Activation of muscarinic K+ channels by arecaidine propargyl ester in isolated guinea-pig atrial myocytes

An integrated platform for simultaneous multi-well field potential recording and Fura-2-based calcium transient ratiometry in human induced pluripotent stem cell (hiPSC)-derived cardiomyocytes

INTRODUCTION

Human induced pluripotent stem cell-derived cardiomyocytes are available from various sources and they are being evaluated for safety testing. Several platforms are available offering different assay principles and read-out parameters: patch-clamp and field potential recording, imaging or photometry, impedance measurement, and recording of contractile force. Routine use will establish which assay principle and which parameters best serve the intended purpose.

METHODS

We introduce a combination of field potential recording and calcium ratiometry from spontaneously beating cardiomyocytes as a novel assay providing a complementary read-out parameter set. Field potential recording is performed using a commercial multi-well multi-electrode array platform. Calcium ratiometry is performed using a fiber optic illumination and silicon avalanche photodetectors. Data condensation and statistical analysis are designed to enable statistical inference of differences and equivalence with regard to a solvent control.

RESULTS

Simultaneous recording of field potentials and calcium transients from spontaneously beating monolayers was done in a nine-well format. Calcium channel blockers (e.g. nifedipine) and a blocker of calcium store release (ryanodine) can be recognized and discriminated based on the calcium transient signal. An agonist of L-type calcium channels, FPL 64176, increased and prolonged the calcium transient, whereas BAY K 8644, another L-type calcium channel agonist, had no effect. Both FPL 64176 and various calcium channel antagonists have chronotropic effects, which can be discriminated from typical "chronotropic" compounds, like (±)isoprenaline (positive) and arecaidine propargyl ester (negative), based on their effects on the calcium transient.

DISCUSSION

Despite technical limitations in temporal resolution and exact matching of composite calcium transient with the field potential of a subset of cells, the combined recording platform enables a refined interpretation of the field potential recording and a more reliable identification of drug effects on calcium handling.

Arecoline inhibits intermediate-conductance calcium-activated potassium channels in human glioblastoma cell lines

Arecoline (ARE) is an alkaloid-type natural product from areca nut. This compound has numerous pharmacological and toxicological effects. Whether this agent interacts with ion channels to perturb functional activity of cells remains unknown. The effects of ARE on ionic currents were studied in glioma cell lines (U373 and U87MG) using patch-clamp technique. Like TRAM-34(1-[(2-chlorophenyl)-diphenylmethyl]pyrazole), ARE suppressed the amplitude of whole-cell voltage-gated K(+) currents in U373 cells elicited by a ramp voltage clamp. In cell-attached configuration, ARE did not modify the single-channel conductance of intermediate-conductance Ca(2+)-activated K(+) (IKCa) channels; however, it did reduce channel activity. Its inhibition of IKCa channels was accompanied by a significant lengthening in the slow component of mean closed time of IKCa channels. Based on minimal kinetic scheme, the dissociation constant (KD) required for ARE-mediated prolongation of mean closed time was 11.2µM. ARE-induced inhibition of IKCa channels was voltage-dependent. Inability of ARE to perturb the activity of large-conductance Ca(2+)-activated K(+) (BKCa) channels was seen. Under current-clamp recordings, ARE depolarized the membrane of U373 cells and DCEBIO reversed ARE-induced depolarization. Similarly, ARE suppressed IKCa-channel activities in oral keratinocytes. This study provides the evidence that ARE block IKCa channels in a concentration, voltage and state-dependent manner. ARE-induced block of IKCa channels is unrelated to the binding of muscarinic receptors. The effects of ARE on these channels may partially be responsible for the underlying cellular mechanisms by which it influences the functional activities of glioma cells or oral keratinocytes, if similar findings occur in vivo.

Betel quid extract promotes oral cancer cell migration by activating a muscarinic M4 receptor-mediated signaling cascade involving SFKs and ERK1/2

Betel quid (BQ) is a widely accepted etiological factor for oral squamous cell carcinoma (OSCC) in Southeast Asia, but how BQ chewing leads to oral carcinogenesis remains to be elucidated. We have previously demonstrated that the activation of Src family kinases (SFKs) is critical for BQ-induced oral cancer cell motility. Here we investigate whether this biological effect is mediated by specific membrane receptors in oral cancer cells. We found that BQ-induced activation of extracellular signal-regulated kinase 1/2 (ERK1/2) and cell migration could be inhibited by atropine, suggesting the involvement of the muscarinic receptor family. The enhanced activities of ERK1/2 and cell migration were significantly counteracted by PD102807, the selective antagonist of muscarinic M4 receptor. Moreover, cold BQ extract effectively competed with a known ligand, [(3)H]-N-methyl scopolamine, for binding to muscarinic M4 receptor in vitro, thereby implying that BQ could activate motility-promoting signaling pathways through direct interaction with the receptor. The requirement of muscarinic M4 receptor for BQ-induced oral cancer cell migration was demonstrated by knockdown of the receptor using RNA interference (RNAi). Remarkably, ectopic expression of muscarinic M4 receptor in two oral cancer cell lines, Ca9-22 and SCC-9, further augmented BQ-induced cell migration by 83% and 99%, respectively. Finally, we verified that BQ-induced oral cancer cell migration was mediated through a muscarinic M4 receptor-->SFKs-->ERK1/2 signaling pathway. Thus, our findings have identified a novel signaling cascade mediating BQ-induced oral cancer cell motility, which could be a therapeutic target for BQ-related oral malignancies.