Pg. Delafuente et al., CONTROL OF PULMONARY VASCULAR SMOOTH-MUSCLE TONE BY SARCOPLASMIC-RETICULUM CA2- THAPSIGARGIN AND CYCLOPIAZONIC ACID( PUMP BLOCKERS ), Pflugers Archiv, 429(5), 1995, pp. 617-624
The effect of thapsigargin (TG) and cyclopiazonic acid (CPA) on the me
chanical activity of the rat pulmonary artery were investigated. In ch
emically (beta-escin)-skinned arterial strips, application of TG (0.1-
1 mu M) or CPA (0.5-10 mu M) prior and throughout the loading procedur
e of the internal Ca2+ stores (0.3 mu M free Ca2+ ions for 8-10 min) c
oncentration dependently inhibited the subsequent contractile response
induced by noradrenaline (NA, 10 mu M) or caffeine (25 mM). In intact
strips repeatedly incubated in a Ca2+-containing solution (2.5 mM for
10 min), followed by incubation in a Ca2+-free solution 12 min before
NA-stimulation, TG and CPA not only inhibited the NA-induced contract
ion but also increased the tension which appeared during the exposure
time to Ca2+. The two phenomena developed with similar time courses. T
he increase in tension during the readmission of Ca2+ ions was not ant
agonized by verapamil (10 mu M) or nifedipine (1 mu M) but was blocked
by La3+ (50 mu M) and Co2+ (1 mM) ions. The amplitude of the verapami
l-insensitive TG (or CPA)-induced contraction was dependent on the ext
ernal [Ca2+][0.1-10 mM, concentration for half maximal effect (EC(50))
= 0.85 mM], not modified by the reduction of the external [Na+] (from
130 to 10 mM) and decreased by depolarization of the strip using K+-r
ich (30-120 mM) solutions. Under the latter condition, 38 +/- 9 and 83
+/- 4% reduction (n = 5) was observed in the presence of 60 and 120 m
M K+ respectively. This contraction was also concentration dependently
inhibited by the tyrosine kinase inhibitors genistein (0.5-50 mu M) a
nd tyrphostin (2-50 mu M). Sr2+ ions, which contracted both depolarize
d intact and skinned strips, failed to replace Ca2+ ions in the verapa
mil-insensitive contraction induced by TG or CPA (n = 4). Finally, TG
(1 mu M) and CPA (10 mu M) did not modify the pCa tension relationship
in skinned strips (n = 5). These results show that the main action of
TG and CPA in rat pulmonary artery is to prevent the refilling of the
internal Ca2+ store. TG and CPA also seem to facilitate a Ca2+ influx
through a specific verapamil-insensitive pathway. The biophysical and
molecular characteristics of this pathway remain to be elucitated, al
though it appears to involve a tyrosine kinase activity.