Regional distribution of potassium currents in the rabbit pulmonary arterial circulation

The response of pulmonary arteries to hypoxia varies as a function of vesse l diameter. Small intrapulmonary resistance arteries are thought to be the main site of hypoxic pulmonary vasoconstriction (HPV), with hypoxia causing minimal contraction or even dilatation in large, conduit vessels. This has been proposed to reflect a differential distribution of morphologically an d electrophysiologically distinct pulmonary artery smooth muscle (PASM) cel ls. We investigated longitudinal heterogeneity in smooth muscle cells isola ted from five regions of the rabbit pulmonary vasculature and could Find no evidence of morphological heterogeneity at the level of the light microsco pe. PASM cells from main (similar to 8 mm outer diameter) acid branch (simi lar to 5 mm) arteries and large (> 400 mu m) intrapulmonary arteries (IPA) were similar in shape and size, as indicated by cell capacitance (similar t o 25 pF). PASM cells from medium (200-400 mu m) and small (< 200 mu m) IPA were significantly smaller (similar to 15 pF), but had the same classical s pindle shape. Cells from all five regions also had similar resting membrane potentials and displayed voltage-activated K+ currents of similar amplitud e when recorded in standard physiological solution. Longitudinal heterogene ity in KC current became apparent when tetraethylammonium ions (TEA; 10 mM) and glibenclamide (10 mu M) were added. The remaining delayed rectifier cu rrent (I-K(V)) doubled in amplitude upon moving down the pulmonary arterial tree from the main artery (9 pA pF(-1) at 40 mV) to the large IPA (17 pA p F(-1)), but remained constant throughout the intrapulmonary vasculature. Th e O-2-sensitive, non-inactivating K+ current (I-K(N)) showed a similar tren d, but was significantly reduced in the smallest IPA, where its amplitude w as comparable with the main artery. Thus the I-K(N)/I-K(V) ratio was relati vely constant, at around 0.14, from the main pulmonary artery to medium IPA , but fell by 50% in the smallest vessels. The amplitude of the TEA-sensiti ve K+ current was similar (similar to 16 pA pF(-1) at 40 mV) at all levels of the pulmonary arterial tree, except in the medium sized vessels where it was 50% smaller. These variations in K+ current expression correlate with reported variations in sensitivity to hypoxia and may contribute to the reg ional heterogeneity of HPV in the rabbit lung.