Simultaneous recordings of cytosolic Ca2+ level and membrane potential and current during the response to thyroliberin in clonal rat anterior pituitary cells.
The response to thyroliberin in prolactin-producing rat GH4C1 clonal cells was studied using fura-2 to monitor the cytosolic Ca2+ level ([Ca2+]i) in single cells, combined with recordings of membrane potential and current. The average value of [Ca2+]i was 109 nM (mean +/- SD, n = 112), and evoked action potentials caused transient elevations of about 100 nM. At higher firing frequencies these transients merged to a sustained elevation. In 100% of the cells thyroliberin caused an instant rise in [Ca2+]i, peaking at 795 +/- 300 nM (n = 112). This first phase of the thyroliberin response was associated with hyperpolarization in current clamp and outward current in voltage clamp, caused by the opening of Ca2(+)-activated K+ channels. In 75% of the cells the initial peak in [Ca2+]i was followed by a prolonged plateau phase at 247 +/- 76 nM (n = 84). In current clamp the second-phase elevation of [Ca2+]i was linked to either a modest depolarization in combination with enhanced firing frequency or a more pronounced depolarization in silent cells. This elevation of [Ca2+]i was reversed by hyperpolarizing current injection. No second-phase elevation of [Ca2+]i was observed during voltage clamp at a holding potential of -50 mV. Short exposure to Ca2(+)-free conditions eliminated the second-phase elevation in [Ca2+]i, whereas the first phase remained intact. Our experiments show a direct relationship between electrical activity and [Ca2+]i in the GH4C1 cells. The second-phase elevation of [Ca2+]i caused by thyroliberin is the result of influx through voltage-sensitive Ca2+ channels, without involving agonist-gated channels.
Iijima
T
T
Electrotechnical Laboratory, Tsukuba-shi, Japan.
Sand
O
O
Sekiguchi
T
T
Matsumoto
G
G
D000818:Animals; D002118:Calcium / Q000378:metabolism*; D015220:Calcium Channels / Q000187:drug effects; D002478:Cells, Cultured; D002999:Clone Cells; D003600:Cytosol / Q000378:metabolism*; D004594:Electrophysiology; D016257:Fura-2; D008564:Membrane Potentials / Q000187:drug effects / Q000502:physiology; D008566:Membranes / Q000187:drug effects / Q000378:metabolism; D010903:Pituitary Gland, Anterior / Q000166:cytology / Q000187:drug effects* / Q000378:metabolism; D011388:Prolactin / Q000097:blood; D051381:Rats; D013050:Spectrometry, Fluorescence; D013973:Thyrotropin-Releasing Hormone / Q000494:pharmacology*
D015220:Calcium Channels; D013973:Thyrotropin-Releasing Hormone; D011388:Prolactin; D002118:Calcium; D016257:Fura-2