Multiple calcium channel subtypes in isolated rat chromaffin cells |
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Authors: | Luis Gandía Ricardo Borges Almudena Albillos Antonio G García |
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Institution: | (1) Departamento de Farmacología, Facultad de Medicina, Universidad Autónoma de Madrid, Arzobispo Morcillo 4, E-28029 Madrid, Spain;(2) Unidad de Farmacología, Facultad de Medicina, Universidad de la Laguna, Tenerife, Spain |
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Abstract: | By using the whole-cell configuration of the patch-clamp technique we have investigated the pharmacological properties of Ca2+ channels in short-term cultured rat chromaffin cells. In cells held at a membrane potential of –80 mV, using 10 mM Ba2+ as the charge carrier, only high-voltage-activated (HVA) Ca2+ channels were found. Ba2+ currents (I
Ba) snowed variable sensitivity to dihydropyridine (DHP) Ca2+ channel agonists and antagonists. Furnidipine, a novel DHP antagonist, reversibly blocked the current amplitude by 22% and 48%, at 1 M and 10 M respectively, during short (15–50 ms) depolarizing pulses to 0 mV. The L-type Ca2+ channel agonist Bay K 8644 (1 M) caused a variable potentiation of HVA currents that could be better appreciated at low rather than at high depolarizing steps. Increase of I
Ba was accompanied by a 20-mV shift in the activation curves for Ca2+ channels towards more hyperpolarizing potentials. Application of the conus toxin -conotoxin GVIA (GVIA; 1 M) blocked 31% of I
Ba; blockade was irreversible upon removal of the toxin from the extracellular medium, -Agatoxin IVA (IVA; 100 nM) produced a 15% blockade of I
Ba. -Conotoxin MVIIC (MVIIC; 5 M) produced a 36% blockade of I
Ba; such blockade seems to be related to both GVIA-sensitive (N-type) and GVIA-resistant Ca2+ channels. The sequential addition of supramaximal concentrations of furnidipine (10 M), GVIA (1 M), IVA (100 nM) and MVIIC (3 M) produced partial inhibition of I
Ba, which were additive. Our data suggest that the whole cell I
Ba in rat chromaffin cells exhibits at least four components. About 50% of I
Ba is carried by L-type Ca2+ channels, 30% by N-type Ca2+channels and 15% by P-type Ca2+ channels. These figures are close to those found in cat chromaffin cells. However, they differ considerably from those found in bovine chromaffin cells where P-like Ca2+channels account for 45% of the current, N-type carry 35% and L-type Ca2+ channels are responsible for only 20–25% of the current. These drastic differences might have profound physiological implications for the relative contribution of each channel subtype to the regulation of catecholamine release in different animal species. |
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Keywords: | Chromaffin cells Ca2+ channels Dihydropyridines -Conotoxin GVIA" target="_blank">gif" alt="ohgr" align="BASELINE" BORDER="0">-Conotoxin GVIA -Agatoxin IVA" target="_blank">gif" alt="ohgr" align="BASELINE" BORDER="0">-Agatoxin IVA -Conotoxin MVIIC" target="_blank">gif" alt="ohgr" align="BASELINE" BORDER="0">-Conotoxin MVIIC |
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