Effects of three different Ca2+ pump ATPase inhibitors on evoked contractions in rabbit aorta and activities of Ca2+ pump ATPases in porcine aorta

Abstract

Using vascular smooth muscle, we describe the actions of three pharmacological tools, cyclopiazonic acid (CPA), thapsigargin (TG) and 2,5-di-(tert-butyl)-1,4-benzohydroquinone (tBHQ), which are presumed to act as selective inhibitors of the sarco-endoplasmic reticulum Ca²⁺-ATPases (SERCAs). In porcine aortic smooth muscle microsomes two Ca²⁺-ATPase activities have been described, one vanadate-sensitive and one vanadate-resistant, representing the Ca²⁺-ATPase activities of the plasma membrane and SERCAs, respectively. In agreement, CPA, TG and tBHQ, in the concentration range 0.1 μM to 0.1 mM, dose-dependently inhibit the Ca²⁺-ATPase activity only in the vanadate-resistant microsomes. However, 0.1 mM tBHQ also significantly inhibited the Ca²⁺-ATPase activity of vanadate-sensitive microsomes. In rabbit aortic rings, all three SERCA inhibitors produced a dose-dependant inhibition of contractions evoked by 20 mM caffeine or 1 μM phenylephrine (PE) in a Ca²⁺-free physiological solution. However, in PE-contracted rings, tBHQ (≥30 μM) also significantly inhibited the ability of cromakalim to induce relaxation. In conclusion, the data suggest that CPA, TG and tBHQ can all act as selective SERCA inhibitors in both porcine and rabbit aortic smooth muscle. However, in contrast to CPA and TG, high concentrations of tBHQ can exhibit some nonspecific effects, which include inhibition of the plasma membrane Ca²⁺-ATPase and possibly K⁺ channels regulated by cromakalim.