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Endogenous pacemaker activity of rat tumour somatotrophs
Author(s) -
Kwiecien Renata,
Robert Christophe,
Can Robert,
Vigues Stephan,
Arnoux Annie,
Kordon Claude,
Hammond Constance
Publication year - 1998
Publication title -
the journal of physiology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.802
H-Index - 240
eISSN - 1469-7793
pISSN - 0022-3751
DOI - 10.1111/j.1469-7793.1998.883bp.x
Subject(s) - apamin , chemistry , nifedipine , charybdotoxin , depolarization , hyperpolarization (physics) , biophysics , electrophysiology , repolarization , patch clamp , channel blocker , endocrinology , membrane potential , medicine , calcium , stereochemistry , biochemistry , biology , organic chemistry , nuclear magnetic resonance spectroscopy
1 Cells derived from a rat pituitary tumour (GC cell line) that continuously release growth hormone behave as endogenous pacemakers. In simultaneous patch clamp recordings and cytosolic Ca 2+ concentration ([Ca 2+ ] i ) imaging, they displayed rhythmic action potentials (44.7 ± 2.7 mV, 178 ± 40 ms, 0.30 ± 0.04 Hz) and concomitant [Ca 2+ ] i transients (374 ± 57 nM, 1.0 ± 0.2 s, 0.27 ± 0.03 Hz). 2 Action potentials and [Ca 2+ ] i transients were reversibly blocked by removal of external Ca 2+ , addition of nifedipine (1 μM) or Ni 2+ (40 μM), but were insensitive to TTX (1 μM). An L‐type Ca 2+ current activated at ‐33.6 ± 0.4 mV (holding potential ( V h ), ‐40 mV), peaked at ‐1.8 ± 1.3 mV, was reduced by nifedipine and enhanced by S ‐(+)‐SDZ 202 791. A T/R‐type Ca 2+ current activated at ‐41.7 ± 2.7 mV ( V h , ‐80 or ‐60 mV), peaked at ‐9.2 ± 3.0 mV, was reduced by low concentrations of Ni 2+ (40 μM) or Cd 2+ (10 μM) and was toxin resistant. Parallel experiments revealed the expression of the class E calcium channel α1‐subunit mRNA. 3 The K + channel blockers TEA (25 mM) and charybdotoxin (10‐100 nM) enhanced spike amplitude and/or duration. Apamin (100 nM) also strongly reduced the after‐spike hyperpolarization. The outward K + tail current evoked by a depolarizing step that mimicked an action potential reversed at ‐69.8 ± 0.3 mV, presented two components, lasted 2‐3 s and was totally blocked by Cd 2+ (400 μM). 4 The slow pacemaker depolarization (3.5 ± 0.4 s) that separated consecutive spikes corresponded to a 2‐ to 3‐fold increase in membrane resistance, was strongly Na + sensitive but TTX insensitive. 5 Computer simulations showed that pacemaker activity can be reproduced by a minimum of six currents: an L‐type Ca 2+ current underlies the rising phase of action potentials that are repolarized by a delayed rectifier and Ca 2+ ‐activated K + currents. In between spikes, the decay of Ca 2+ ‐activated K + currents and a persistent inward cationic current depolarize the membrane, activate the T/R‐type Ca 2+ current and initiate a new cycle.