Quinacrine affinity of endocrine cell systems containing dense core vesicles as visualized by fluorescence microscopy

Quinacrine is not a vital fluorescent probe for vesicular ATP storage

Quinacrine, a fluorescent amphipathic amine, has been used as a vital fluorescent probe to visualize vesicular storage of ATP in the field of purinergic signaling. However, the mechanism(s) by which quinacrine represents vesicular ATP storage remains to be clarified. The present study investigated the validity of the use of quinacrine as a vial fluorescent probe for ATP-storing organelles. Vesicular nucleotide transporter (VNUT), an essential component for vesicular storage and ATP release, is present in very low density lipoprotein (VLDL)-containing secretory vesicles in hepatocytes. VNUT gene knockout (Vnut^-/-) or clodronate treatment, a VNUT inhibitor, disappeared vesicular ATP release (Tatsushima et al., Biochim Biophys Acta Molecular Basis of Disease 2021, e166013). Upon incubation of mice's primary hepatocytes, quinacrine accumulates in a granular pattern into the cytoplasm, sensitive to 0.1-μM bafilomycin A₁, a vacuolar ATPase (V-ATPase) inhibitor. Neither Vnut^-/- nor treatment of clodronate affected quinacrine granular accumulation. In vitro, quinacrine is accumulated into liposomes upon imposing inside acidic transmembranous pH gradient (∆pH) irrespective of the presence or absence of ATP. Neither ATP binding on VNUT nor VNUT-mediated uptake of ATP was affected by quinacrine. Consistently, VNUT-mediated uptake of quinacrine was negligible or under the detection limit. From these results, it is concluded that vesicular quinacrine accumulation is not due to a consequence of its interaction with ATP but due to ∆pH-driven concentration across the membranes as an amphipathic amine. Thus, quinacrine is not a vital fluorescent probe for vesicular ATP storage.

Vesicular nucleotide transporter-mediated ATP release regulates insulin secretion

Extracellular ATP plays a critical role in regulating insulin secretion in pancreatic β cells. The ATP released from insulin secretory vesicles has been proposed to be a major source of extracellular ATP. Currently, the mechanism by which ATP accumulates into insulin secretory granules remains elusive. In this study, the authors identified the expression of a vesicular nucleotide transporter (VNUT) in mouse pancreas, isolated mouse islets, and MIN6 cells, a mouse β cell line. Immunohistochemistry and immunofluorescence revealed that VNUT colocalized extensively with insulin secretory granules. Functional studies showed that suppressing endogenous VNUT expression in β cells by small hairpin RNA knockdown greatly reduced basal- and glucose-induced ATP release. Importantly, knocking down VNUT expression by VNUT small hairpin RNA in MIN6 cells and isolated mouse islets dramatically suppressed basal insulin release and glucose-stimulated insulin secretion (GSIS). Moreover, acute pharmacologic blockade of VNUT with Evans blue, a VNUT antagonist, greatly attenuated GSIS in a dose-dependent manner. Exogenous ATP treatment effectively reversed the insulin secretion defect induced by both VNUT knockdown and functional inhibition, indicating that VNUT-mediated ATP release is essential for maintaining normal insulin secretion. In contrast to VNUT knockdown, overexpression of VNUT in β cells resulted in excessive ATP release and enhanced basal insulin secretion and GSIS. Elevated insulin secretion induced by VNUT overexpression was reversed by pharmacologic inhibition of P2X but not P2Y purinergic receptors. This study reveals VNUT is expressed in pancreatic β cells and plays an essential and novel role in regulating insulin secretion through vesicular ATP release and extracellular purinergic signaling.

ATP synthesis and storage

Since 1929, when it was discovered that ATP is a substrate for muscle contraction, the knowledge about this purine nucleotide has been greatly expanded. Many aspects of cell metabolism revolve around ATP production and consumption. It is important to understand the concepts of glucose and oxygen consumption in aerobic and anaerobic life and to link bioenergetics with the vast amount of reactions occurring within cells. ATP is universally seen as the energy exchange factor that connects anabolism and catabolism but also fuels processes such as motile contraction, phosphorylations, and active transport. It is also a signalling molecule in the purinergic signalling mechanisms. In this review, we will discuss all the main mechanisms of ATP production linked to ADP phosphorylation as well the regulation of these mechanisms during stress conditions and in connection with calcium signalling events. Recent advances regarding ATP storage and its special significance for purinergic signalling will also be reviewed.

Methods for imaging Renin-synthesizing, -storing, and -secreting cells

Renin-producing cells have been the object of intense research efforts for the past fifty years within the field of hypertension. Two decades ago, research focused on the concept and characterization of the intrarenal renin-angiotensin system. Early morphological studies led to the concept of the juxtaglomerular apparatus, a minute organ that links tubulovascular structures and function at the single nephron level. The kidney, thus, appears as a highly "topological organ" in which anatomy and function are intimately linked. This point is reflected by a concurrent and constant development of functional and structural approaches. After summarizing our current knowledge about renin cells and their distribution along the renal vascular tree, particularly along glomerular afferent arterioles, we reviewed a variety of imaging techniques that permit a fine characterization of renin synthesis, storage, and release at the single-arteriolar, -cell, or -granule level. Powerful tools such as multiphoton microscopy and transgenesis bear the promises of future developments of the field.

Cellular Distribution and Functions of P2 Receptor Subtypes in Different Systems

This review is aimed at providing readers with a comprehensive reference article about the distribution and function of P2 receptors in all the organs, tissues, and cells in the body. Each section provides an account of the early history of purinergic signaling in the organ?cell up to 1994, then summarizes subsequent evidence for the presence of P2X and P2Y receptor subtype mRNA and proteins as well as functional data, all fully referenced. A section is included describing the plasticity of expression of P2 receptors during development and aging as well as in various pathophysiological conditions. Finally, there is some discussion of possible future developments in the purinergic signaling field.

Effects of amines, monensin and nigericin on the renin release from isolated superfused rat glomeruli

Renin release (RR) in vitro has been shown to depend upon exocytosis, which is brought about by osmotically induced swelling of the acidic secretory granules. Since granule acidity has been suggested to be responsible for the exocytosis of other secretory granules (the chemiosmotic hypothesis), experiments were designed to test its possible significance in the RR from isolated superfused rat glomeruli. Each experiment comprised 5-6 series each of 14 consecutive 12 min periods. Changes in the extracellular pH from 7.4 to 7.8 by an increase in the concentration of bicarbonate inhibited the RR transiently. Alkalinization of the cell interior was achieved with weak permeable bases and ionophores. At low concentrations (5 mM NH4Cl; 0.2 mM chloroquine) the weak bases caused a delayed inhibition of the RR, while at higher concentrations (15 and 30 mM NH4Cl; 10 mM methylamine) the inhibitory effect was overlaid with a transient stimulation. 1.5 mM NH4Cl and 10 and 20 microM chloroquine had no effect. Addition of 10 microM of the Na-H ionophore monensin also caused a transient stimulation followed by a progressive inhibition. 0.1 microM monensin had no effect. The above procedures cause increases in both the granular and the cytosolic pH. The K-H ionophore nigericin will cause an increase in the granular pH but a decrease in the cytosolic pH because of the prevailing ionic gradients. Since the effect of 10 microM nigericin was similar to that of monensin, it is concluded that the above effects are due to the increase in the intragranular pH. Thus, the maintenance of a low intragranular pH is of importance for a continuous RR.(ABSTRACT TRUNCATED AT 250 WORDS)

Quinacrine accumulation in pancreatic islet cells of rat and mouse: relationship to functional activity and effects on basal and stimulated insulin secretion

The fluorescent acridine derivative, quinacrine, was found to accumulate in rat and mouse pancreatic islet cells storing insulin, glucagon, pancreatic polypeptide, or somatostatin. Following administration of large doses of tolbutamide via an oro-gastric tube, the intensity of quinacrine fluorescence of insulin cells was substantially reduced. Similarly, the pancreatic insulin content was lowered. In contrast, the fluorescence intensity of the glucagon, pancreatic polypeptide and somatostatin cells appeared unaffected. Basal plasma insulin levels in the mouse were slightly elevated following quinacrine administration (25%). Glucose-stimulated insulin release was markedly enhanced (51%) in quinacrine-pretreated animals, whereas insulin release induced by cholinergic stimulation was unaffected. The results show that quinacrine accumulates in the various pancreatic islet cells. The drug seems to be confined to the secretory granules and affects the insulin response to glucose but not that to cholinergic stimulation, suggesting that these secretagogues act through different or partly different secretory pathways.

Fluorescence histochemical methods for the study of peptide hormone-producing cells

Fluorescence histochemical methods for the demonstration of specific residues in peptides and proteins are reviewed: Formaldehyde-ozone for NH2-terminal tryptophan, formaldehyde-HCl for tryptophan regardless of position in the peptide, OPT for NH2-terminal histidine, formaldehyde-fluorescamine for "protected" amino groups, nitroso-naphthol for tyrosine, and phenanthrenequinone for arginine residues. The methods are potent in demonstrating granule-stored material in peptide hormone-producing cells. Also quinacrine, the fluorescent anti-malaria agent, binds to granular components, as yet unidentified, in several endocrine cell types. In many cases the fluorescence histochemical methods seem to demonstrate peptides and proteins distinct from the known hormones.

Release of [14C]quinacrine from peripheral and central nerves

Incubations of intestinal mouse smooth muscle sheets including Auerbach's plexus and slices of the spinal cord in buffer containing low concentrations of labelled quinacrine ([14C]QC) result in a high affinity binding of QC to certain nerve fibres. Various means of depolarization, such as veratridine, high potassium and electrical field stimulation, were used to assess quantitatively the release of QC from nerves. Under optimal release conditions, all 3 types of depolarizations induced a clearcut increase in radioactivity of a continuously superfused buffer. When intestinal muscle sheets were incubated in high concentrations of QC (5 . 10(-6)--5 . 10(-5) M) the depolarization-induced release was blocked. Similarly, high concentrations of QC (10(-5) M) blocked release of [3H]noradrenaline from adrenergic nerves. Low QC concentrations did not affect the depolarization induced release of [3H]noradrenaline from adrenergic nerves but caused a moderately increased spontaneous overflow of noradrenaline. The present data, together with previous studies, permit the conclusion that QC in low concentrations can be used to label selectively a population of non-adrenergic nerve fibres. This binding is closely related to the transmitter storage and release mechanisms and can be used to study the activity of such nerves.

Juxtaglomerular cell activity during hemorrhage and ischemia as revealed by quinacrine histofluorescence

Quinacrine (QC) binds with high affinity to the intracellular storage granules of juxtaglomerular cells (JG-cells) in the afferent arteriolus of the glomerulus of the kidney. The present study tests whether QC bound to JG-cells can be released. The cells were stimulated by renal ischemia and hemorrhagic shock combined with immobilization stress. 1 h after onset of renal ischemia QC-JGI (modified Hartroft & Hartroft 1953) in 14C-QC-treated rats had decreased to about 40% in the ischemic kidney compared to a not ligated control kidney. The 14C-contents in the ischemic kidney had decreased to 33% of that in the untouched control kidney. Hemorrhagic shock was obtained by bleeding into a reservoir for 15 min or 1 h. Rats who received QC or 14C-QC 1 h before onset of bleeding showed no change in QC-JGI (15 min shock) or 14C-contents (1 h shock) as compared to controls. This was probably due to formation of new QC-binding granules, which took up still circulating quinacrine thereby masking a release. If the time between the QC injection and the onset of shock was extended to about 15 h, when circulating amounts of QC are very low, a decrease of QC-JGI (about 30% of controls) was seen in the kidneys of the shocked rats. The results are compatible with the possibility that QC in vivo bound to granules of JG-cells could be released together with the content of the granules following stimuli known to induce renin release. Quinacrine-binding therefore possibly provides a new method to study endocrine cells in the way it has been used in the present study as a marker of JG-cell activity.