Pharmacological evidence that potentiation of plasmalemmal Ca²⁺-extrusion is functionally coupled to inhibition of SR Ca²⁺-ATPases in vascular smooth muscle cells

Abstract

Cyclopiazonic acid (CPA), a specific inhibitor of sarcoplasmic reticulum (SR) Ca²⁺-ATPases, causes slowly developing and subsequently diminishing characteristic contractions in vascular smooth muscle, and the second application of CPA has incompletely repeatable effects, depending on the vessel type. The objective of the present study was to examine the mechanisms underlying the significant decrease of CPA-induced contractions upon the second application. A pharmacological intervention of Ca²⁺ extrusion process as a strategy was performed to modulate vasoconstrictor effects of CPA in rat aortic ring preparations. CPA-induced contractions, expressed as percentages of the contractions induced by KCl (80 mM), were significantly decreased from 44.1 ± 5.7 to 7.6 ± 1.8 % (P < 0.001) upon the second application. The contractions, however, were completely repeatable in the presence of vanadate, an inhibitor of ATPases, but not of ouabain, an inhibitor of Na⁺-pumps. Strikingly, CPA-induced contractions were sustained and completely repeatable in Na⁺-free and low Na⁺ medium. Furthermore, we found that the contractions were completely repeatable in the presence of 2′,4′-dichlorobenzamil, an inhibitor of the forward mode of Na⁺/Ca²⁺ exchangers, but not of KBR7943, an inhibitor of the reverse mode of Na⁺/Ca²⁺ exchangers. Our findings indicate that CPA by inducing a transient rise in cytosolic Ca²⁺ level causes a long-lasting upregulation of plasma membrane (PM) Ca²⁺ extruders and thus leads to a diminished contraction upon its second application in blood vessels. This suggests that there is a functional coupling between PM Ca²⁺ extruders and SR Ca²⁺-ATPases in rat aortic smooth muscle cells.