Inhibitors of Na-K-ATPase in human urine: effects of ouabain-like factors and of vanadium-diascorbate on calcium mobilization in rat vascular smooth muscle cells

Endogenous ouabain-like factors (OLF) may play a role in the pathogenesis of volume-dependent hypertension by raising intracellular free calcium ([Ca2+]i) as a consequence of inhibition of the sodium pump. In previous studies we described the presence of two low molecular (Mr ≈ 400) inhibitors of Na-K-ATPase in human urine, ie, a more polar OLF-1 and a more apolar OLF-2. We subsequently identified the active compound in OLF-2 as vanadium (VIV)-diascorbate (Mr 416). OLF-1, OLF-2, and V-diascorbate inhibited dose-dependently porcine Na-K-ATPase in vitro. In the present study we investigated the effects of urinary OLF-1, OLF-2, and V-diascorbate on calcium mobilization, ie, on [Ca2+]i in cultured rat vascular smooth muscle (VSM) cells in comparison to the effects of ouabain, angiotensin II (A II), and arginine-vasopressin (AVP). [Ca2+]i was determined by the fura-2 method. OLF-1 and OLF-2 (each ≈ 10−4 mol/L), obtained as single spots by thin-layer chromatography, produced a rise in [Ca2+]i in VSM cells from 45 ± 7 to 99 ± 22 and from 48 ± 9 to 92 ± 2 nmol/L (each n = 5; P< .05), respectively, after 3 min. V-diascorbate also increased [Ca2+]i slowly and dose-dependently, eg, from 56 ± 14 to 102 ± 15 nmol/L at a concentration of 10−6 mol/L (n = 5; P< .05) after 3 min. A similar slow rise in [Ca2+]i from 53 ± 10 to 185 ± 3 nM (n = 5; P< .05) after 3 min was found with ouabain (10−6 mol/L). As standard vasoconstrictor, All (10−8 mol/L) rapidly increased [Ca2+]i from 23 ± 4 to 846 ± 50 nmol/L (n = 7; P< .01) within 30 sec. This effect was enhanced to 1389 ± 161 nM (n = 7; P< .01) when VSM cells were preincubated with V-diascorbate (10−6 mol/L) for 10 min. AVP (10−7 mol/L) also rapidly increased [Ca2+]i to 418 ± 11 nmol/L within 30 sec (n = 7; P< .01). This effect was enhanced in the presence of OLF-2 (≈10−4 mol/L) or ouabain (10−6 mol/L) to 523 ± 14 and 560 ± 19 nmol/L, respectively (each n = 7); P< .01). The calcium channel blocker verapamil, the intracellular calcium release blocker TMB-8, and the unselective cation channel blocker Ni2+ partly blunted the A II- or AVP-induced rise in [Ca2+]i and prevented the OLF-2- and V-diascorbate–induced increase in [Ca2+]i. Thus, OLF-1, OLF-2 and V-diascorbate, the active component of OLF-2, reveal effects similar to those of ouabain on [Ca2+]i in VSM cells, ie, they produce a slow rise in [Ca2+]i subsequent to inhibition of the sodium pump. The physiologic and pathologic roles of these and additional OLF in body fluid and blood pressure regulation and in hypertension have yet to be evaluated.