Effects of Cl^– Substitution on Electrophysiological Properties, Ca²⁺ Influx and Prolactin Secretion of Rat Lactotropes in vitro

Abstract

In this study, we compared the effects of different chloride (Cl^–) substitutes – methane sulfonate (CH₃SO^–₃), bromide (Br^–), nitrate (NO^–₃), thiocyanate (SCN^–) and perchlorate (ClO^–₄) – on the secretory activity and calcium current activation of rat lactotropes in primary culture. We observed that CH₃SO^–₃ decreased basal prolactin (PRL) secretion. Br^– had no effect, whereas the more lyotropic anions, such as NO^–₃, SCN^– and ClO^–₄, increased basal PRL secretion. The latter three substitutes induced a significant shift in the voltage dependence of T-type calcium channel activation towards hyperpolarized values. However, this shift alone cannot explain the increase in secretion. Anion permeability studies also demonstrated that the organic anion CH₃SO^–₃ was less permeant than Cl^–, whereas monovalent inorganic anions were more permeant, with the following anion permeability sequence: SCN^– > ClO^–₄ > NO^–₃ > Br^–. In conclusion, deprivation of Cl^– ions has converse consequences on basal and induced secretion; permeating anions result in a transient increase in intracellular Ca²⁺ ions. This process involves voltage-dependent Ca²⁺ channels. We propose that an alteration in intracellular anion concentrations may influence the activation of internal effectors such as G proteins or channel proteins and, therefore, interfere with exocytosis. These effects are correlated with an external action of lyotropic anions, particularly NO^–₃, ClO^–₄ and SCN^–, on the gating properties of T-type calcium channels, probably through changes in cell surface charges. The results demonstrate the modulatory effect of anions on the secretory activity of rat lactotropes and underline the specific role played by chloride in stimulus-secretion coupling.

[Cl−]imodulation of Ca2+-regulated exocytosis in ACh-stimulated antral mucous cells of guinea pig

“…In antral mucous cells, ACh accumulates cAMP via PGE 2 production (34). PKI (1 M) decreased the frequency of ACh-stimulated exocytotic events by ϳ50%, as previously reported (8,32,34) (Fig. 10A).…”

Cytosolic Cl− ions in the regulation of secretory and endocytotic activity in melanotrophs from mouse pituitary tissue slices

“…The Cl − ‐dependent modulation of Ca 2+ entry described in lactotrophs (Garcia et al 1999) is not predominant in melanotrophs. In fact, the amplitude of VACCs remained unaffected by the changes in [Cl − ] i .…”

“…Cytosolic Cl − ions enhance Ca 2+ ‐regulated secretion in a variety of endocrine cell types, including mammalian melanotrophs, lactotrophs and pancreatic β‐cells (Rupnik & Zorec, 1992; Garcia et al 1999; Barg et al 2001). In previous studies, three major mechanisms for Cl − ‐dependent stimulation of secretion have been proposed: (i) Cl − modulates the activity of G‐proteins involved in the regulated exocytosis (Rupnik & Zorec, 1995); (ii) Cl − influences Ca 2+ entry through VACCs by either direct interaction with channel proteins or by modulation of cytosolic effectors such as G‐proteins (Garcia et al 1999); (iii) Cl − uptake into exocytotic vesicles during granular acidification is crucial for vesicle priming (Barg et al 2001).…”

“…The modulation of Ca 2+ ‐dependent exocytosis of hormone‐containing dense core vesicles (DCVs) by cytosolic Cl − ions has been previously described in several neuroendocrine cell types (Rupnik & Zorec, 1992; Garcia et al 1999; Barg et al 2001). In these studies, different mechanisms underlying the Cl − dependence of secretion have been proposed, including the modulation of GTP‐binding proteins (Rupnik & Zorec, 1995), Ca 2+ influx (Garcia et al 1999) and vesicle priming (Barg et al 2001).…”

“…Furthermore, the basal prolactin secretion of lactotrophs was decreased by Cl − ‐deficient extracellular solution. The replacement of extracellular Cl − by lyotropic anions shifted the activation and peak voltage of T‐type Ca 2+ currents towards more negative potentials (Garcia et al 1999). The evidence of anion‐dependent modulation of VACCs has also been provided in the retinal photoreceptors of salamander, where lowering of extracellular [Cl − ] by lyotropic anions reduced the amplitude of L‐type Ca 2+ currents and caused a hyperpolarizing shift in their activation and peak voltage.…”