Histamine and hypoxic pulmonary hypertension. A quantitative study

The principal pathways involved in the in vivo modulation of hypoxic pulmonary vasoconstriction, pulmonary arterial remodelling and pulmonary hypertension

Hypoxic pulmonary vasoconstriction (HPV) serves to optimize ventilation-perfusion matching in focal hypoxia and thereby enhances pulmonary gas exchange. During global hypoxia, however, HPV induces general pulmonary vasoconstriction, which may lead to pulmonary hypertension (PH), impaired exercise capacity, right-heart failure and pulmonary oedema at high altitude. In chronic hypoxia, generalized HPV together with hypoxic pulmonary arterial remodelling, contribute to the development of PH. The present article reviews the principal pathways in the in vivo modulation of HPV, hypoxic pulmonary arterial remodelling and PH with primary focus on the endothelin-1, nitric oxide, cyclooxygenase and adenine nucleotide pathways. In summary, endothelin-1 and thromboxane A₂ may enhance, whereas nitric oxide and prostacyclin may moderate, HPV as well as hypoxic pulmonary arterial remodelling and PH. The production of prostacyclin seems to be coupled primarily to cyclooxygenase-1 in acute hypoxia, but to cyclooxygenase-2 in chronic hypoxia. The potential role of adenine nucleotides in modulating HPV is unclear, but warrants further study. Additional modulators of the pulmonary vascular responses to hypoxia may include angiotensin II, histamine, serotonin/5-hydroxytryptamine, leukotrienes and epoxyeicosatrienoic acids. Drugs targeting these pathways may reduce acute and/or chronic hypoxic PH. Endothelin receptor antagonists and phosphodiesterase-5 inhibitors may additionally improve exercise capacity in hypoxia. Importantly, the modulation of the pulmonary vascular responses to hypoxia varies between species and individuals, with hypoxic duration and age. The review also define how drugs targeting the endothelin-1, nitric oxide, cyclooxygenase and adenine nucleotide pathways may improve pulmonary haemodynamics, but also impair pulmonary gas exchange by interference with HPV in chronic lung diseases.

Hypoxic Pulmonary Vasoconstriction

It has been known for more than 60 years, and suspected for over 100, that alveolar hypoxia causes pulmonary vasoconstriction by means of mechanisms local to the lung. For the last 20 years, it has been clear that the essential sensor, transduction, and effector mechanisms responsible for hypoxic pulmonary vasoconstriction (HPV) reside in the pulmonary arterial smooth muscle cell. The main focus of this review is the cellular and molecular work performed to clarify these intrinsic mechanisms and to determine how they are facilitated and inhibited by the extrinsic influences of other cells. Because the interaction of intrinsic and extrinsic mechanisms is likely to shape expression of HPV in vivo, we relate results obtained in cells to HPV in more intact preparations, such as intact and isolated lungs and isolated pulmonary vessels. Finally, we evaluate evidence regarding the contribution of HPV to the physiological and pathophysiological processes involved in the transition from fetal to neonatal life, pulmonary gas exchange, high-altitude pulmonary edema, and pulmonary hypertension. Although understanding of HPV has advanced significantly, major areas of ignorance and uncertainty await resolution.

Pulmonary vascular actions of the antihistamine oxatomide during hypoxia

The role of histamine in the cardiovascular responses to hypoxia has not been resolved. Oxatomide, an orally active anti-allergic drug, has been reported to inhibit mast cell discharge and to antagonize any histamine which is released. This drug was administered to six dogs to determine the effect of inhibiting histamine release on the pulmonary vascular and cardiac responses to hypoxia. Oxatomide potentiated hypoxic pulmonary vasoconstriction, but reduced the cardiac output and stroke volume responses to hypoxia. These findings suggest that histamine is released during hypoxia and acts to oppose hypoxic vasoconstriction. In addition, histamine released from peripheral tissues may play an important role in mediating the increased cardiac output observed during acute hypoxia. By utilizing infusions of histamine, oxatomide was also found to be a histamine H1-receptor antagonist.

Adrenalectomy prevents hypoxic lung histamine accumulation

The lung histamine concentration of rats exposed to chronic hypobaric hypoxia was higher than in normoxic sea-level control rats. This hypoxia-induced lung histamine increase was absent in adrenalectomized chronically hypoxic rats. Adrenalectomy, per se, did not influence lung histamine content. The results suggest that the lung histamine content of chronically hypoxic rats is under hormonal control. This control mechanism is not evident under normoxic conditions.