A. Tse et Ak. Lee, Voltage-gated Ca2+ channels and intracellular Ca2+ release regulate exocytosis in identified rat corticotrophs, J PHYSL LON, 528(1), 2000, pp. 79-90
1. The patch clamp technique was used in conjunction with a fluorescent Ca2
+ indicator (indo-1, or indo-1FF) to measure simultaneously cytosolic Ca2concentration ([Ca2+](i)) and exocytosis (changes in membrane capacitance)
in single, identified rat corticotrophs.
2. Exocytosis could be stimulated by extracellular Ca2+ entry (via voltage-
gated Ca2+ channels). A train of depolarizations could exhaust the pool of
readily releasable granules and the pool replenished with it time constant
of 42 s (at 22-25 degreesC).
3. Recordings from cells with 0.5 mM intracellular cAMP showed that the amp
litude of the depolarization-triggered exocytosis, the Ca2+ sensitivity of
exocytosis, as well as the rate of replenishment of the readily releasable
pool, were similar to the controls.
4. Exocytosis could also Le stimulated by intracellular Ca2+ release from t
he inositol 1,4,5-trisphosphate (IP3)-sensitive store (via flash photolysis
of caged IP3). At comparable [Ca2+](i), extracellular Ca2+ entry and intra
cellular Ca2+ release had similar efficacy in triggering exocytosis.
5. The rate of exocytosis triggered via depolarization or intracellular Ca2
+ release was much faster than that triggered via uniform elevation of [Ca2
+](i) (Ca2+ dialysis or flash photolysis of caged Ca2+).
6. The above findings suggest that both intracellular Ca2+ release and volt
age-gated extracellular Ca2+ entry generate a spatial Ca2+ gradient, such t
hat the local [Ca2+] near the exocytic sites was similar to3-fold higher th
an the mean cytosolic [Ca2+]. However, neither cAMP nor the spatial Ca2+ gr
adient generated during depolarization could account for the high efficacy
of corticotropin-releasing hormone (CRH) in stimulating adrenocorticotropic
hormone (ACTH) secretion from corticotrophs.