betadex and Pituitary-Neoplasms

The effects of (2-hydroxypropyl)-β-cyclodextrin (HPβCD), a cyclic oligomer, on membrane electroporation-induced inward current (I(MEP)) in pituitary tumor (GH(3)) cells were experimentally and analytically characterized. Depletion of membrane cholesterol by exposing cells to HPβCD (2 mM) increased the activation time constant of delayed rectifier K(+) current. Such maneuver resulted in a significant reduction of I(MEP) density. 2,2'-Azo-bis(2-amidinopropane) dihydrochloride (AAPH), an initiator of free radicals, increased the magnitude of I(MEP). AAPH-stimulated I(MEP) was not reversed by the blockers of Ca(2+)-activated K(+) channels, but by LaCl(3) or MnCl(2). However, in HPβCD-treated cells, the ability of AAPH to enhance I(MEP) was abolished. Under such maneuver, the gating charge of I(MEP) activation was increased by 2 fold, along with a hyperpolarized shift of the activation curve by 30 mV. No change in single-channel conductance of MEP-induced channels during cell exposure to HPβCD was demonstrated. The energy change of I(MEP) in untreated and HPβCD-treated cells was estimated to be -17.7 and -44.8 kJ/mol, respectively, and the perturbation of free energy following HPβCD treatment was -27.1 kJ/mol. Based on an MEP model, cell exposure to HPβCD increased the edge energy of the electropore size. By use of a two barrier-one site barrier model, HPβCD treatment can increase both the peak height and well depth of the barrier profile. Taken together, depletion of membrane cholesterol by HPβCD can elevate the edge energy of pore formation, thereby decreasing the I(MEP) magnitude. The channel-suppressing properties during membrane cholesterol depletion with HPβCD might thus contribute to the underlying mechanisms by which such maneuver alters neuronal or neuroendocrine function.

Topics: 2-Hydroxypropyl-beta-cyclodextrin; Amidines; Animals; Anticholesteremic Agents; beta-Cyclodextrins; Cell Line, Tumor; Cell Membrane; Cholesterol; Electroporation; Ion Channel Gating; Membrane Potentials; Pituitary Neoplasms; Rats

The effects of changes in membrane cholesterol on ion currents were investigated in pituitary GH3 cells. Depletion of membrane cholesterol by exposing cells to methyl-beta-cyclodextrin (MbetaCD), an oligosaccharide, resulted in an increase in the density of Ca2+-activated K+ current (IK(Ca)). However, no significant change in IK(Ca) density was demonstrated in GH3 cells treated with a mixture of MbetaCD and cholesterol. Cholesterol depletion with MbetaCD (1.5 mg/ml) slightly suppressed the density of voltage-dependent L-type Ca2+ current. In inside-out patches recorded from MbetaCD-treated cells, the activity of large-conductance Ca2+-activated K+ (BK(Ca)) channels was enhanced with no change in single-channel conductance. In MbetaCD-treated cells, voltage-sensitivity of BK(Ca) channels was increased; however, no change in Ca2+-sensitivity could be demonstrated. A negative correlation between adjacent closed and open times in BK(Ca) channels was observed in MbetaCD-treated cells. In inside-out patches from MbetaCD-treated cells, dexamethasone (30 microM) applied to the intracellular surface did not increase BK(Ca)-channel activity, although caffeic acid phenethyl ester and cilostazol still opened its probability effectively. However, no modification in the activity of ATP-sensitive K+ channels could be seen in MbetaCD-treated cells. Current-clamp recordings demonstrated that the cholesterol depletion maneuver with MbetaCD reduced the firing of action potentials. Therefore, the increase in BK(Ca)-channel activity induced by membrane depletion may influence the functional activities of neurons or neuroendocrine cells if similar results occur in vivo.

Topics: Action Potentials; Animals; Apamin; beta-Cyclodextrins; Caffeic Acids; Calcium; Cholesterol; Cilostazol; Dexamethasone; Diazoxide; Glyburide; Indoles; Ion Channel Gating; Large-Conductance Calcium-Activated Potassium Channels; Membrane Lipids; Phenylethyl Alcohol; Pituitary Neoplasms; Rats; Tetrazoles; Tumor Cells, Cultured