Long-term induction of a unique C1- current by endothelin-1 in an epithelial cell line from rat lung: evidence for regulation of cytoplasmic calcium.
1. Using conventional microelectrodes, the perforated patch clamp technique and fluorescence microscopy with fura-2, we investigated the relationship between the cell membrane potential, whole-cell currents and the free cytoplasmic Ca2+ concentration ([Ca2+]i) in response to 10 nM endothelin-1 (ET) in a rat respiratory epithelial cell line (L2). 2. Microelectrode experiments revealed that ET caused an immediate depolarization of the cell membrane potential (Vm) by 25 mV, which was unaffected by Na+ replacement with N-methyl-D-glucamine+ (NMDG+) or by omission of bath Ca2+. In contrast, ET depolarized the cells by 61 mV in the presence of low C1- (6 mM), resulting in a complete breakdown of Vm. 3. In perforated patch clamp experiments, the ET-induced whole-cell current (IET) exhibited a slight outward rectification with a reversal potential (Vrev) of -22.7 mV. IET was reduced by 85 % in low C1- (6 mM), but was unaffected by Ca2+ removal, Na+ replacement with NMDG+, pipette K+ replacement with Cs+ or 1 mM Ni2+ in the bath. 4. IET was unaffected by (+)-isradipine (100 nM), a specific L-type Ca2+ channel (L-VDCC) blocker. Transient inward Sr2+ currents through L-VDCCs were blocked by ET. 5. ET induced a biphasic Ca2+ signal, consisting of a 'peak' and a 'plateau' elevation of [Ca2+]i. Simultaneous patch clamp and fura-2 measurements revealed that IET coincided with intracellular Ca2+ release but clearly outlasted the elevation of [Ca2+]i. When the rise of [Ca2+]i was prevented by pretreatment with thapsigargin in a Ca2+-free bath, both activation time and amplitude of IET were reduced. Under these conditions, ET caused a decrease of [Ca2+]i. 6. The C1- channel blocker mefenamic acid (MFA) had a dual, concentration-dependent effect on both IET and the ET-induced 'plateau' elevation of [Ca2+]i: an increase at 10 microM, but an almost complete block at 100 microM. The effect of MFA on IET preceded the effect on [Ca2+]i. 7. The ET-induced 'plateau' [Ca2+]i fell below control values in a low-C1- (6 mM) solution. 8. These data indicate an amplifying function of intracellular Ca2+ release on an otherwise Ca2+-independent, unique C1- current by ET. Moreover, this C1- current appears to be functionally coupled with dihydropyridine (DHP)-insensitive Ca2+ entry, suggesting a modulatory role for long-lasting effects of ET.
Department of Physiology, University of Innsbruck, Fritz-Pregl-Strasse 3, A-6020 Innsbruck, Austria.
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N
Mair
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M
Frick
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A
Meraner
H
H
Schramek
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P
Dietl
D000818:Animals; D002118:Calcium / Q000378:metabolism* / Q000494:pharmacology; D002121:Calcium Channel Blockers / Q000494:pharmacology; D015220:Calcium Channels / Q000187:drug effects / Q000502:physiology; D020746:Calcium Channels, L-Type; D002460:Cell Line; D002462:Cell Membrane / Q000187:drug effects / Q000502:physiology; D018118:Chloride Channels / Q000037:antagonists & inhibitors / Q000187:drug effects / Q000502:physiology*; D003593:Cytoplasm / Q000378:metabolism; D019332:Endothelin-1 / Q000494:pharmacology / Q000502:physiology*; D004847:Epithelial Cells / Q000166:cytology / Q000187:drug effects / Q000502:physiology*; D017275:Isradipine / Q000494:pharmacology; D007700:Kinetics; D008168:Lung / Q000502:physiology; D008528:Mefenamic Acid / Q000494:pharmacology; D008536:Meglumine / Q000494:pharmacology; D008839:Microelectrodes; D018408:Patch-Clamp Techniques; D051381:Rats; D019284:Thapsigargin / Q000494:pharmacology
D002121:Calcium Channel Blockers; D015220:Calcium Channels; D020746:Calcium Channels, L-Type; D018118:Chloride Channels; D019332:Endothelin-1; D008528:Mefenamic Acid; D019284:Thapsigargin; D008536:Meglumine; D002118:Calcium; D017275:Isradipine