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Pflügers Archiv

, Volume 431, Issue 1, pp 76–83 | Cite as

Intracellular CI modulates Ca2+-induced exocytosis from rat melanotrophs through GTP-binding proteins

  • Marjan Rupnik
  • Robert Zorec
Original Article Molecular and Cellular Physiology

Abstract

We used the whole-cell patch-clamp technique to monitor changes in membrane capacitance (Cm) to study the influence of cytosolic concentration ([Cl]i) on the secretory activity of rat melanotrophs. The sensitivity of the secretory machinery to Ca2+ was enhanced in the presence of a high [Cl]i. The free concentration of Ca2+ required for half-maximal secretory activity was reduced from 3.2 μM at 4 mM [Cl]i to 0.7 μM at 154 mM [Cl]i. To study whether the modulation of secretory activity by Cl involves guanosine 5′-triphosphate-(GTP-) binding proteins, cells were dialysed with non-hydrolysable GTP and GDP analogues, fluoroaluminate (A1F4), or were pretreated with pertussis toxin. With guanosine 5′-O-(3-thiotriphosphate) (GTP[γ-S], 100 μM) the maximal rate of Cm increase (dCm/dt) was enhanced at 4 and 14 mM [Cl]i, but it was not affected at 154 mM [Cl]i. In contrast, the secretory response, measured as a percentage of resting Cm 10 min after the start of recordings, was reduced at 154 mM [Cl]i, but not affected at 4 mM [Cl]i. Only with 154 mM [Cl]i did intracellular dialysis of cells with guanosine 5′-O-(2-thiodiphosphate) (GDP[β-S], 500 μM) inhibit dCmdt as well as relative secretory responses. The presence of AlF4 (30 μM) or a 7-h pretreatment of cells with pertussis toxin (250 ng/ml) significantly reduced both the maximal dCm/dt and relative secretory responses, but only in the presence of 154 mM [Cl]i. Since the effects of GDP[β-S], AlF4, and pertussis toxin pretreatment were only detected with a high [Cl]i, we conclude that modulation by Cl of secretory activity of rat melanotrophs is mediated through GTP-binding proteins. Furthermore, the effects of AlF4 and pertussis toxin indicate a role of heterotrimeric GTP-binding proteins in the secretory activity of melanotrophs.

Key words

Chloride ions GTP-binding proteins Calcium ions Exocytosis 

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Copyright information

© Springer-Verlag 1995

Authors and Affiliations

  • Marjan Rupnik
    • 1
  • Robert Zorec
    • 1
  1. 1.Laboratory of Neuroendocrinology, Institute of Pathophysiology, School of MedicineUniversity of LjubljanaLjubljanaSlovenia

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