Reduction by (-)-cicletanine of the vascular reactivity to angiotensin II in rats

Myocardial PKC beta2 and the sensitivity of Na/K-ATPase to marinobufagenin are reduced by cicletanine in Dahl hypertension

Marinobufagenin (MBG), an endogenous ligand of alpha-1 Na/K-ATPase, becomes elevated and contributes to hypertension in NaCl-loaded Dahl-S rats (DS). Protein kinase C (PKC) phosphorylates alpha-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 alpha-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 alpha-1 Na/K-ATPase levels, and sensitivity to MBG in hypertensive DS. Seven DS received 50 mg x kg(-1) x 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 alpha-1 Na/K-ATPase protein, and PKC beta2 and delta. The sensitivity of Na/K-ATPase to MBG was enhanced at the level of high-affinity binding sites (IC50, 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 alpha-1 Na/K-ATPase protein and MBG levels were unchanged. In cicletanine-treated rats, PKC beta2 was not increased, the sensitivity of Na/K-ATPase to MBG was decreased (IC50=20 micromol/L), and phorbol diacetate-induced alpha-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 alpha-1 Na/K-ATPase is a likely target for cicletanine treatment.

Renal-protective effect of nondepressor dose of cicletanine in diabetic rats with hypertension

We assessed the renal and cardiac benefits of cicletanine (CIC), a furopyridine derivative drug with diuretic and antihypertensive properties, in diabetic spontaneously hypertensive rats with renal impairment. Uninephrectomized streptozotocin (STZ)-diabetic spontaneously hypertensive Izmo rats (SHRIzm) (10 weeks old) were randomly assigned to receive vehicle or CIC (100 mg/kg/day, orally), and age-matched, uninephrectomized STZ diabetic Wistar-Kyoto Izmo rats (WKYIzm) were assigned to receive vehicle for up to 12 weeks. Blood pressure increased progressively in diabetic SHRIzm but not in diabetic WKYIzm. Urinary albumin excretion increased significantly in both diabetic SHRIzm and diabetic WKYIzm throughout the experiment. The antihypertensive effect of CIC was not significantly observed in diabetic SHRIzm. However, the subdepressor doses of CIC significantly decreased urinary albumin excretion, serum creatinine, and blood urea nitrogen in diabetic SHRIzm. These results were confirmed by morphological analysis of kidneys in each group of rats. The index of focal glomerular sclerosis (FGS) in diabetic SHRIzm was significantly higher than that in diabetic WKYIzm. The CIC treatment significantly and effectively protected against an increase in the index of FGS in diabetic SHRIzm. Moreover, CIC treatment significantly attenuated the increase in the heart weight to body weight ratio in diabetic SHRIzm. Treatment with CIC did not affect urinary and blood glucose concentrations at this dose. These results suggest that CIC has a renal-protective action, which is not related to improvement of diabetes or improvement of high blood pressure in diabetic rats with hypertension. The action might be due to the reduction of intraglomerular capillary pressure or protection of the renal glomerular vascular endothelial cell injury and mesangial cell injury through stimulation of PGI2 generation or elimination of free radicals, although the mechanism remains to be further investigated.

Cicletanine reverses vasoconstriction induced by the endogenous sodium pump ligand, marinobufagenin, via a protein kinase C dependent mechanism

RATIONALE

Cicletanine (CIC), an anti-hypertensive compound with direct vascular and natriuretic actions, is especially effective in salt-sensitive hypertension, in which dysregulation of the sodium pump plays an important pathogenic role, and digitalis-like cardiotonic steroids contribute to increased vascular tone. The purpose of the present study was to investigate whether, and by what mechanisms, cicletanine antagonizes the vasoconstrictor effects of cardiotonic steroids in isolated human arteries.

METHODS

The effects of cicletanine on vascular tone were studied in isolated, endothelium-denuded rings of 2nd-3rd-order branches of human mesenteric arteries pre-contracted with bufodienolide marinobufagenin (MBG), an Na/K-ATPase inhibitor, or endothelin-1 (ET-1). Na/K-ATPase activity was measured in sarcolemmal membranes from the mesenteric artery. Activity of rat brain protein kinase C (PKC) was measured using the PepTag phosphorylation assay.

RESULTS

MBG and ET-1 both induced sustained vasoconstriction in human mesenteric artery rings, and cicletanine relaxed rings pre-contracted with either MBG (EC50 = 11 +/- 2 micromol/l) or ET-1 (EC50 = 6.4 +/- 1.1 micromol/l). Although 8-Br-cGMP (100 micromol/l) caused complete vasorelaxation of arterial rings pre-contracted with ET-1, it did not affect the MBG-induced vasoconstriction. An activator of PKC, phorbol diacetate (PDA) (50 nmol/l), attenuated CIC-induced vasorelaxation of mesenteric artery rings pre-contracted with MBG (EC50 > 100 micromol/l), but not rings pre-contracted with ET-1 (EC50 = 6.5 +/- 1.2 micromol/l). In mesenteric artery sarcolemma, 100 nmol/l MBG inhibited the Na/K-ATPase by 68 +/- 5% and cicletanine (100 micromol/l) attenuated this Na/K-ATPase inhibition by 85 +/- 6%. In the PepTag PKC assay, cicletanine produced a concentration-dependent inhibition of rat brain PKC activity (IC50 45 +/- 11 micromol/l). In the presence of 50 nmol/l PDA, 100 micromol/l cicletanine did not antagonize the Na/K-ATPase inhibition by MBG, and did not inhibit the PKC from rat brain.

CONCLUSIONS

Cicletanine antagonizes vasoconstriction induced by Na/K-ATPase inhibition via a PKC-dependent mechanism that does not involve inhibition of cyclic GMP phosphodiesterase (cGMP-PDE). This mechanism of action may be relevant to the greater potency of cicletanine in salt-sensitive hypertension in which plasma levels of endogenous digitalis-like cardiotonic steroids are elevated. Our findings also suggest that PKC is an important factor for cardiotonic steroid-Na/K-ATPase interactions on the vascular tone, and is therefore a potential target for therapeutic intervention in hypertension.

Effects of oxazepam and acetaminophen on cicletanine metabolism in rat hepatocytes and liver microsomes

Cicletanine, a racemic furopyridine derivative synthesized as racemate, is used as an antihypertensive agent. Its two enantiomers are involved in the pharmacological effects of the drug. Cicletanine is metabolized by conjugation enzyme systems (phase II) into sulfoconjugated or glucuroconjugated enantiomers. As oxazepam and acetaminophen are widely prescribed, especially to elderly patients, these two drugs may be co-administered with cicletanine. The metabolic profile and the kinetics of biotransformation were studied by using rat hepatocytes and liver microsomes. Cicletanine was extensively metabolized by rat hepatocytes. More than 80% of the drug was biotransformed after a 3 h incubation. The formation of glucuroconjugated metabolites was characterized by the following kinetic parameters, i.e. Vmax = 2.05 +/- 0.21 nmol/min/mg protein and Km = 287 +/- 6.7 microM for (-)-cicletanine, and Vmax = 1.44 +/- 0.12 nmol/min/mg protein and K(m) = 171 +/- 4.1 microM for (+)-cicletanine. Oxazepam inhibited the glucuronidation of cicletanine in both rat hepatocytes and liver microsomes with a competitive-type inhibition, i.e. K(i) = 129 +/- 7.5 and 152 +/- 19.7 microM for (-)-cicletanine and (+)-cicletanine, respectively. The co-incubation of acetaminophen with cicletanine showed that only sulfoconjugation was inhibited in rat hepatocytes. Glucuronidation was not modified by acetaminophen. As natriuric activity is due to sulfoconjugated (+)-cicletanine, acetaminophen could potentially modulate in vivo the pharmacological effect of cicletanine. The data of the in vitro study reported here suggested an interaction between cicletanine and oxazepam or cicletanine and acetaminophen. However, the clinical impact of such a drug interaction needs further evaluation.

Cicletanine: new insights into its pharmacological actions

Cicletanine ((+/-)3-(4-chlorophenyl)-1,3-dihydro-7-hydroxy-6-methylfuro-[3,4-c ] pyridine) 3-(4-chlorophenyl)-1,3-dihydro-7-hydroxy-6-methylfuro-[3,4-c] pyridine) is a novel antihypertensive agent that has been shown to possess vasorelaxant, natriuretic, and diuretic properties in preclinical and clinical studies. The mechanism(s) by which cicletanine induces these biological effects has not been definitely established, although it appears to differ from that of other classes of antihypertensive drugs. The salidiuretic activity appears to be the result of an action of the sulfoconjugated metabolite of cicletanine, which inhibits the apical Na+-dependent Cl-/HCO3- anion exchanger in the distal convoluted tubule. The mechanism of the vasodilating effect of cicletanine seems to be complex; it may include stimulation of vascular prostaglandin synthesis, inhibition of the low Km cyclic GMP phosphodiesterases, and blockade of Ca2+ channels either directly or indirectly through a K+-channel opening effect. The drug has also been shown to interact with alpha-adrenergic, vascular histamine, and muscarinic receptors. We have also reviewed the other vascular effects of the drug, such as stimulation of nitric oxide synthesis and inhibition of both myosin light chain kinase and protein kinase C. Cicletanine protects cardiovascular and renal systems against the injuries induced by hypertension, in addition to its lowering of arterial pressure. Similarly to the vasorelaxant action of cicletanine, the various properties of the drug likely contribute to its protective effect against injury in hypertension.

Renal-protective effect of non-depressor dose of cicletanine in streptozotocin diabetic rats

OBJECTIVE

To assess the renal benefits of cicletanine (CIC) in diabetic rats with renal impairment.

METHODS

Hemi-nephrectomized streptozotocin-diabetic Wistar-Kyoto Izmo rats (WKYIzm) (10 weeks old) were randomly assigned to receive vehicle or a low or high dose of CIC (30 or 100 mg/kg per day, orally) for 12 weeks.

RESULTS

The blood pressure was raised slightly but not significantly in this model. An anti-hypertensive effect of CIC was not significantly observed. However, the sub-depressor doses of CIC significantly and dose-dependently decreased urinary albumin excretion. These results were confirmed by morphological analysis of kidneys in each group of rats. CIC treatment significantly and effectively protected against an increase in the percentage of focal glomerular sclerosis. CIC did not affect urinary and blood glucose concentrations at either dose.

CONCLUSIONS

These results suggest that CIC has a renal-protective action, which is not related to improvement of diabetes or of high blood pressure in this model. The action might be due to the reduction of intraglomerular capillary pressure, although the mechanism remains to be further investigated.