Modulation of the unitary exocytic event amplitude by cAMP in rat melanotrophs

Sikdar, Sujit K and Kreft, Marko and Zorec, Robert (1998) Modulation of the unitary exocytic event amplitude by cAMP in rat melanotrophs. In: The Journal of Physiology, 511 (03). pp. 851-859.

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Abstract

Secretory responses were measured in single rat pituitary melanotrophs as the relative increase in membrane capacitance (C-m) 8 min after the start of dialysis with solutions containing 0.45 mu M Ca2+. In the added presence of cAMP (0.2 mM) in the patch pipette solution, capacitance responses increased 2- to 3-fold in comparison with controls. 2. To study whether cAMP-dependent mechanisms affect cytosolic calcium activity ([Ca2+](i)), dibutyryl cyclic AMP (dbcAMP, 10 mM) was added to intact melanotrophs and [Ca2+](i) was measured using fura-2 AM. Addition of dbcAMP caused a transient reduction in [Ca2+](i) to 82 +/- 21 nM from a resting value of 100 +/- 19 nM (mean +/- S.E.M., n = 32, P < 0.002), indicating that the cAMP-induced increase in secretory activity was not the result of cAMP acting to increase [Ca2+](i), which then increased secretory activity. 3. To investigate whether cAMP affects the secretory apparatus directly, the interaction of a single secretory granule with the plasmalemma was monitored by measuring discrete femtofarad steps in C-m. The signal-to-noise ratio of recordings was increased by preincubating the cells with a hydrophobic anion, dipicrylamine. 4. Recordings of unitary exocytic events (discrete 'on' steps in C-m) showed that the amplitude of 'on' steps - a parameter correlated to the size of exocytosing secretory granules increased from 4.2 +/- 0.2 fF (n = 356) in controls to 7.9 +/- 0.2 fF in the presence of cAMP (n = 329, P < 0.001), while the frequency of unitary exocytic events was similar in controls and in the presence of cAMP. 5. The results suggest that a cAMP-dependent mechanism mediates the fusion of larger granules with the plasmalemma.

Item Type:	Journal Article
Publication:	The Journal of Physiology
Publisher:	Cambridge University Press
Additional Information:	Copyright of this article belongs to Cambridge University Press.
Department/Centre:	Division of Biological Sciences > Molecular Biophysics Unit
Date Deposited:	22 Dec 2009 07:15
Last Modified:	19 Sep 2010 05:26
URI:	http://eprints.iisc.ac.in/id/eprint/18976

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