MULTIPLE CYTOSOLIC CALCIUM SIGNALS AND MEMBRANE ELECTRICAL EVENTS EVOKED IN SINGLE ARGININE VASOPRESSIN-STIMULATED CORTICOTROPHS

The action of arginine vasopressin (AVP) on cytosolic free Ca2+ concen tration ([Ca2+]i) was investigated in single rat pituitary corticotrop hs using indo-1 microfluorimetry, in part in combination with the moni toring of membrane electrical events with the perforated patch-clamp t echnique. In corticotrophs showing the series of short-lived [Ca2+]i r ises (transient pattern) in response to corticotropin-releasing factor , 100 nM AVP evoked either the transient pattern or a [Ca2+]i spike fo llowed by a sustained plateau (spike/plateau pattern). Not all cortico trophs responded to changes in AVP concentration in the same manner. S ome cells exhibited a concentration-dependent increase in [Ca2+]i tran sient activity, whereas others showing the spike/plateau at high AVP c oncentrations responded to low agonist concentrations by two [Ca2+]i r esponses: a slow rising step or two to three sinusoidal-like oscillati ons. Combined [Ca2+]i and patch-clamp recordings as well as manipulati on of extracellular Ca2+ showed that both transient pattern and the pl ateau of spike/plateau response depended on Ca2+ entry mainly through voltage-gated, dihydropyridine-sensitive Ca2+ channels. By contrast, s tep, oscillations, and spike were due to Ca2+ release from internal st ores. These Ca2+-mobilizing responses caused the activation of Ca2+-ac tivated, apamin-sensitive K+ channels, which led to a membrane hyperpo larization. These results reveal cell-specific [Ca2+]i signals and ass ociated electrical events in individual AVP-stimulated corticotrophs.