Myocardial PKC β2 and the Sensitivity of Na/K-ATPase to Marinobufagenin Are Reduced by Cicletanine in Dahl Hypertension

Abstract

Marinobufagenin (MBG), an endogenous ligand of α-1 Na/K-ATPase, becomes elevated and contributes to hypertension in NaCl-loaded Dahl-S rats (DS). Protein kinase C (PKC) phosphorylates α-1 Na/K-ATPase and increases its MBG sensitivity. Cicletanine, an antihypertensive compound with PKC-inhibitory activity, reverses MBG-induced Na/K-ATPase inhibition and vasoconstriction. We hypothesized that increased PKC levels in sodium-loaded hypertensive DS would sensitize α-1 Na/K-ATPase to MBG and that PKC inhibition by cicletanine would produce an opposite effect. We studied the effects of cicletanine on systolic blood pressure, left ventricular PKC isoforms, cardiac α-1 Na/K-ATPase levels, and sensitivity to MBG in hypertensive DS. Seven DS received 50 mg · kg −1 · d −1 cicletanine, and 7 DS received vehicle during 4 weeks of an 8% NaCl diet. Vehicle-treated rats exhibited an increase in blood pressure, left ventricular mass, MBG excretion (74±11 vs 9±1 pmol/24 h, P <0.01), myocardial α-1 Na/K-ATPase protein, and PKC β2 and δ. The sensitivity of Na/K-ATPase to MBG was enhanced at the level of high-affinity binding sites (IC 50 , 0.8 vs 4.4 nmol/L, P <0.01). Cicletanine-treated rats exhibited a 56-mm Hg reduction in blood pressure ( P <0.01) and a 30% reduction in left ventricular weight, whereas cardiac α-1 Na/K-ATPase protein and MBG levels were unchanged. In cicletanine-treated rats, PKC β2 was not increased, the sensitivity of Na/K-ATPase to MBG was decreased (IC 50 =20 μmol/L), and phorbol diacetate–induced α-1 Na/K-ATPase phosphorylation was reduced versus vehicle-treated rats. In vitro cicletanine treatment of sarcolemma from vehicle-treated rats also desensitized Na/K-ATPase to MBG, indicating that this effect was not solely attributable to a reduction in blood pressure. Thus, PKC-induced phosphorylation of cardiac α-1 Na/K-ATPase is a likely target for cicletanine treatment.