Haug, T. M. | bioRxiv Science

bioRxiv2019

Gonadotropes in medaka grow long extensions with varicosity-like swellings, projecting towards each other and blood vessels.

Accumulating evidence in the scientific literature indicates that some pituitary cell types are organized in complex networks. Previous observations have indicated that this may also be the case in medaka (Oryzias latipes), where long cellular extensions with varicosity-like swellings are formed by luteinizing hormone (Lh)-producing gonadotropes expressing green fluorescent protein. In this study, immunofluorescence of intact pituitaries reveal that Lh beta polypeptides are mainly located in the varicosity-like swellings and at the extremity of the extensions. Some extensions approach nearby Lh-producing cells, and other extensions are in close contact with blood vessels. To investigate whether these extensions may contribute to network formation, we followed their development using confocal and fluorescent microscopy on primary cultures. During the first two days in culture, the extensions initiated the formation of homotypic cellular networks and clustering. The extensions were classified as either major or minor. Major extensions were several cell diameters long, dependent on microtubules, and displaying varicosity-like swellings at regular intervals. Minor extensions typically protruded from the major, were significantly shorter and thinner, and dependent on actin. The swellings were dependent on both microtubules and actin. Flash photolysis of caged Ca2+ showed that the signal was propagated along the major extensions, intensifying in each swelling, indicating a continuous structure. However, the Ca2+ signal did not transfer to the next cell in the network, but was transferred between cells merged at their somas. In summary, Lh-producing gonadotropes in medaka display a complex cellular structure of extensions, possibly linked to communication with blood vessels and/or other gonadotrope cells.

cell biology↗

bioRxiv2018

BK channels have opposite effects on sodium versus calcium-mediated action potentials in endocrine pituitary cells.

Pituitary endocrine cells fire action potentials (APs) to regulate their cytosolic Ca2+ concentration and hormone secretion rate. Depending on animal species, cell type, and biological conditions, pituitary APs are generated either by TTX-sensitive Na+ currents (INa), high-voltage activated Ca2+ currents (ICa), or by a combination of the two. Previous computational models of pituitary cells have mainly been based on data from rats, where INa is largely inactivated at the resting potential, and spontaneous APs are exclusively mediated by ICa. As a part of the previous modeling studies, a paradoxical role was identified for the big conductance K+ current (IBK), which was found to prolong the duration of ICa-mediated APs, and sometimes give rise to pseudo-plateau bursts, contrary to what one would expect from a hyperpolarizing current. Unlike in rats, spontaneous INa-mediated APs are consistently seen in pituitary cells of several other animal species, including several species of fish. In the current work we develop the, to our knowledge, first computational model of a pituitary cell that fires INa-mediated APs. Although we constrain the model to experimental data from gonadotrope cells in the teleost fish medaka (Oryzias latipes), it may likely provide insights also into other pituitary cell types that fire INa-mediated APs. In the current work, we use the model to explore how the effect of IBK depends on the AP generating mechanisms of pituitary cells. We do this by comparing simulations on the medaka gonadotrope model (two versions thereof) with simulations on a previously developed model of a rat pituitary cell. Interestingly, we find that IBK has the opposite effect on APs in the two models, i.e. it reduces the duration of already fast INa-mediated APs in the medaka model, and prolongs the duration of already slow ICa-mediated APs in the rat model.\n\nAuthor summaryExcitable cells elicit electrical pulses called action potentials (APs), which are generated and shaped by a combination of ion channels in the cell membrane. While neurons use APs for interneuronal communication and heart cells use them to generate heart-beats, pituitary cells use APs to regulate their cytosolic Ca2+ concentration, which in turn controls their hormone secretion rate. The amount of Ca2+ that enters the pituitary cell during an AP depends strongly on how long it lasts, and it is therefore important to understand the mechanisms that control this. Depending on animal species and biological conditions, pituitary APs may be initiated either by Ca2+ channels or Na+ channels. Here, we explore the differences between the two scenarios by comparing simulations on two different computer models: (i) a previously developed model which fires Na+-based APs, adapted to data from pituitary cells in rats, and (ii) a novel model that fires Ca2+-based APs, adapted to data from pituitary cells in the fish medaka. Interestingly, we find that the role of big conductance K+ (BK) channels, which are known to affect the duration of the AP, are opposite in the two models, i.e., they act to prolong Ca2+-based APs while they act to shorten Na+-based APs.

cell biology↗