Veratridine activates a silent sodium channel in rat isolated aorta

Pre-clinical safety and therapeutic efficacy of a plant-based alkaloid in a human colon cancer xenograft model

A high-throughput drug screen revealed that veratridine (VTD), a natural plant alkaloid, induces expression of the anti-cancer protein UBXN2A in colon cancer cells. UBXN2A suppresses mortalin, a heat shock protein, with dominant roles in cancer development including epithelial–mesenchymal transition (EMT), cancer cell stemness, drug resistance, and apoptosis. VTD-dependent expression of UBXN2A leads to the deactivation of mortalin in colon cancer cells, making VTD a potential targeted therapy in malignant tumors with high levels of mortalin. VTD was used clinically for the treatment of hypertension in decades past. However, the discovery of newer antihypertensive drugs and concerns over potential neuro- and cardiotoxicity ended the use of VTD for this purpose. The current study aims to determine the safety and efficacy of VTD at doses sufficient to induce UBXN2A expression in a mouse model. A set of flow-cytometry experiments confirmed that VTD induces both early and late apoptosis in a dose-dependent manner. In vivo intraperitoneal (IP) administration of VTD at 0.1 mg/kg every other day (QOD) for 4 weeks effectively induced expression of UBXN2A in the small and large intestines of mice. Liquid chromatography–tandem mass spectrometry (LC–MS/MS) assays on tissues collected from VTD-treated animals demonstrated VTD concentrations in the low pg/mg range. To address concerns regarding neuro- and cardiotoxicity, a comprehensive set of behavioral and cardiovascular assessments performed on C57BL/6NHsd mice revealed that VTD generates no detectable neurotoxicity or cardiotoxicity in animals receiving 0.1 mg/kg VTD QOD for 30 days. Finally, mouse xenograft experiments in athymic nude mice showed that VTD can suppress tumor growth. The main causes for the failure of experimental oncologic drug candidates are lack of sufficient safety and efficacy. The results achieved in this study support the potential utility of VTD as a safe and efficacious anti-cancer molecule.

Physiologic increases in extracellular sodium salt enhance coronary vasoconstriction and Ca2+ entry

High dietary sodium salt has been suggested to increase the risk of coronary vasospasm and coronary artery disease. However, whether high-sodium salt directly affects the mechanisms of coronary artery contraction is unclear. This study investigated whether physiologic and supraphysiologic increases in extracellular concentrations of sodium chloride ([NaCl]e) enhance the Ca2+ handling mechanisms of coronary smooth muscle contraction. Isometric contraction and 45Ca2+ influx were measured in endothelium-denuded porcine coronary artery strips incubated in Krebs solution (2.5 mM Ca2+) containing increasing [NaCl]e (120, 121, 123, 126, 130, 140, and 150 mM). Increasing [NaCl]e for 30 min did not increase the resting coronary tone or 45Ca2+ influx. 5-Hydroxytryptamine (5-HT) caused concentration-dependent increases in contraction and 45Ca2+ influx. Preincubation of coronary strips in increasing [NaCl]e for 30 min did not change the median effective dose of 5-HT. However, the magnitude of the 5-HT contraction and 45Ca2+ influx was significantly increased at 121-126 mM [NaCl]e. Preincubation with 2,4-dichlorobenzamil (10-5 M), inhibitor of the Na+/Ca2+ exchanger, or KB-R7943 (10-5 M), selective inhibitor of the reverse mode of the Na+/Ca2+ exchanger, abolished the increases in 5-HT contraction and 45Ca2+ influx at 121-126 mM [NaCl]e. Preincubation in Krebs solution containing 120 mM NaCl plus 1-6 mM LiCl or N-methyl-d-glucamine did not increase 5-HT contraction or 45Ca2+ influx. Higher [NaCl]e (140-150 mM) increased 5-HT-induced 45Ca2+ influx but inhibited 5-HT contraction. 5-HT (10-5 M)- and caffeine (25 mM)-induced contraction in Ca2+-free (2 mM EGTA) solution, a measure of Ca2+ release from the intracellular stores, was not affected by small increases in [NaCl]e (121-126 mM) but was inhibited at higher [NaCl]e (130-150 mM). Thus increases in [NaCl]e within the physiologic range enhance coronary smooth muscle contraction to 5-HT by a mechanism possibly involving Ca2+ entry via the reverse mode of the Na+/Ca2+ exchanger, but not Ca2+ release from the intracellular stores. The reduction of coronary contraction with supraphysiologic [NaCl]e in both Ca2+-containing and Ca2+-free Krebs could be related to excessive increases in ionic strength and may mask significant coronary vasoconstrictor effects of physiologic increases in [NaCl]e.

Na(+) entry via store-operated channels modulates Ca(2+) signaling in arterial myocytes

In many nonexcitable cells, hormones and neurotransmitters activate Na(+) influx and mobilize Ca(2+) from intracellular stores. The stores are replenished by Ca(2+) influx via "store-operated" Ca(2+) channels (SOC). The main routes of Na(+) entry in these cells are unresolved, and no role for Na(+) in signaling has been recognized. We demonstrate that the SOC are a major Na(+) entry route in arterial myocytes. Unloading of the Ca(2+) stores with cyclopiazonic acid (a sarcoplasmic reticulum Ca(2+) pump inhibitor) and caffeine induces a large external Na(+)-dependent rise in the cytosolic Na(+) concentration. One component of this rise in cytosolic Na(+) concentration is likely due to Na(+)/Ca(2+) exchange; it depends on elevation of cytosolic Ca(2+) and is insensitive to 10 mM Mg(2+) and 10 microM La(3+). Another component is inhibited by Mg(2+) and La(3+), blockers of SOC; this component persists in cells preloaded with 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid to buffer Ca(2+) transients and prevent Na(+)/Ca(2+) exchange-mediated Na(+) entry. This Na(+) entry apparently is mediated by SOC. The Na(+) entry influences Na(+) pump activity and Na(+)/Ca(2+) exchange and has unexpectedly large effects on cell-wide Ca(2+) signaling. The SOC pathway may be a general mechanism by which Na(+) participates in signaling in many types of cells.

Regulation of Ca2+ homeostasis by atypical Na+ currents in cultured human coronary myocytes

Primary cultured human coronary myocytes (HCMs) derived from ischemic human hearts express an atypical voltage-gated tetrodotoxin (TTX)-sensitive sodium current (I(Na)). The whole-cell patch-clamp technique was used to study the properties of I(Na) in HCMs. The variations of intracellular calcium ([Ca2+]i) and sodium ([Na+]i) were monitored in non-voltage-clamped cells loaded with Fura-2 or benzofuran isophthalate, respectively, using microspectrofluorimetry. The activation and steady-state inactivation properties of I(Na) determined a "window" current between -50 and -10 mV suggestive of a steady-state Na+ influx at the cell resting membrane potential. Consistent with this hypothesis, the resting [Na+]i was decreased by TTX (1 micromol/L). In contrast, it was increased by Na+ channel agonists that also promoted a large rise in [Ca2+]i. Veratridine (10 micromol/L), toxin V from Anemonia sulcata (0.1 micromol/L), and N-bromoacetamide (300 micromol/L) increased [Ca2+]i by 7- to 15-fold. This increase was prevented by prior application of TTX or lidocaine (10 micromol/L) and by the use of Na(+)-free or Ca(2+)-free external solutions. The Ca(2+)-channel antagonist nicardipine (5 micromol/L) blocked the effect of veratridine on [Ca2+]i only partially. The residual component disappeared when external Na+ was replaced by Li+ known to block the Na+/Ca2+ exchanger. The resting [Ca2+]i was decreased by TTX in some cells. In conclusion, I(Na) regulates [Ca2+]i in primary cultured HCMs. This regulation, effective at baseline, involves a tonic control of Ca2+ influx via depolarization-gated Ca2+ channels and, to a lesser extent, via a Na+/Ca2+ exchanger working in the reverse mode.

Veratridine actions on two types of fast Na+ channels in human uterine leiomyosarcoma cells

In human uterine leiomyosarcoma cell line (SK-UT-1B), we previously demonstrated two types of fast Na+ current (INa(f)) induced by serum, based on different time course of current decay: fast-inactivating and slow-inactivating. To further clarify the properties of these currents, we studied the effects of veratridine, which is known to modify the inactivation process of INa(f), using whole-cell voltage clamp. Bath application of veratridine (100 microM) produced a decrease in peak INa(f) (Ipeak), simultaneous with increase in the steady-state current (Iss) and tail current (Itail). These effects of veratridine were observed in only slow-inactivating INa(f). The induction of Iss and Itail was completely reversed by washout of veratridine within 5 min, whereas the decreased Ipeak did not recover even after 15 min of washout. These findings suggest that the fast Na+ channels in this cell line may have two binding sites for veratridine: a high-affinity site, involved in the decrease in Ipeak (possibly due to a decrease in conductance), and a low-affinity site, related to the appearance of Iss and Itail (due to a long opening of the channels). It is concluded that the two types of INa(f) in this cell line have different sensitivity to veratridine and the fast Na+ channels may have two binding sites for veratridine.