Quantal Release Analysis of Electrochemically Active Molecules Using Single-Cell Amperometry

Single-cell amperometry is a powerful technique that permits the detection of electrochemically active transmitters, such as catecholamines, histamine, or serotonin, released by exocytosis from secretory cells.Amperometry has two main characteristics that make it ideal for the study of exocytosis at the single-cell level with single-vesicle resolution quantal release. (i) It is noninvasive. The carbon fiber microelectrode can be carefully positioned on plasma membrane of a single cell, allowing the detection of the oxidation current of the secreted molecules. (ii) High temporal resolution and sensitivity. Exocytosis can be monitored with a real-time resolution that allows the determination of the kinetics release with an attomol detection sensitivity, which ensures an accurate calculation of the amount of transmitter released.Here, we compile some recommendations and advices to perform amperometry quantal analysis.

Faculty Rating of the Importance and Availability of Organizational Mentoring Climate

Organizational climate is the shared perception of and the meaning attached to the policies, practices, and procedures employees experience. University faculty can assess their organizational mentoring climate (OMC) using recently published, reliable, and valid OMC importance (OMCI) and availability (OMCA) scales. Factors affecting the OMC's importance and availability are, however, not known. By studying these factors, organizational leaders can determine whether and how to change the OMC to improve faculty mentoring outcomes. In this cross-sectional study, 300 faculty from the University of New Mexico (Main, Health Sciences Center [HSC] and branch campuses) and Arizona State University (a non-HSC campus) completed the online OMCI and OMCA scales, each with three subscales: Organizational Expectations, Mentor-Mentee Relationships, and Resources. OMCI scale items were rated from very unimportant (1) to very important (5); and, for OMCA, -1 (no), 0 (don't know), 1 (yes). The study used linear regression analysis after normalizing the scales to M=0 and SD=1. Although not explicitly targeted for recruitment, the respondents were predominantly women, non-Hispanic White, senior, tenure-track faculty members who were neither providing mentoring nor receiving mentoring. In the multivariable models, women faculty attached greater importance to mentoring climate components than men. HSC faculty and those receiving mentoring reported greater availability of mentoring climate components than their respective counterparts. Underrepresented minority (URM) faculty did not rate OMCI or OMCA differently than non-URM faculty. Faculty subgroups in this study attached varying levels of importance to the OMC and rated the availability of climate components differently. Factors impacting the importance of the OMC differed from those affecting the perceived availability of the climate components. Based on their relative importance and lack of availability, organizational leaders should create, modify and implement structures, programs, and policies to improve organizational mentoring expectations, mentor-mentee relationships, and mentoring resources, thereby strengthening their OMC.

Adrenergic chromaffin cells are adrenergic even in the absence of epinephrine

Adrenal chromaffin cells release epinephrine (EPI) and norepinephrine (NE) into the bloodstream as part of the homeostatic response to situations like stress. Here we utilized EPI-deficient mice generated by knocking out (KO) the phenylethanolamine N-methyltransferase (Pnmt) gene. These Pnmt-KO mice were bred to homozygosis but displayed no major phenotype. The lack of EPI was partially compensated by an increase in NE, suggesting that EPI storage was optimized in adrenergic cells. Electron microscopy showed that despite the lack of EPI, chromaffin granules retain their shape and general appearance. This indicate that granules from adrenergic or noradrenergic cells preserve their characteristics even though they contain only NE. Acute insulin injection largely reduced the EPI content in wild-type animals, with a minimal reduction in NE, whereas there was only a partial reduction in NE content in Pnmt-KO mice. The analysis of exocytosis by amperometry revealed a reduction in the quantum size (-30%) and I_max (-21%) of granules in KO cells relative to the wild-type granules, indicating a lower affinity of NE for the granule matrix of adrenergic cells. As amperometry cannot distinguish between adrenergic or noradrenergic cells, it would suggest even a larger reduction in the affinity for the matrix. Therefore, our results demonstrate that adrenergic cells retain their structural characteristics despite the almost complete absence of EPI. Furthermore, the chromaffin granule matrix from adrenergic cells is optimized to accumulate EPI, with NE being a poor substitute. Open Science: This manuscript was awarded with the Open Materials Badge For more information see: https://cos.io/our-services/open-science-badges/.

Distinct patterns of exocytosis elicited by Ca2+, Sr2+ and Ba2+ in bovine chromaffin cells

Three divalent cations can elicit secretory responses in most neuroendocrine cells, including chromaffin cells. The extent to which secretion is elicited by the cations in intact depolarized cells was Ba²⁺ > Sr²⁺ ≥ Ca²⁺, contrasting with that elicited by these cations in permeabilized cells (Ca²⁺ > Sr²⁺ > Ba²⁺). Current-clamp recordings show that extracellular Sr²⁺ and Ba²⁺ cause membrane depolarization and action potentials, which are not blocked by Cd²⁺ but that can be mimicked by tetra-ethyl-ammonium. When applied intracellularly, only Ba²⁺ provokes action potentials. Voltage-clamp monitoring of Ca²⁺-activated K⁺ channels (K_Ca) shows that Ba²⁺ reduces outward currents, which were enhanced by Sr²⁺. Extracellular Ba²⁺ increases cytosolic Ca²⁺ concentrations in Fura-2-loaded intact cells, and it induces long-lasting catecholamine release. Conversely, amperometric recordings of permeabilized cells show that Ca²⁺ promotes the longest lasting secretion, as Ba²⁺ only provokes secretion while it is present and Sr²⁺ induces intermediate-lasting secretion. Intracellular Ba²⁺ dialysis provokes exocytosis at concentrations 100-fold higher than those of Ca²⁺, whereas Sr²⁺ exhibits an intermediate sensitivity. These results are compatible with the following sequence of events: Ba²⁺ blocks K_Ca channels from both the outside and inside of the cell, causing membrane depolarization that, in turn, opens voltage-sensitive Ca²⁺ channels and favors the entry of Ca²⁺ and Ba²⁺. Although Ca²⁺ is less permeable through its own channels, it is more efficient in triggering exocytosis. Strontium possesses both an intermediate permeability and an intermediate ability to induce secretion.

ATP: The crucial component of secretory vesicles

The colligative properties of ATP and catecholamines demonstrated in vitro are thought to be responsible for the extraordinary accumulation of solutes inside chromaffin cell secretory vesicles, although this has yet to be demonstrated in living cells. Because functional cells cannot be deprived of ATP, we have knocked down the expression of the vesicular nucleotide carrier, the VNUT, to show that a reduction in vesicular ATP is accompanied by a drastic fall in the quantal release of catecholamines. This phenomenon is particularly evident in newly synthesized vesicles, which we show are the first to be released. Surprisingly, we find that inhibiting VNUT expression also reduces the frequency of exocytosis, whereas the overexpression of VNUT drastically increases the quantal size of exocytotic events. To our knowledge, our data provide the first demonstration that ATP, in addition to serving as an energy source and purinergic transmitter, is an essential element in the concentration of catecholamines in secretory vesicles. In this way, cells can use ATP to accumulate neurotransmitters and other secreted substances at high concentrations, supporting quantal transmission.

Preparation and culture of adrenal chromaffin cells

Cultured chromaffin cells have been used for almost 40 years in the study of different cell functions using biochemical, electrophysiological, pharmacological, and toxicological approaches. Chromaffin cells are essentially secretory cells that are used to model sympathetic neurons or neuroendocrine cells. In this chapter, we describe the most common methods currently used to isolate and culture chromaffin cells from the animals used most commonly: cows, rats, and mice. We also provide some advice on the use of these cells in the laboratory.

Vesicular Ca(2+) mediates granule motion and exocytosis

Secretory vesicles of chromaffin cells are acidic organelles that maintain an increasing pH gradient towards the cytosol (5.5 vs. 7.3) that is mediated by V-ATPase activity. This gradient is primarily responsible for the accumulation of large concentrations of amines and Ca(2+), although the mechanisms mediating Ca(2+) uptake and release from granules, and the physiological relevance of these processes, remain unclear. The presence of a vesicular matrix appears to create a bi-compartmentalised medium in which the major fractions of solutes, including catecholamines, nucleotides and Ca(2+), are strongly associated with vesicle proteins, particularly chromogranins. This association appears to be favoured at acidic pH values. It has been demonstrated that disrupting the pH gradient of secretory vesicles reduces their rate of exocytosis and promotes the leakage of vesicular amines and Ca(2+), dramatically increasing the movement of secretory vesicles and triggering exocytosis. In this short review, we will discuss the data available that highlights the importance of pH in regulating the association between chromogranins, vesicular amines and Ca(2+). We will also address the potential role of vesicular Ca(2+) in two major processes in secretory cells, vesicle movement and exocytosis.

Chromogranins A and B are key proteins in amine accumulation, but the catecholamine secretory pathway is conserved without them

Chromogranins are the main soluble proteins in the large dense core secretory vesicles (LDCVs) found in aminergic neurons and chromaffin cells. We recently demonstrated that chromogranins A and B each regulate the concentration of adrenaline in chromaffin granules and its exocytosis. Here we have further studied the role played by these proteins by generating mice lacking both chromogranins. Surprisingly, these animals are both viable and fertile. Although chromogranins are thought to be essential for their biogenesis, LDCVs were evident in these mice. These vesicles do have a somewhat atypical appearance and larger size. Despite their increased size, single-cell amperometry recordings from chromaffin cells showed that the amine content in these vesicles is reduced by half. These data demonstrate that although chromogranins regulate the amine concentration in LDCVs, they are not completely essential, and other proteins unrelated to neurosecretion, such as fibrinogen, might compensate for their loss to ensure that vesicles are generated and the secretory pathway conserved.

Chromogranins as regulators of exocytosis

Chromogranins (Cgs) constitute the main protein component in the vesicular matrix of large dense core vesicles (LDCV). These acidic proteins have been implicated in several physiological processes such as vesicle sorting, the generation of bioactive peptides and the accumulation of soluble species inside LDCV. This latter feature of Cgs accounts for the ability of vesicles to concentrate catecholamines and Ca(2+). Indeed, the low affinity and high capacity of Cgs to bind solutes at the low pH of the LDCV lumen seems to be behind the delay in the neurotransmitter exit towards the extracellular milieu after vesicle fusion. The availability of new mouse strains lacking Cgs in combination with the arrival of several techniques for the direct monitoring of exocytosis (like amperometry, patch-amperometry and intracellular electrochemistry), have helped advance our understanding of how these granins concentrate catecholamines and Ca(2+) in LDCV, and how they influence the kinetics of exocytosis. In this review, we will discuss the roles of Cgs A and B in maintaining the intravesicular environment of secretory vesicles and in exocytosis, bringing together the most recent findings from adrenal chromaffin cells.

[Efficacy of sugammadex in the reversal of neuromuscular blockade induced by rocuronium in long-duration surgery: under inhaled vs. intravenous anesthesia]

BACKGROUND AND OBJECTIVE

Sugammadex reverses neuromuscular blockade induced by aminosteroid agents by encapsulating these agents. The objective of this study was to compare the efficacy and safety of sugammadex to reverse a rocuronium-induced neuromuscular blockade in long-duration surgery in association with inhaled or intravenous anesthesia.

PATIENTS AND METHODS

We performed a randomized, double-blind, multicenter trial of 20 ASA 1-3 patients aged between 18 and 69 years and scheduled for elective surgery lasting at least 120 minutes. Anesthesia was induced with remifentanil and rocuronium at a dosage of 0.6 mg x kg(-1), and neuromuscular function was monitored by means of acceleromyography. After randomization, anesthesia was maintained with sevoflurane or with propofol for total intravenous anesthesia. Patients in both groups also received an infusion of remifentanil for analgesia and rocuronium to maintain a block of greater than 90%. After surgery, sugammadex was administered at a dosage of 2 mg x kg(-1) on reappearance of the second train-of-four (TOF) twitch (T2) and the times until recovery of T4/T1 ratios of 0.7, 0.8, and 0.9 (main endpoints). Mean arterial pressure and heart rate were recorded at baseline and after 2, 5, 10, and 30 minutes (secondary outcome measures).

RESULTS

Although less rocuronium was consumed in the sevoflurane group than in the propofol group and the time between the start of sugammadex administration until recovery of a TOF ratio of 0.9 was shorter for the sevoflurane group than for propofol group (mean [SD], 1.46 [0.30] minutes and 1.89 [0.62] minutes, respectively), these differences were not significant. No signs of recurarization or associated adverse effects were observed.

CONCLUSIONS

Sugammadex effectively and safely reverses a rocuronium-induced neuromuscular blockade in less than 2 minutes in long-duration surgery performed under both inhaled and intravenous anesthesia. The interaction of neuromuscular blocking agents with sevoflurane appears not to affect the reversal time of sugammadex in such operations.