Modulation of arterial Na+-K+-ATPase-induced [Ca2+](i) reduction and relaxation by norepinephrine, ET-1, and PMA

Na+-K+-ATPase plays a major role in regulating membrane potential and vascu lar tone. We analyzed the modulation by norepinephrine (NE), endothelin-1 ( ET-1), and phorbol 12-myristate 13-acetate (PMA) of Na+-K+-ATPase-induced c ytoplasmic free Ca2+ concentration ([Ca2+](i)) reduction and relaxation in isolated endothelium-denuded piglet mesenteric arteries. KCl (0.2-8.8 mM)-i nduced [Ca2+](i) reduction and relaxation in arteries incubated in K+-free solution were used as functional indicators of Na+-K+-ATPase activity. KCl- induced relaxations after exposure to K+-free solution were associated with a reduction in [Ca2+](i), as measured by fura 2 fluorescence. However, KCl reduced [Ca2+](i) below resting values, whereas force was reduced to near resting values. NE, ET-1, and PMA inhibited the relaxant effects of KCl, an d this effect was attenuated by the protein kinase C inhibitor staurosporin e but not by the phospholipase A(2) inhibitor quinacrine. However, ET-1 and PMA potentiated the [Ca2+](i)-reducing effect of KCl. In conclusion, ET-1, PMA, and NE are functional inhibitors of Na+-K+-ATPase activity in endothe lium-denuded piglet mesenteric arteries, even when the direct effect on the enzyme activity may be stimulatory rather than inhibitory. This can be exp lained because ET-1, PMA, and NE induce Ca2+ sensitization for smooth muscl e contraction, and therefore relaxations do not parallel the reductions in [Ca2+](i) after the activation of Na+-K+-ATPase.