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-Adrenergic stimulation of cytosolic Ca²⁺ oscillations and exocytosis in identified rat corticotrophs

Amy Tse and Frederick W. Tse

1. The patch clamp technique was used in conjunction with a fluorescent Ca²⁺ indicator (indo-1, or indo-1FF) to measure simultaneously cytosolic Ca²⁺ concentration ([Ca²⁺]_i), ionic current and changes in membrane capacitance in single rat corticotrophs identified with the reverse haemolytic plaque assay.

2. Application of the adrenocorticotropin (ACTH) secretagogue noradrenaline (NA; norepinephrine), triggered [Ca²⁺]_i oscillation in corticotrophs via -adrenergic receptors and the guanosine trisphosphate (GTP) binding protein-coupled phosphoinositide pathway.

3. Simultaneous measurement of [Ca²⁺]_i and capacitance shows that exocytosis was triggered during the first cycle of NA-induced [Ca²⁺]_i oscillation and the mean increase in cell membrane surface area was 1·4 ± 0·3 % (n = 6).

4. When Ca²⁺ was directly released from the inositol 1,4,5 trisphosphate (IP₃)-sensitive store via flash photolysis of caged IP₃, the mean increase in cell surface area was 1·5 ± 0·5 % (n = 6). Thus, NA-stimulated ACTH secretion in rat corticotrophs is closely coupled to intracellular Ca²⁺ release.

5. Large and rapid elevation of [Ca²⁺]_i (>15 µM) via flash photolysis of caged Ca²⁺ triggered two phases of exocytosis: a rapid exocytic burst that was complete in ~100 ms and a slow burst that continued for many seconds.

6. The rapid exocytic burst reflected the exhaustion of a pool of readily releasable granules and, on average, increased the cell surface by 2·8 ± 0·1 % (n = 14).

7. We suggest that the relatively weak exocytic response in corticotrophs during intracellular Ca²⁺ release may be partially attributed to a smaller pool of readily releasable granules.

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