Role of Histamine in Hypoxic Pulmonary Hypertension in the Rat

The present investigation was undertaken to see if any of the naturally occurring vasoactive substances was likely to act as a mediator for the pulmonary vasoconstrictor response to acute alveolar hypoxia. Rats were treated with agents that deplete local stores of vasoactive amines or inhibit their synthesis. Lungs from these animals were isolated, ventilated, and perfused with homologous blood at constant volume inflow. Their pressor responses to 2to 3-minute periods of ventilation hypoxia (2% O2) were observed. The histamine-releasing agent 48/80 or the histidine decarboxylase inhibitor NSD 1055, or both, depleted 73% of the histamine in the lung but had no consistent effect on the pressor responses to hypoxia. When 48/80 (0.2 to 1 mg) was given in vitro, histamine in the lung was reduced to 10% of normal, or less, and the pressor response to alveolar hypoxia was completely abolished. Reserpine, guanethidine, or alpha-methyl-tyrosine reduced catecholamine stores in heart tissue (as an index of general tissue changes) by a maximum of 90% and reserpine decreased serotonin in the lung by 93% without inhibiting the hypoxic pressor responses. The findings strengthen the concept that histamine mediates the pressor response to acute alveolar hypoxia in the rat. ADDITIONAL