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J Physiol. 2000 Jun 15;525 Pt 3:669-80.

Voltage-independent calcium entry in hypoxic pulmonary vasoconstriction of intrapulmonary arteries of the rat.

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1
Department of Respiratory Medicine and Allergy, GKT School of Medicine, King's College London, Guy's Campus, London SE1 9RT, UK.

Abstract

It has been proposed that hypoxic pulmonary vasoconstriction (HPV) is mediated via K+ channel inhibition and Ca2+ influx through voltage-gated channels. HPV depends strongly on the degree of preconstriction, and we therefore examined the effect of Ca2+ channel blockade on tension and intracellular [Ca2+] ([Ca2+]i) during HPV in rat intrapulmonary arteries (IPAs), whilst maintaining preconstriction constant. We also investigated the role of intracellular Ca2+ stores. HPV demonstrated a transient constriction (phase I) superimposed on a sustained constriction (phase II). Nifedipine (1 microM) partially inhibited phase I, but did not affect phase II. In arteries exposed to 80 mM K+ and nifedipine or diltiazem the rises in tension and [Ca2+]i were blunted during phase I, but were unaffected during phase II. At low concentrations (< 3 microM), La3+ almost abolished the phase I constriction and rise in [Ca2+]i, but had no effect on phase II, or constriction in response to 80 mM K+. Phase II was inhibited by higher concentrations of La3+ (IC50 approximately 50 microM). IPA treated with thapsigargin (1 microM) in Ca2+-free solution to deplete Ca2+ stores showed sustained constriction upon re-exposure to Ca2+ and an increase in the rate of Mn2+ influx, suggesting capacitative Ca2+ entry. The concentration dependency of the block of constriction by La3+ was similar to that for phase I of HPV. Pretreatment of IPA with 30 microM CPA reduced phase I by > 80 %, but had no significant effect on phase II. We conclude that depolarization-mediated Ca2+ influx plays at best a minor role in the transient phase I constriction of HPV, and is not involved in the sustained phase II constriction. Instead, phase I appears to be mainly dependent on capacitative Ca2+ entry related to release of thapsigargin-sensitive Ca2+ stores, whereas phase II is supported by Ca2+ entry via a separate voltage-independent pathway.

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