Kinetics of serotonin uptake in the intact lung

Is Serotonin the Missing Link between COVID-19 Course of Severity in Patients with Diabetes and Obesity?

COVID-19 is an intriguing infectious condition with multisystemic manifestations and variable outcomes that are influenced by the concomitant presence of non-communicable diseases, such as obesity, diabetes, and cardiovascular disease, which were previously well established epidemics and therefore are considered global syndemics. Although an enormous progress towards understanding mechanisms of SARS-CoV-2 infection leading to COVID-19 has been made, there are still many areas of uncertainty to clarify. Systemic diseases are characterized by common links that allow integrating apparently unrelated disease manifestations. The authors launch the provocative hypothesis that serotonin is the putative mediator linking the lung, gut, cardiac, neurological, and other systemic manifestations that characterize severe COVID-19 in individuals with diabetes and obesity. In support of a role for serotonin in the mechanisms leading to disease severity are the similarities between acute and post-acute COVID-19 manifestations and neuroendocrine tumors presenting with carcinoid syndrome. Scientific discussion is set by highlighting the available clues that support this working hypothesis to trigger future research aimed at unravelling the molecular pathways underlying SARS-CoV-2 infection that are still far from being fully disclosed.

Modeling serotonin uptake in the lung shows endothelial transporters dominate over cleft permeation

A four-region (capillary plasma, endothelium, interstitial fluid, cell) multipath model was configured to describe the kinetics of blood-tissue exchange for small solutes in the lung, accounting for regional flow heterogeneity, permeation of cell membranes and through interendothelial clefts, and intracellular reactions. Serotonin uptake data from the Multiple indicator dilution "bolus sweep" experiments of Rickaby and coworkers (Rickaby DA, Linehan JH, Bronikowski TA, Dawson CA. J Appl Physiol 51: 405-414, 1981; Rickaby DA, Dawson CA, and Linehan JH. J Appl Physiol 56: 1170-1177, 1984) and Malcorps et al. (Malcorps CM, Dawson CA, Linehan JH, Bronikowski TA, Rickaby DA, Herman AG, Will JA. J Appl Physiol 57: 720-730, 1984) were analyzed to distinguish facilitated transport into the endothelial cells (EC) and the inhibition of tracer transport by nontracer serotonin in the bolus of injectate from the free uninhibited permeation through the clefts into the interstitial fluid space. The permeability-surface area products (PS) for serotonin via the inter-EC clefts were ~0.3 ml·g⁻¹·min⁻¹, low compared with the transporter-mediated maximum PS of 13 ml·g⁻¹·min⁻¹ (with Km = ~0.3 μM and Vmax = ~4 nmol·g⁻¹·min⁻¹). The estimates of serotonin PS values for EC transporters from their multiple data sets were similar and were influenced only modestly by accounting for the cleft permeability in parallel. The cleft PS estimates in these Ringer-perfused lungs are less than half of those for anesthetized dogs (Yipintsoi T. Circ Res 39: 523-531, 1976) with normal hematocrits, but are compatible with passive noncarrier-mediated transport observed later in the same laboratory (Dawson CA, Linehan JH, Rickaby DA, Bronikowski TA. Ann Biomed Eng 15: 217-227, 1987; Peeters FAM, Bronikowski TA, Dawson CA, Linehan JH, Bult H, Herman AG. J Appl Physiol 66: 2328-2337, 1989) The identification and quantitation of the cleft pathway conductance from these studies affirms the importance of the cleft permeation.

Analysis Methods and Recent Advances in Nonlinear Pharmacokinetics from In Vitro through In Loci to In Vivo

An attempt has been made to review the nonlinearities in the disposition in vitro, in situ, in loci and in vivo mainly from a theoretical point of view. Parallel Michaelis-Menten and linear (first-order) eliminations are often observed in the cellular uptake, metabolism and efflux of drugs. The well-stirred and parallel-tube models are mainly adopted under steady-state conditions in perfusion experiments, whereas distribution, tank-in-series and dispersion models are often used under nonsteady-state conditions with a pulse input. The analysis of the nonlinear local disposition in loci is reviewed from two points of view, namely an indirect method involving physiologically based pharmacokinetics (PBPK) and a direct (two or three samplings) method using live animals. The nonlinear global pharmacokinetics in vivo is reviewed with regard to absorption, elimination (metabolism and excretion) and distribution.

Hormone metabolism by the fish gill

The fish gill, like the mammalian lung, is ideally situated to process circulating biomolecules because: 1) the gill is the only organ perfused by the entire cardiac output, 2) the in-series positioning of branchial and systemic circulations permits "conditioning" of blood immediately before systemic perfusion and 3) gill microcirculation is extensive, providing substantial endothelial/pillar cell surface in contact with plasma. In addition, two or three distinct circulatory pathways within the gill may differentially affect plasma substrates, raising the possibility of vasoactive control of hormone titers. Hormones may be activated or inactivated by the gill, the latter involving extraction (uptake) from the plasma, metabolism by enzymes on the endothelial surface without uptake or uptake plus intracellular metabolism. Over 60% of angiotensin I (ANG I) is activated to angiotensin II (ANG II) in a single transit through the gill lamellae by pillar cell angiotensin-converting enzyme, whereas both ANG I and II are inactivated by the non-lamellar filamental vasculature. Gills may accumulate and store (uptake 1) or degrade (uptake 2) catecholamines via intracellular monoamine oxidase and catechol-O-methyl transferase enzymes, and they show substrate preference for norepinephrine over epinephrine. Similar processes may exist for serotonin. Atrial natriuretic peptides are efficiently (60-90%) extracted from plasma in vivo by C-type clearance receptors. Fifty percent of an endothelin-1 bolus is removed in a single transit through the gill circulation, arginine vasotocin extraction is modest and bradykinin is virtually unaffected. Arachidonic acid is completely extracted by the gill, whereas extraction of prostaglandins I2 and E2 is only 13 and 5%, respectively. Intense cytochrome P450 immunofluorescence in the pillar cells suggests that the gill vasculature may be an important site of detoxification and production of biologically active epoxides. Thus, gills appear to be potent and selective effectors of hormonal signals.

Efficient numerical methods for nonlinear-facilitated transport and exchange in a blood-tissue exchange unit

The analysis of experimental data obtained by the multiple-indicator method requires complex mathematical models for which capillary blood-tissue exchange (BTEX) units are the building blocks. This study presents a new, nonlinear, two-region, axially distributed, single capillary, BTEX model. A facilitated transporter model is used to describe mass transfer between plasma and intracellular spaces. To provide fast and accurate solutions, numerical techniques suited to nonlinear convection-dominated problems are implemented. These techniques are the random choice method, an explicit Euler-Lagrange scheme, and the MacCormack method with and without flux correction. The accuracy of the numerical techniques is demonstrated, and their efficiencies are compared. The random choice, Euler-Lagrange and plain MacCormack method are the best numerical techniques for BTEX modeling. However, the random choice and Euler-Lagrange methods are preferred over the MacCormack method because they allow for the derivation of a heuristic criterion that makes the numerical methods stable without degrading their efficiency. Numerical solutions are also used to illustrate some nonlinear behaviors of the model and to show how the new BTEX model can be used to estimate parameters from experimental data.

Evaluation of the functional integrity of endothelium in perfused lungs

Prostacyclin (PGI2) formation and the saturable uptake of 5-hydroxytryptamine (5HT) were studied as indices of endothelial integrity in isolated lungs. 5HT uptake was characterized by its kinetic parameters Km and Vmax. These were calculated from multiple indicator dilution data on the basis of an organ model of 5HT uptake. Perfused dog lung lobes were exposed to plasma activated with yeast (YAP) or zymosan (ZAP). YAP induced a transient elevation of Km. This increase probably reflects endothelial injury. Vmax remained unchanged, suggesting that the perfused endothelial surface remained stable. PGI2 biosynthesis was negligible in the control period, but started immediately after exposure to ZAP or YAP. It was proportional to the transient elevation of Km and to the pulmonary oedema. The data suggest that PGI2 might be a marker of severe endothelial distress.

Prostacyclin biosynthesis and reduced 5-HT uptake after complement-induced endothelial injury in the dog isolated lung

1. Pulmonary prostacyclin (PGI2) biosynthesis was evaluated in relation to endothelial integrity before and after complement activation in isolated plasma-perfused lung lobes of the dog. 2. The plasma was activated with zymosan (ZAP, n = 4), yeast cells (YAP, n = 4) or yeast with 3 microM indomethacin (Indo + YAP, n = 3). Immunoreactive 6-oxo-prostaglandin F1 alpha (i-6-oxo-PGF1 alpha) and thromboxane B2 (iTXB2) were measured to monitor PGI2 and TXA2 biosynthesis. 3. The kinetic parameters Km and Vmax of 5-hydroxytryptamine (5-HT) uptake were calculated on the basis of multiple indicator diffusion data to evaluate endothelial integrity. 4. YAP and ZAP induced a biphasic increase of the arterial perfusion pressure. The immediate pressure peak was partly mediated by TXA2 and the TXB2 was subsequently cleared by the lung. 5. The apparent Vmax of 5-HT uptake remained constant throughout the experiment. Thus, complement activation did not affect the number of endothelial 5-HT carrier sites available to the perfusate. 6. The apparent Km of 5-HT uptake was enhanced in 9 lungs exposed to activated plasma complement for 20 min. This decreased affinity for 5-HT probably reflects endothelial injury. It was transient as the apparent Km had returned to the baseline value after 60 min. 7. PGI2 clearance and biosynthesis were virtually absent in the control period. PGI2 formation increased drastically after infusion of ZAP or YAP and was proportional to the endothelial injury expressed as elevated Km or pulmonary oedema. Thus, PGI2 biosynthesis might be a marker of severe endothelial distress.

Kinetics of uptake and metabolism by endothelial cell from indicator dilution data

Certain substrates are rapidly taken up and/or metabolized by pulmonary endothelial cells in a saturable process. When such a substrate and a reference indicator are included in a bolus which is injected into the blood flowing into the lung, the extraction ratio, E(t), curves measured in the pulmonary venous outflow are asymmetric with respect to the reference indicator curve. If a sufficient quantity of substrate is included in the bolus, the extraction curves are concave upward. The shapes of the E(t) curves contain information regarding the chemical-physical processes which govern the fate of the substrate during its single passage through the lung. To interpret the shapes, computer simulations are used to illustrate separately the effects of the uptake of substrate into the cell, the returning flux of the substrate from the cell, the saturation phenomena of the extraction process, and the perfusion heterogeneity of the capillaries. Lastly, a simple analytical method for estimating the organ kinetic parameters of the extraction process is presented.