Bisphenol A (BPA) is a widespread environmental contaminant detected in urine of 93 % of investigated US population. Recent epidemiological studies found correlation between BPA exposure and diseases including cardiovascular and neuronal disorders. BPA targets include hormone receptors and voltage-dependent ion channels. T-type calcium channels are important regulatory elements in both cardiovascular and neuronal system. Therefore, we investigated effects of BPA on T-type calcium channels. Calcium current flowing through recombinant T-type calcium channels expressed in HEK 293 cells was measured using whole-cell patch clamp. BPA inhibited the current through individual T-type calcium channel subtypes in a concentration-dependent manner with two distinguishable components in these concentration-dependencies. Nanomolar concentrations of BPA inhibited calcium current through T-type calcium channels in the order of efficiency CaV3.2 ≥ CaV3.1 > CaV3.3 without affecting voltage dependence and kinetics of channel gating. Micromolar concentrations of BPA accelerated kinetics of current decay, shifted voltage dependence of steady-state inactivation towards more negative values and inhibited current amplitudes. We suggest that BPA acts as a modifier of channel gating and directly plugs conductive channel pore at high concentration. Concentration range in which inhibition was observed corresponds to concentrations detected in human fluids and therefore may be relevant for evaluation of health effects of BPA.

The interaction of N, N-dimethyl-1-dodecanamine N-oxide (C 12 NO) with egg yolk phosphatidylcholine (EYPC) liposomes containing cholesterol (CHOL) was studied. The perturbation of CHOL-EYPC bilayers in unilamellar liposomes (ULL) was observed by the leakage of fluorescence probe calcein. Weak leakage is observed at low surfactant concentration c C12NO (minimal perturbation of the bilayer) followed by an intensive leakage at a middle c C12NO (creation of pores). No change in fluorescence intensity was measured at high c C12NO (calcein totally released from liposomes). The higher CHOL amount in the bilayer, the more surfactant is needed to create pores in the bilayer. Solubilization of CHOL-EYPC ULL induced by C 12 NO was studied turbidimetrically. The solubilization curve consists of three parts: saturation of bilayer at low c C12NO (liposomes are preserved), followed by solubilization (liposome -mixed micelle transition) and post-solubilization. The C 12 NO concentration needed for the onset of the solubilization raise with the increase of n CHOL :n EYPC . The structure of liposomes is still preserved at total calcein release for all n CHOL :n EYPC .

Dedicated to prof. P. Balgavý on the occasion of his 75th birthday.

The effect of N-dodecyl-N,N-dimethylamine N-oxide (C12NO) on the unilamellar liposomes prepared from egg yolk phosphatidylcholine (EYPC) was studied using turbidimetric and fluorescence spectroscopic methods. The concentrations of C12NO causing saturation and total solubilization of membrane were evaluated turbidimetrically. Liposomes filled with hydrophilic fluorescent probe calcein were used for the fluorescence leakage measurements. Dependence of fluorescence intensity on C12NO concentration can be divided into three linear sections. The turning points between individual sections represent two concentrations of C12NO which induce formation of small defects, specifically large pores in liposomal membrane. These pores are wide enough to enable calcein to be washed out from liposomes completely. We compared results obtained by the two methods used. Measurements of fluorescent probe leakage showed that free diffusion of calcein through pores in EYPC bilayer was observed at such C12NO concentrations when the form of liposome is still preserved and the saturation of bilayer is not yet finished. Higher C12NO concentrations are needed for solubilization, a phase transition from bilayers to micelles.