The possible role of
sarcoplasmic reticulum (SR)
Ca2+ stores in
hypoxic pulmonary vasoconstriction (
HPV) is not well understood. In order to assess the possible role of
intracellular Ca2+ release from SR
Ca2+ stores in
HPV, we examined the effects of: (1)
ryanodine (10 microM) depletion of
intracellular Ca2+ stores, and (2)
thapsigargin (THAPS, 2 microM) or
cyclopiazonic acid (CPA, 10 microM) depletion of
intracellular Ca2+ stores on
HPV in canine
pulmonary artery. 2 Isometric tension was measured from
arterial ring suspended in Krebs-Henseliet
solution (K-H) bubbled with 95%O2/5%
CO2. Hypoxia was induced by bubbling
phenylephrine (PE, 1 microM) precontracted rings with 95%N2/5%
CO2.
HPV was observed in both intact and endothelial-denuded
arteries and expressed as % of maximal
KCl contraction (% Tkmax) = 21.3 +/- 3.2%; n = 13 and 21.7 +/- 4%; n = 4 respectively. 3 When SR
caffeine sensitive
Ca2+ stores were depleted by pretreatment with
ryanodine and brief
caffeine (15 mM) exposure, the hypoxic response was significantly reduced to 19.1 +/- 9.2% of the control hypoxic contraction (n = 7; p < 0.001) with little or no effect on PE or
KCl contractions. On the other
hand, in normoxic rings pretreated with THAPS or CPA, the PE responses were significantly reduced (% Tkmax = 18.2 +/- 3.1% compared to 39.0 +/- 3.9% in control; n = 16; P < 0.001; %Tkmax = 3.4 +/- 1.6% compared to 49.9 +/- 7.9% in control; n = 6; P < 0.001; respectively) with no significant effect on caffeine-induced contractions, suggesting that both THAPS and CPA preferentially deplete InsP3-sensitive
Ca2+ stores, without affecting the caffeine-sensitive
Ca2+ store; consistent with the existence of separate and independent InsP3 and caffeine-sensitive
Ca2+ stores in this preparation. 4 When hypoxia was induced in the presence of THAPS or CPA, developed tension was significantly larger than control (% Tkmax = 64.5 +/- 6.0%; n = 16; P < 0.05%; %Tkmax = 78.2 +/- 15%; n = 6; P < 0.05; respectively), was partially blocked by
nisoldipine (10 microM) and
ryanodine (% Tkmax = 20.3 +/- 3.7%; n = 6), and nearly completely blocked by SK&F 96365 (50 microM). However, the actions of SK&F 96365 appeared to be nonselective since this compound also significantly reduced contractions elicited by
KCl, PE and
caffeine. 5 Finally, evidence was obtained suggesting: (a) that at least some of the
Ca2+ released from the caffeine- and ryanodine-sensitive
Ca2+ stores by hypoxia may be taken up and buffered by the InsP3-sensitive
Ca2+ stores, and (b) the apparent dependence of
HPV on
extracellular Ca2+ entry pathways may be partially due to the dependence of the
Ca2+ content of
intracellular SR
Ca2+ stores on sarcolemmal
Ca2+ entry pathways. 6 These data suggest that caffeine- and ryanodine-sensitive SR
Ca2+ stores contribute significantly to
HPV under normal conditions and, in the presence of THAPS or CPA, an additional nisoldipine- and ryanodine-insensitive
Ca2+ entry pathway is evoked by hypoxia.
