Endogenous pacemaker activity of rat tumour somatotrophs.
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 Ca2+ concentration ([Ca2+]i) imaging, they displayed rhythmic action potentials (44.7 +/- 2.7 mV, 178 +/- 40 ms, 0.30 +/- 0.04 Hz) and concomitant [Ca2+]i transients (374 +/- 57 nM, 1.0 +/- 0.2 s, 0.27 +/- 0.03 Hz). 2. Action potentials and [Ca2+]i transients were reversibly blocked by removal of external Ca2+, addition of nifedipine (1 microM) or Ni2+ (40 microM), but were insensitive to TTX (1 microM). An L-type Ca2+ current activated at -33.6 +/- 0.4 mV (holding potential (Vh), -40 mV), peaked at -1.8 +/- 1.3 mV, was reduced by nifedipine and enhanced by S-(+)-SDZ 202 791. A T/R-type Ca2+ current activated at -41.7 +/- 2.7 mV (Vh, -80 or -60 mV), peaked at -9.2 +/- 3.0 mV, was reduced by low concentrations of Ni2+ (40 microM) or Cd2+ (10 microM) and was toxin resistant. Parallel experiments revealed the expression of the class E calcium channel alpha1-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 Cd2+ (400 microM). 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 Ca2+ current underlies the rising phase of action potentials that are repolarized by a delayed rectifier and Ca2+-activated K+ currents. In between spikes, the decay of Ca2+-activated K+ currents and a persistent inward cationic current depolarize the membrane, activate the T/R-type Ca2+ current and initiate a new cycle.
Kwiecien
R
R
Unite de Dynamique des Systemes Neuroendocriniens, INSERM U159, 2 ter rue d'Alesia, 75014 Paris, France.
Robert
C
C
Cannon
R
R
Vigues
S
S
Arnoux
A
A
Kordon
C
C
Hammond
C
C
D000200:Action Potentials / Q000187:drug effects / Q000502:physiology; D000818:Animals; D016375:Antisense Elements (Genetics); D001030:Apamin / Q000494:pharmacology; D001464:Barium / Q000493:pharmacokinetics; D001683:Biological Clocks / Q000502:physiology*; D002104:Cadmium / Q000494:pharmacology; D002118:Calcium / Q000378:metabolism / Q000493:pharmacokinetics; D002121:Calcium Channel Blockers / Q000494:pharmacology; D015220:Calcium Channels / Q000502:physiology; D018999:Charybdotoxin / Q000494:pharmacology; D003198:Computer Simulation; D003600:Cytosol / Q000378:metabolism; D004095:Dihydropyridines / Q000494:pharmacology; D013006:Growth Hormone / Q000378:metabolism*; D013007:Growth Hormone-Releasing Hormone / Q000378:metabolism; D009532:Nickel / Q000494:pharmacology; D009543:Nifedipine / Q000494:pharmacology; D018408:Patch-Clamp Techniques; D010455:Peptides / Q000494:pharmacology; D010911:Pituitary Neoplasms; D011188:Potassium / Q000378:metabolism; D051381:Rats; D019837:Ryanodine Receptor Calcium Release Channel / Q000502:physiology; D012964:Sodium / Q000494:pharmacology; D013111:Spider Venoms / Q000494:pharmacology; D019789:Tetraethylammonium / Q000494:pharmacology; D013779:Tetrodotoxin / Q000494:pharmacology; D014407:Tumor Cells, Cultured / Q000737:chemistry / Q000502:physiology; D020867:omega-Agatoxin IVA; D020866:omega-Conotoxin GVIA
D016375:Antisense Elements (Genetics); D002121:Calcium Channel Blockers; D015220:Calcium Channels; D004095:Dihydropyridines; D010455:Peptides; D019837:Ryanodine Receptor Calcium Release Channel; D013111:Spider Venoms; D020867:omega-Agatoxin IVA; D002104:Cadmium; D018999:Charybdotoxin; D001030:Apamin; D001464:Barium; D013779:Tetrodotoxin; D019789:Tetraethylammonium; C038806:1,4-dihydropyridine; D009532:Nickel; D013006:Growth Hormone; D013007:Growth Hormone-Releasing Hormone; D020866:omega-Conotoxin GVIA; D012964:Sodium; D009543:Nifedipine; D011188:Potassium; D002118:Calcium