stilbenes and 1-9-dideoxyforskolin

To test the hypothesis that P-glycoprotein enhances swelling currents through regulation of volume-sensitive Cl- channels [recently termed VSOAC (volume-sensitive osmolyte and anion channel)], a human uterine sarcoma cell line (MES-SA) and its doxorubicin-selected counterpart (Dx5) were studied. P-glycoprotein mRNA and protein levels were detected only in Dx5 cells. However, whole cell patch-clamp experiments showed that swollen Dx5 cells (n = 5) produced smaller VSOAC currents than MES-SA cells (n = 4; 106 +/- 26 pA/pF vs. 232 +/- 76 pA/pF at 90 mV). In radioisotopic efflux experiments, both swelling-activated 125I (Cl-) currents (n = 15) and 86Rb (K+) currents (n = 8) were found to be two-to fourfold smaller in the Dx5 (high P-glycoprotein) cells. Inhibitors of P-glycoprotein showed no specificity for the doxorubicin-selected cells (Dx5). Dideoxyforskolin (100 microM) blocked swelling-activated 125I efflux equally in both cell lines, whereas 100 microM verapamil had no effect. Thus, in this cell line, selection for P-glycoprotein expression is associated with reduced swelling currents. These findings suggest that P-glycoprotein expression does not directly facilitate VSOAC.

Topics: ATP Binding Cassette Transporter, Subfamily B, Member 1; Body Fluids; Chlorides; Colforsin; Doxorubicin; Drug Resistance, Multiple; Electric Conductivity; Female; Humans; Patch-Clamp Techniques; Potassium; RNA, Messenger; Stilbenes; Tumor Cells, Cultured; Verapamil

Swelling-induced chloride currents may contribute to cardiac electrical activity and cell volume regulation. Identification of selective blockers would aid in understanding the functional contribution(s) of this current.. Dog atrial cells were used to investigate the pharmacologic properties of the swelling-induced chloride current. Whole cell patch clamp was used. Swelling-induced chloride current was activated by osmotic stress. Initially, the chloride selectivity and calcium independence of the swelling-induced current in dog atrial cells was demonstrated. Subsequently, a number of putative chloride channel blockers were examined. Anthracene-9-carboxylic acid (1 mM) and dideoxyforskolin (100 microM) and extracellular cAMP (5 mM) were found to partially inhibit the swelling-induced chloride current (approximately 50%, 80%, and 10% inhibition, respectively). Niflumic acid (100 microM), nitrophenylpropylamino benzoate (NPPB; 10 to 40 microM), and (+) 2-[(2-cyclopentyl-6,7-dichloro-2,3-dihydro-2-methyl-1-oxy-1H-inden -5-yl)oxy d acetic acid (indanyloxyacetic acid; IAA-94; 100 microM) could fully inhibit the swelling-induced chloride current without decreasing cell size. DIDS (100 microM) and dinitrostilbene disulfonic acid (DNDS; 5 mM) fully inhibited outward currents but only partially inhibited inward current.. Niflumic acid, IAA-94, and NPPB were identified as full blockers of cardiac swelling-induced chloride current. Nonspecific effects were identified for each of the full blockers. Experiments that use these agents as functional antagonists should be carefully designed and interpreted with caution.

Topics: Animals; Anthracenes; Atrial Function; Calcium; Chloride Channels; Colforsin; Dogs; Electrophysiology; Glycolates; Heart Atria; Niflumic Acid; Nitrobenzoates; Patch-Clamp Techniques; Stilbenes