Effects of osmotic swelling on voltage-gated calcium channel currents in rat anterior pituitary cells.
ABSTRACT Decrease in extracellular osmolarity ([Os]e) results in stimulation of hormone secretion from pituitary cells. Different mechanisms can account for this stimulation of hormone secretion. In this study we examined the possibility that hyposmolarity directly modulates voltage-gated calcium influx in pituitary cells. The effects of hyposmolarity on L-type (IL) and T-type (IT) calcium currents in pituitary cells were investigated by using two hyposmotic stimuli, moderate (18-22% decrease in [Os]e) and strong (31-32% decrease in [Os]e). Exposure to moderate hyposmotic stimuli resulted in three response types in IL (a decrease, a biphasic effect, and an increase in IL) and in increase in IT. Exposure to strong hyposmotic stimuli resulted only in increases in both IL and IT. Similarly, in intact pituitary cells (perforated patch method), exposure to either moderate or strong hyposmotic stimuli resulted only in increases in both IL and IT. Thus it appears that the main effect of decrease in [Os]e is increase in calcium channel currents. This increase was differential (IL were more sensitive than IT) and voltage independent. In addition, we show that these hyposmotic effects cannot be explained by activation of an anionic conductance or by an increase in cell membrane surface area. In conclusion, this study shows that hyposmotic swelling of pituitary cells can directly modulate voltage-gated calcium influx. This hyposmotic modulation of IL and IT may contribute to the previously reported hyposmotic stimulation of hormone secretion. The mechanisms underlying these hyposmotic effects and their possible physiological relevance are discussed.
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ABSTRACT: Membrane stretch modulates the activity of voltage-gated channels (VGCs). These channels are nearly ubiquitous among eukaryotes and they are present, too, in prokaryotes, so the potential ramifications of VGC mechanosensitivity are diverse. In situ traumatic stretch can irreversibly alter VGC activity with lethal results but that is pathology. This chapter discusses the reversible responses of VGCs to stretch, with the general relation of stretch stimuli to other forms of lipid stress, and briefly, with some irreversible stretch effects (=stretch trauma). A working assumption throughout is that mechanosensitive (MS) VGC motions-that is, motions that respond reversibly to bilayer stretch-are susceptible to other forms of lipid stress, such as the stresses produced when amphiphilic molecules (anesthetics, lipids, alcohols, and lipophilic drugs) are inserted into the bilayer. Insofar as these molecules change the bilayer's lateral pressure profile, they can be termed bilayer mechanical reagents (BMRs). The chapter also discusses the MS VGC behavior against the backdrop of eukaryotic channels more widely accepted as "MS channels"--namely, the transient receptor potential (TRP)-based MS cation channels.Current Topics in Membranes - CURR TOP MEMBR. 01/2007; 59:297-338.
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ABSTRACT: Increased extracellular osmolarity ([Os]e) suppresses stimulated hormone secretion from anterior pituitary cells. Ca2+ influx may mediate this effect. We show that increase in [Os]e (by 18-125%) differentially suppresses L-type and T-type Ca2+ channel currents (IL and IT, respectively); IL was more sensitive than IT. Hyperosmotic suppression of IL depended on the magnitude of increase in [Os]e and was correlated with the percent decrease in pituitary cell volume, suggesting that pituitary cell shrinkage can modulate L-type currents. The hyperosmotic suppression of IL and IT persisted after incubation of pituitary cells either with the actin-disrupter cytochalasin D or with the actin stabilizer phalloidin, suggesting that the actin cytoskeleton is not involved in this modulation. The hyperosmotic suppression of Ca2+ influx was not correlated with changes in reversal potential, membrane capacitance, and access resistance. Together, these results suggest that the hyperosmotic suppression of Ca2+ influx involves Ca2+ channel proteins. We therefore recorded the activity of L-type Ca2+ channels from cell-attached patches while exposing the cell outside the patch pipette to hyperosmotic media. Increased [Os]e reduced the activity of Ca2+ channels but did not change single-channel conductance. This hyperosmotic suppression of Ca2+ currents may therefore contribute to the previously reported hyperosmotic suppression of hormone secretion.AJP Cell Physiology 02/2006; 290(1):C222-32. · 3.71 Impact Factor
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ABSTRACT: This study demonstrates that a significant proportion of high voltage-activated (HVA) Ca(2+) influx in native rat anterior pituitary cells is carried through non-L-type Ca(2+) channels. Using whole-cell patch-clamp recordings and specific Ca(+2) channel toxin blockers we show that ~35% of the HVA Ca(2+) influx in somatotrophs and lactotrophs is carried through Ca(v) 2.1, Ca(v) 2.2 and Ca(v) 2.3 channels, and that somatotrophs and lactotrophs share similar proportions of these non-L-type Ca(+2) channels. Furthermore, experiments on mixed populations of native anterior pituitary cells revealed that the fraction of HVA Ca(+2) influx carried through these non-L-type Ca(2+) channels might even be higher (~46%), suggesting that non-L-type channels exist in the majority of native anterior pituitary cells. Using western blotting, immunoblots for α(1C) , α(1D) , α(1A) , α(1B) and α(1E) Ca(+2) channel subunits were identified in native rat anterior pituitary cells. Additionally, using reverse transcriptase (RT)-PCR cDNA transcripts for α(1C) , α(1D) , α(1A) and α(1B) Ca(+2) channel subunits were identified. Transcripts for α(1E) were non-specific and transcripts for α(1S) were not detected at all (control). Altogether these results clearly demonstrate the existence of multiple HVA Ca(+2) channels in the membrane of rat native anterior pituitary cells. Whether these channels are segregated among different membrane compartments was further investigated in flotation assays, demonstrating that Ca(v) 2.1, Ca(v) 1.2 and caveolin-1 were mostly localized in light fractions of Nycodenz gradients, i.e., in lipid raft domains. Ca(v) 1.3 channels were distributed among both light and heavy fractions of the gradients, i.e., among raft and nonraft domains whereas Ca(v) 2.2 and Ca(v) 2.3 channels mostly among nonraft domains. In summary, we demonstrate here multiple pathways for HVA Ca(2+) influx through L-type and non-L-type Ca(2) channels in the membrane of native anterior pituitary cells. Compartmentalization of these channels among raft and nonraft membrane domains might be essential for their proper regulation by separate receptors and signaling pathways. © 2012 The Authors. Journal of Neuroendocrinology © 2012 British Society for Neuroendocrinology.Journal of Neuroendocrinology 08/2012; · 3.51 Impact Factor