Letter: Prostaglandins and the pulmonary vasoconstrictor response to alveolar hypoxia

Normocarbic hyperventilation fails to induce pulmonary vasodilation

The pulmonary vasodilator effect of increased rate of mechanical ventilation, with and without respiratory alkalosis, was studied in chronically instrumented newborn lambs. Pulmonary hypertension was first induced by ventilating with a hypoxic gas mixture. Subsequent respiratory alkalosis caused significant decreases in pulmonary arterial pressure and pulmonary vascular resistance. When normocarbia was re-established by adding carbon dioxide to the inspired gas, with the ventilator rate being held constant, the pressure and resistance returned to the baseline hypertensive state. Therefore, mechanical factors, either direct or indirect, appear to be of minor importance in the mechanism of pulmonary vasodilation secondary to frequency-induced hyperventilation.

Hypoxia stimulates prostacyclin generation by dog lung in vitro

We have investigated the possibility that pulmonary biosynthesis of prostacyclin and thromboxane A2 (TXA2) may be affected by variations in PO2. Fresh lung homogenated from 8 dogs were incubated for 1 hour at 37 degrees C in Krebs-Ringer solution, at high (492 mmHg) or low (53 mmHg) PO2. After incubation, the stable metabolites of prostacyclin (6-keto-PGF1) and of TXA2 (TXB2) were measured by radioimmunoassay. The basal ("pre-incubation") levels of these metabolites, measured in tubes in which biosynthesis was arrested by the addition of indomethacin (10 g/ml), were 0.76 +/- 0.15 and 0.76 +/- 0.11 ng/mg wet wt. for 6-keto-PGF1 alpha, in high and low PO2, respectively, and 0.97 +/- 0.09 and 0.82 +/- 0.15 x 10(-1) ng/mg wet wt. for TXB2, in high and low PO2 respectively. Synthesis during incubation in other tubes was estimated by subtracting basal values from those measured at the end of incubation. More 6-keto-PGF1 alpha was formed in homogenates exposed to low PO2 (3.01 +/- 0.45) than in those exposed kept at high PO2 (1.89 +/- 0.37, p less than 0.001), but equal amounts of TXB2 were bound under both conditions. These results suggest that hypoxia may stimulate pulmonary prostacyclin synthesis; a pulmonary vasodilator, prostacyclin may help modulate hypoxic vasoconstriction in the lung.

Possible relationship between pulmonary hypertension and prostaglandins

This study was performed on 20 children with congenital heart defect and left-to-right shunt. These patients were divided into two groups. Group I consisted of 10 patients with normal pulmonary arterial pressure while group II consisted of 10 patients with pulmonary hypertension. Prostaglandin Like Materials (PLMs) in pulmonary venous and arterial blood which were taken during cardiac catheterization were extracted and biologically assayed for Prostaglandin-Like Activities (PGLA) on rat stomach fundus. In all patients of group I, the ratio of pulmonary venous to pulmonary arterial PGLA levels (PV/PA-PGLA) was found to be less than one. However in group II, this ratio was less than one in only two patients. In 8 out of the patients in group II, the ratio was either 1 or greater. These data confirmed the early concepts suggesting the role of prostaglandins (PG) in regulating normal pulmonary circulation. In addition these data have suggested that PGs may have an important role in the development of pulmonary hypertension in some cardiac patients.

Prostaglandins, their intermediates and precursors: cardiovascular actions and regulatory roles in normal and abnormal circulatory systems

The characterization of newly found unstable metabolites of arachidonic acid has provided new perspectives for cardiovascular regulatory mechanisms and new insights into disorders of the circulatory system. Since these intermediates are often more potent on and more specific for cardiovascular structures than the classical prostaglandins, they are more likely candidates as physiologic mediators of circulatory events. Their instability in vitro need not preclude these roles; on the contrary, the limited pharmacology described to date suggests that they function purely as local hormones. As such, changes in the rate of generation of these unstable but potent compounds would provide an excellent control system. The stable prostaglandins may represent only overflow of degradation products of the active mediators associated with pathologic events. For example, the dicovery of prostacyclin and the realization that this prostaglandin and not PGE2 is the primary metabolite of arachidonic acid in blood vessels emphasizes the need to reinterpret many of the previously held hypotheses that proposed that prostaglandins of the E series contributed to the regulation of vessel tone and blood pressure, Moreover, the contribution made by abnormal prostaglandin mechanisms to hypertensive disease should now take into account that a deficiency of prostacyclin and not PGE2 could be a major factor causing the elevated tension developed in vascular smooth muscle and the augmented vessel responsiveness to stimuli associated with hypertension.

Prostaglandin content in blood and lung tissue during alveolar hypoxia

The aim of the present work was to investigate whether prostaglandins (PGs) are synthetized and released from isolated blood-perfused rat and cat lungs secondary to vasoconstriction induced by alveolar hypoxia. The lungs were perfused with autologous blood with constant volume inflow via the pulmonary artery in a recirculating system. They were ventilated with constant volume positive pressure, and acute alveolar hypoxia was induced by ventilation with a gas containing 2% O2. A superfusion bioassay technique was used to measure PG-like activity in the perfusate from the lungs, the blood being re-oxygenated before reaching the assay tissues. The oxygenator prevented the perfusate hypoxia induced by ventilation hypoxia to affect the bioassay tissues. The assay tissues were rat stomach strip, rat colon and chick rectum. They were sensitive to calibrating doses of 0.5--1 ng/ml PGE2 and 1--2 ng/ml PGF2alpha. In another series of experiments PGs of the F-series were measured in lung tissue from normoxic and hypoxic lungs with radioimmunoassay technique. No increase in PG-like activity could be detected in the venous effluent by means of bioassay during hypoxia, nor was the lung tissue content of immunoactive PGF increased by hypoxia. The present findings indicate that alveolar hypoxia does not stimulate PG-synthesis in lungs, refuting that PGs are important mediators of the pulmonary vasoconstrictor response to alveolar hypoxia. It is concluded that PGs play no significant role in producing the pressor response to alveolar hypoxia.

A role for prostaglandins in the regulation of the placental blood flows

A model is proposed for the regulation of the placental blood flows to the near-term pregnancy. The model has three features. 1) The maternal uterine and fetal placental tissues can synthesize constrictor and dilator prostaglandins. 2) Prostaglandins can cross the placenta. 3) There must exist a prostaglandin which has a vascodilating action in one of the placental circulations and a vasoconstricting action in the other circulation. Evidence is provided to indicate that the sheep, prostaglandin E2 (PGE2) can cross the placenta and has a vasodilating action in the uterine placental circulation and a vasoconstricting action in the umbilical placental circulation. The placenta and the lung are compared and PGE2 is shown to have similar actions in each of these organs.