Synergistic action of NS-004 and internal Ca2+ concentration in modulating pulmonary artery K+ channels

Pulmonary Circulation Under Pressure: Pathophysiological and Therapeutic Implications of BK Channel

The large-conductance Ca2+-activated K+ (BK) channel is widely expressed in the pulmonary blood vessels and plays a significant role in regulating pulmonary vascular tonus. It opens under membrane depolarization, increased intracellular Ca+2 concentration, and chronic hypoxia, resulting in massive K+ efflux, membrane hyperpolarization, decreased L-type Ca+2 channel opening, and smooth muscle relaxation. Several reports have demonstrated an association between BK channel dysfunction and pulmonary hypertension (PH) development. Decreased BK channel subunit expression and impaired regulation by paracrine hormones result in decreased BK channel opening, increased pulmonary vascular resistance, and pulmonary arterial pressure being the cornerstone of PH. The resulting right ventricular pressure overload ultimately leads to ventricular remodeling and failure. Therefore, it is unsurprising that the BK channel has arisen as a potential target for treating PH. Recently, a series of selective, synthetic BK channel agonists have proven effective in attenuating the pathophysiological progression of PH without adverse effects in animal models.

CFTR activation in human bronchial epithelial cells by novel benzoflavone and benzimidazolone compounds

Activators of the CFTR Cl- channel may be useful for therapy of cystic fibrosis. Short-circuit current (Isc) measurements were done on human bronchial epithelial cells to characterize the best flavone and benzimidazolone CFTR activators identified by lead-based combinatorial synthesis and high-throughput screening. The 7,8-benzoflavone UCcf-029 was a potent activator of Cl- transport, with activating potency (<1 microM) being much better than other flavones, such as apigenin. The benzimidazolone UCcf-853 gave similar Isc but with lower potency (5-20 microM). In combination, the effect induced by maximal UCcf-029 and UCcf-029, UCcf-853, and apigenin increased strongly with increasing basal CFTR activity: for example, Kd for activation by UCcf-029 decreased from >5 to <0.4 microM with increasing basal Isc from approximately 4 microA/cm2 to approximately 12 microA/cm2. This dependence was confirmed in permeabilized Fischer rat thyroid cells stably expressing CFTR. Our results demonstrate efficacy of novel CFTR activators in bronchial epithelia and provide evidence that activating potency depends on basal CFTR activity.

Some structural changes on triazolyl-benzotriazoles and triazolyl-benzimidazolones as potential potassium channel activators. III

This paper reports the synthesis and pharmacological evaluation of some compounds, obtained by structural modifications of 1,2,3-triazolyl-benzotriazoles and 1,2,3-triazolyl-benzimidazolones, which had shown activity as potential activators of the big-conductance calcium-activated potassium channels (BK(Ca)). Changes have concerned the introduction of a hinderer substituent in the 5-position of the benzimidazolone (4a, b) and benzotriazole (5a, b) rings, opening of the benzimidazolone ring (7) and substitution of the 1,2,3-triazole ring with a 2-hydroxyphenyl ring (10). Furthermore a series of 3-aryl-benzotriazin-4-one derivatives (13a-e) has been studied, which appears as a modification and/or combination of the benzimidazolone and benzotriazole rings. Only compound 10 shows interesting activity, while the other structural modifications either do not increase (compounds 4 and 5) or reduce (compounds 7 and 13) the pharmacological activity. However, these results provide useful information about structure-activity relationships.

New 5-substituted-1-(2-hydroxybenzoyl)-benzotriazoles, potassium channel activators. IV

This paper reports the synthesis of a series of new 5-substituted-1-(2-hydroxybenzoyl)-benzotriazoles, which have been tested for their activity as possible activators of potassium channels. In rat aortic rings, the 'opened' derivatives 1a-f, intermediates of synthesis, showed vasorelaxing properties, with appreciable values of potency. However, the most remarkable effects were recorded for the 2-hydroxybenzoylbenzotriazoles 3a-f, which showed full vasorelaxing efficacy and high potency values. The introduction of a 2-hydroxybenzyl substituent in the 1 position of the benzotriazole ring (compound 7) strongly decreased the activity, showing the importance of the electron-acceptor carbonyl function. The best compound, 3b, was further investigated, in order to evaluate the possible mechanism of action involved in the vasodilator activity. In the vascular model, different potassium channel blockers inhibited the effects of the compound, and an increase of the levels of membrane depolarisation induced a significant reduction of the recorded responses. Compound 3b was also tested in a model of isolated rat heart, retroperfused through the aorta and submitted to a global ischemia/reperfusion cycle. In such an experimental condition, 3b showed an interesting cardioprotective activity. All the above observations are in agreement with the hypothesis of a mechanism linked to the activation of potassium channels.

Triazolyl-benzimidazolones and triazolyl-benzotriazoles: new potential potassium channel activators. II

This paper reports the synthesis and pharmacological evaluation of a series of 5-substituted-triazolyl benzotriazoles (2a-f) and the corresponding series of 5-substituted-triazolyl-benzimidazolones (6a-f), as potential activators of the big-conductance calcium-activated potassium channels (BK(Ca)). The synthesis and structure demonstration of the stock compounds of the two series have been described in our previous works, as well as the common starting compounds 4-carboxamido-5-(4-substituted-2amino-anilino)-1,2,3-triazoles (1a-f). The triazolyl-benzotriazoles were obtained by diazotization, while the triazolyl-benzimidazolones were obtained by thermal intramolecular cyclization of ethoxycarbonylamino derivatives or directly with phosgene. Benzimidazolone compounds generally showed little effect whilst the compounds with a benzotriazole ring showed full efficacy, with vasorelaxing properties and potency parameters a little lower than that of the reference compound NS 1619. These effects were significantly reduced by an increased membrane depolarization. This depolarization-sensitive response is in agreement with the pharmacodynamic hypothesis of activation of potassium channels.

5-(4'-Substituted-2'-nitroanilino)-1,2,3-triazoles as new potential potassium channel activators. I

By the hypothesised correlation with the large conductance Ca(++)-activated potassium channel (BK(Ca)) openers NS 004 and NS 1619, bearing a benzimidazolone ring, a series of new 5-(4'-substituted-2'-nitroanilino)-1,2,3-triazoles were synthesised and tested on in vitro isolated vascular preparation. The compounds were prepared starting from the appropriately substituted 2-nitro-phenylazides by 1,3-dipolar cycloaddition reaction to cyanoacetamide and following Dimroth isomerisation of the corresponding 1-arylsubstituted-5-amino-1,2,3-triazoles. The analogous 5-(4'-substituted-2'-amino-anilino)-1,2,3-triazoles were also prepared to assess the role of the nitro group in the pharmacophoric model. Almost all the nitro compounds showed a vasorelaxant activity on endothelium-denuded rat aortic rings with a potency comparable to that recorded for the reference compound NS 1619. Such a vasorelaxing activity was significantly reduced by the increase of the level of membrane depolarisation and by the potassium channel blocker 4-aminopyridine with a pharmacodynamic behaviour consistent with a potassium channel activation.

Pharmacology of CFTR chloride channel activity

Pharmacology of CFTR Chloride Channel Activity. Physiol. Rev. 79, Suppl.: S109-S144, 1999. - The pharmacology of cystic fibrosis transmembrane conductance regulator (CFTR) is at an early stage of development. Here we attempt to review the status of those compounds that modulate the Cl- channel activity of CFTR. Three classes of compounds, the sulfonylureas, the disulfonic stilbenes, and the arylaminobenzoates, have been shown to directly interact with CFTR to cause channel blockade. Kinetic analysis has revealed the sulfonylureas and arylaminobenzoates interact with the open state of CFTR to cause blockade. Suggestive evidence indicates the disulfonic stilbenes act by a similar mechanism but only from the intracellular side of CFTR. Site-directed mutagenesis studies indicate the involvement of specific amino acid residues in the proposed transmembrane segment 6 for disulfonic stilbene blockade and segments 6 and 12 for arylaminobenzoate blockade. Unfortunately, these compounds (sulfonylureas, disulfonic stilbenes, arylaminobenzoate) also act at a number of other cellular sites that can indirectly alter the activity of CFTR or the transepithelial secretion of Cl-. The nonspecificity of these compounds has complicated the interpretation of results from cellular-based experiments. Compounds that increase the activity of CFTR include the alkylxanthines, phosphodiesterase inhibitors, phosphatase inhibitors, isoflavones and flavones, benzimidazolones, and psoralens. Channel activation can arise from the stimulation of the cAMP signal transduction cascade, the inhibition of inactivating enzymes (phosphodiesterases, phosphatases), as well as the direct binding to CFTR. However, in contrast to the compounds that block CFTR, a detailed understanding of how the above compounds increase the activity of CFTR has not yet emerged.

Spontaneous transient outward currents and delayed rectifier K+ current: effects of hypoxia

Single smooth muscle cells of rabbit intrapulmonary artery were voltage clamped using the perforated-patch configuration of the patch-clamp technique. We observed spontaneous transient outward currents (STOCs) and a steady-state outward current. Because STOCs were tetraethylammonium sensitive and activated by Ca2+ influx, they were believed to represent activation of Ca2+-activated K+ channels. The steady-state outward current, which was sensitive to 4-aminopyridine, was the delayed rectifier K+ current. In cells voltage clamped at 0 mV, we found that STOCs were not randomly distributed in amplitude but were composed of multiples of 1.57 +/- 0.56 pA/pF. The mean frequency of STOCs was 5.51 +/- 3.49 Hz. Ryanodine (10 microM), caffeine (5 mM), thapsigargin (200 nM), and hypoxia (PO2 = 10 mmHg) decreased STOCs. The effect of hypoxia on STOCs was partially reversible only if the experiment was conducted in the presence of thapsigargin. Hypoxia and thapsigargin decrease steady-state outward current. Thapsigargin and removal of external Ca2+ abolished the effect of hypoxia, suggesting that hypoxia decreases steady-state outward current by a Ca2+-dependent mechanism.

Effects of K+ channel inhibitors and antagonists on NS-004 evoked relaxations in guinea-pig isolated trachea

The smooth muscle relaxant responses to NS-004, an activator of charybdotoxin-sensitive, large conductance Ca(2+)-dependent K+ channels (BKCa) were studied on the basal spontaneous tone in guinea-pig trachea in vitro. The sensitivity of these responses to a range of K+ channel inhibitors and antagonists were also evaluated. NS-004 (0.1-30 microM) evoked concentration-related relaxations (pIC50 5.48 +/- 0.13) on the spontaneous tone in guinea-pig tracheal rings, suspended in Krebs bicarbonate solution, with a maximum response not different to that to aminophylline (1 microM). Charybdotoxin (0.03 and 0.1 microM) or iberiotoxin (0.1 microM) significantly displaced the NS-004 concentration-response curve to the right of control with no change in maximum response. In contrast, glibenclamide (1.0 microM) apamin (0.1 microM) and dofetilide (1.0 microM) each failed to modify the responses to NS-004 on spontaneous tone in guinea-pig trachea. These results suggest that relaxations in guinea-pig tracheal smooth muscle to the substituted benzimidazolone, NS-004, involve the activation of BKCa channels.

On the mechanism of the differential effects of NS004 and NS1608 in smooth muscle cells from guinea pig bladder

Our recent study revealed that the reported BK(Ca) channel openers NS004 (5-trifluoromethyl-(5-chloro-2-hydroxyphenyl)-1,3-dihydro-2H-benzimidazo le-2-one) and its analog NS1608 (N-(3-(trifluoromethyl)phenyl)-N'-(2-hydroxy-5-chlorophenyl)urea) produced a differential pattern of action on the BK(Ca) channel in porcine coronary arterial cells (Hu, S., H.S. Kim and C.A. Fink, 1995, Differential effects of the BK(Ca) channel openers NS004 and NS1608 in porcine coronary arterial cells, Eur. J. Pharmacol. 294, 357). In this study, using the patch-clamp method on the smooth muscle cells from guinea pig bladder detrusor, the activity profile of NS004 and NS1608 on the whole-cell BK(Ca) current (I(BK)) was examined and found to be similar to that in coronary arterial cells. NS004 did not significantly modify I(BK) at concentrations below 5 microM, but robustly augmented I(BK) at concentrations greater than 20 microM. With a minimum effective concentration of 0.5 microM, NS1608 caused potentiation of I(BK) with a bell-shaped concentration-response relationship. The activation was maximized between 5-10 microM and substantially attenuated at higher concentrations. The behavior of single BK(Ca) channels in the presence of NS004 and NS1608 was scrutinized to elucidate the mechanism underlying their distinct patterns of action. The channel open-state probability (NPo) was increased by NS004 at 0.5, 5 and 50 microM to a respective 1.75 +/- 0.38, 4.05 +/- 0.90, and 15.01 +/- 3.66 fold (n = 7) of the control by increasing the open time and the frequency of opening while having no effect on the single BK(Ca) channel conductance. NS1608 at 0.5 and 5 microM increased NPo to 1.69 +/- 0.43 and 18.03 +/- 6.64 fold of the control (n = 4), respectively. NS1608 at higher concentrations repeatedly blocked channel openings as evidenced by the highly flickering open state, though the overall open time and frequency of opening remained high. The diminished activation of I(BK) by 50 microM NS1608 manifested therefore a net result of these two opposing actions on the single channels. Thus, the two structurally related BK(Ca) channel openers interact distinctly with the channel gating components.