CONTROL OF PULMONARY VASCULAR SMOOTH-MUSCLE TONE BY SARCOPLASMIC-RETICULUM CA2+ PUMP BLOCKERS - THAPSIGARGIN AND CYCLOPIAZONIC ACID

The effect of thapsigargin (TG) and cyclopiazonic acid (CPA) on the mechanical activity of the rat pulmonary artery were investigated. In chemically (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 procedure of the internal Ca2+ stores (0.3 mu M free Ca2+ ions for 8-10 min) concentration 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 contraction but also increased the tension which appeared during the exposure time to Ca2+. The two phenomena developed with similar time courses. The increase in tension during the readmission of Ca2+ ions was not antagonized 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 verapamil-insensitive TG (or CPA)-induced contraction was dependent on the external [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+-rich (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 mM K+ respectively. This contraction was also concentration dependently inhibited by the tyrosine kinase inhibitors genistein (0.5-50 mu M) and tyrphostin (2-50 mu M). Sr2+ ions, which contracted both depolarized intact and skinned strips, failed to replace Ca2+ ions in the verapamil-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, although it appears to involve a tyrosine kinase activity.