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.