Almitrine is a respiratory stimulant whose effect of increasing arterial PO₂ appeared to be independent of its effects on lung ventilation. We (and others) hypothesized that almitrine could increase blood oxygenation by potentiating the hypoxic pulmonary vasoconstriction. In this study we found that almitrine in a low dose (0.25 µg/ml ) selectively potentiates hypoxic pulmonary vasoconstriction without changing the baseline perfusion pressure, while higher doses increase baseline pressure which then does not rise further with hypoxia (Figure).

Low dose almitrine potentiates pulmonary vasoconstrictor response to moderate acute hypoxia, while higher doses cause pulmonary vasoconstriction even in normoxia. Isolated rat lungs were perfused with blood at constant flow rate (0.04 ml/min/g body weight), so that increases in perfusion pressure reflect vasoconstriction. *P<0.05 the pressor response at 0.25 µg/ml almitrine differs from that with no almitrine. **P<0.05 the value differs from that with no almitrine.

We found that isolated pulmonary arterial myocytes form three distinct populations characterized by size, shape, and electrophysiological properties.Based on the predominant type of potassium (K) channel, we call them K_Ca cells, K_DR cells, and mixed cells (Figure 1). Hypoxia, the most important physiological regulator of the pulmonary circulation, acts predominantly on the K_DR channels (prevalent in the K_DR cells), causing their closure (Figure 2). This leads to depolarization and vasoconstriction. Consequently, the hypoxic pulmonary vasoconstriction occurs in the small pulmonary arteries, where K_DR cells are more numerous than in the large pulmonary arteries, where the vasoconstrictor response to hypoxia is only transient, followed by vasodilation (Figure 3). On the other hand, the calcium-dependent K channels, predominant in the K_Ca cells, are activated by another endogenous regulator, nitric oxide (NO), leading to membrane hyperpolarization and vasorelaxation.

Figure 1: Three types of pulmonary arterial smooth muscle cells.

The large, elongated cells with K current dominated by the calcium-dependent K channel (KCa) have relatively small, spiky K currents and are more common in the large than in the small pulmonary arteries (the right-hand graph). The smaller cells with K currents dominated by the delayed rectifier K channel (KDR) have large, smooth K currents, a perinuclear bulge, and are prevalent in the small pulmonary arteries. The mixed cells are a distinct population with intermediary features.

Figure 2: Acute hypoxia reversibly decreases the open probability of a K channel of a pulmonary arterial myocyte with the KDR phenotype. The patch pipette was attached to the membrane of a vesicle removed from the myocyte so that three individual K channels were in the pipette's mouth. Tiny perforations were made in the membrane of the vesicle by amphotericin to allow diffusion of the internal pipette solution into the vesicle. Opening of one of the channels caused the recorded current to suddently rise from the level marked "c" to the level marked by the next horizontal line above "c". Opening of 2 or 3 channels at the same time shows as the current being at the level of the third or fourth mark from the bottom, respectively.

Figure 3: Hypoxia causes a sustained constriction of small resistance pulmonary arteries but only a transient vasoconstriction follwed by sustained relaxation in large, conduit pulmonary artery studied in vitro. The rings of the pulmonary arteries were dissected, mounted on stainless steel wires in an organ bath and their response to hypoxia was measured without preconstriction. The data are averaged (± SEM) tension traces from 35 rings/group.

The electrophysiological data in this study were acquired by Drs. Jimmy Huang, Helen Reeve and Evangelos Michelakis. The pulmonary arterial rings data were acquired by Dr. Simona Tolarova.

We wanted to know which of the many types of potassium channels is closed by hypoxia in the pulmonary circulation, thus leading to hypoxic pulmonary vasoconstriction. We tested the antibodies against two of the most likely candidate channels, Kv1.5 and Kv2.1. We found that the Kv1.5 antibodies reduced the increase in intracellular calcium ([Ca²⁺]_i), caused in pulmonary artery smooth muscle cells by hypoxia, and reduced hypoxic pulmonary vasoconstriction in isolated lungs. This indicates that the Kv1.5 channel is important in the mechanism of hypoxic pulmonary vasoconstriction. Kv2.1 antibodies caused pulmonary vasoconstriction in normoxia, indicating a role in maintaining normal low basal tension in the pulmonary vessels.