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

Abstract

1. 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. 2. HPV demonstrated a transient constriction (phase I) superimposed on a sustained constriction (phase II). Nifedipine (1 μM) 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. 3. At low concentrations (< 3 μM), 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 ~ 50 μM). 4. IPA treated with thapsigargin (1 μM) 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 μM CPA reduced phase I by > 80%, but had no significant effect on phase II. 5. 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.