Integrative understanding of hypoxic pulmonary vasoconstriction using in vitro models: from ventilated/perfused lung to single arterial myocyte

Abstract

Contractile response of a pulmonary artery (PA) to hypoxia (hypoxic pulmonary vasoconstriction; HPV) is a unique physiological reaction. HPV is beneficial for the optimal distribution of blood flow to differentially ventilated alveolar regions in the lung, thereby preventing systemic hypoxemia. Numerous in vitro studies have been conducted to elucidate the mechanisms underlying HPV. These studies indicate that PA smooth muscle cells (PASMCs) sense lowers the oxygen partial pressure (PO₂) and contract under hypoxia. As for the PO₂-sensing molecules, a variety of ion channels in PASMCs had been suggested. Nonetheless, the modulator(s) of the ion channels alone cannot mimic HPV in the experiments using PA segments and/or isolated organs. We compared the hypoxic responses of PASMCs, PAs, lung slices, and total lungs using a variety of methods (e.g., patch-clamp technique, isometric contraction measurement, video analysis of precision-cut lung slices, and PA pressure measurement in ventilated/perfused lungs). In this review, the relevant results are compared to provide a comprehensive understanding of HPV. Integration of the influences from surrounding tissues including blood cells as well as the hypoxic regulation of ion channels in PASMCs are indispensable for insights into HPV and other related clinical conditions.

Keywords: K+ channel; hypoxic pulmonary vasoconstriction; oxygen; patch clamp; pulmonary artery; smooth muscle.

Fig. 1

A hypoxic pulmonary vasoconstriction study using a ventilated/perfused lung model in rodents. (A) A schematic drawing of the experiment. The rodent ventilator is connected to a tracheal cannula, and either normoxic [O₂ pressure (PO₂), 21%] or hypoxic gas (PO₂, 3%) is passed through it. Perfusion of the pulmonary vascular system is achieved using a peristaltic pump connected to the right ventricle (i.e., pulmonary artery; PA) as an inlet and to the left atrium (i.e., pulmonary vein) as an outlet. Our system uses rat or mouse erythrocytes (closed circles in A). PA pressure is measured using a pressure transducer connected to the inlet tubing using a three-way connector. (B) Photo taken during the ventilated/perfused lung experiment in a mouse. LA, left atrium.

Fig. 2

Isometric contraction measurement using a pulmonary arterial (PA) ring. (A) A view of a rat lung. The third branch of the PA (arrow) was dissected and trimmed under a stereomicroscope. (B) PA rings placed in a Mulvany-type myograph using tungsten wires connected to two jaws (arrow). Although not shown here, the chamber fluid is directly bubbled with either normoxic [O₂ pressure (PO₂), 21%] or hypoxic (PO₂, 3%) gas. (C) A representative recording of simultaneously measured PO₂ and PA tone. An incremental change of PA tone (hypoxic pulmonary vasoconstriction) was discernable at PO₂ below 7%.

Fig. 3

Procedure for the preparation of lung slices (precision-cut lung slices; PCLSs) and video analysis of airway lumens using PCLSs. The PCLS chamber was perfused with a physiological solution enabling drug application and washout. In addition, the temperature of the experimental solutions was maintained using water jacket circulation systems and a semitight cover.

Fig. 4

A schematic drawing of a whole-cell patch-clamp recording and representative traces of membrane currents. Hypoxic perfusate [O₂ pressure (PO₂), 3%] decreased the amplitude of the outward K⁺ current (voltage-gated K⁺ channel current). PASMC, pulmonary artery smooth muscle cell.

Fig. 5

The current model of the cellular mechanism of hypoxic pulmonary vasoconstriction in a rat pulmonary artery (PA). Relevant ion channels are displayed. Under normoxia, the membrane potential of the smooth muscle of the PA is held at approximately −50 mV because of the TASK-like background current of a K⁺ channel. Hypoxic conditions initially decrease TASK activity. When combined with TXA₂, activation of NSC induces membrane depolarization up to the threshold voltage for activation of K_v channels (Step 1). In addition to the NSC activation, hypoxic inhibition of the K_v current further depolarizes the membrane potential (Step 2). As the membrane potential depolarizes above −40 mV, the activation of VOCC_L eventually allows for Ca²⁺ influx for contraction of smooth muscles. K_v, voltage-gated K⁺ channel; NSC, nonselective cation channel; TASK-1, background-type K⁺ channel with a two-pore domain (K2P); TXA₂, thromboxane A₂; VOCC_L, voltage-gated L-type Ca²⁺ channels.