Upregulation of Na+and Ca2+transporters in arterial smooth muscle from ouabain-induced hypertensive rats

Abstract

Prolonged ouabain administration (25 μg·kg⁻¹·day⁻¹for 5 wk) induces “ouabain hypertension” (OH) in rats, but the molecular mechanisms by which ouabain elevates blood pressure are unknown. Here, we compared Ca²⁺signaling in mesenteric artery smooth muscle cells (ASMCs) from normotensive (NT) and OH rats. Resting cytosolic free Ca²⁺concentration ([Ca²⁺]_cyt; measured with fura-2) and phenylephrine-induced Ca²⁺transients were augmented in freshly dissociated OH ASMCs. Immunoblots revealed that the expression of the ouabain-sensitive α₂-subunit of Na⁺pumps, but not the predominant, ouabain-resistant α₁-subunit, was increased (2.5-fold vs. NT ASMCs) as was Na⁺/Ca²⁺exchanger-1 (NCX1; 6-fold vs. NT) in OH arteries. Ca²⁺entry, activated by sarcoplasmic reticulum (SR) Ca²⁺store depletion with cyclopiazonic acid (SR Ca²⁺-ATPase inhibitor) or caffeine, was augmented in OH ASMCs. This reflected an augmented expression of 2.5-fold in OH ASMCs of C-type transient receptor potential TRPC1, an essential component of store-operated channels (SOCs); two other components of some SOCs were not expressed (TRPC4) or were not upregulated (TRPC5). Ba²⁺entry activated by the diacylglycerol analog 1-oleoyl-2-acetyl- sn-glycerol [a measure of receptor-operated channel (ROC) activity] was much greater in OH than NT ASMCs. This correlated with a sixfold upregulation of TRPC6 protein, a ROC family member. Importantly, in primary cultured mesenteric ASMCs from normal rats, 72-h treatment with 100 nM ouabain significantly augmented NCX1 and TRPC6 protein expression and increased resting [Ca²⁺]_cytand ROC activity. SOC activity was also increased. Silencer RNA knockdown of NCX1 markedly downregulated TRPC6 and eliminated the ouabain-induced augmentation; silencer RNA knockdown of TRPC6 did not affect NCX1 expression but greatly attenuated its upregulation by ouabain. Clearly, NCX1 and TRPC6 expression are interrelated. Thus, prolonged ouabain treatment upregulates the Na⁺pump α₂-subunit-NCX1-TRPC6 (ROC) Ca²⁺signaling pathway in arterial myocytes in vitro as well as in vivo. This may explain the augmented myogenic responses and enhanced phenylephrine-induced vasoconstriction in OH arteries ( 83 ) as well as the high blood pressure in OH rats.