjusted to match the O2 demand of local tissues. Endothelium-generated NO diffuses to the smooth muscle of microvessels and produces vasodilation that increases circulatory time in the capillaries and allows a more effective O2 extraction in the tissues. Once within the tissue, O2 diffuses to mitochondria where it is reduced in an exergonic process coupled to ATP synthesis. Both, O2 and ATP are the two most homeostatic intracellular species. In heart and muscle, both species show unchanged levels with 25-100 times increases in work load and ATP turnover rate. The linear rates of O2 uptake shown by tissue slices and perfused organs are interpreted as a fast switching of mitochondria between metabolic state 3 (with a fast rate of O2 uptake and ATP synthesis) and state 4 (with a slow rate of O2 uptake and no ADP phosphorylation). Endogenous mitochondrial NO, produced by mtNOS, sustains the concept of a physiological functional activity of this enzyme in regulating mitochondrial and cellular O2 uptake.