Assessing the ability of zebrafish scales to contribute to the short-term homeostatic regulation of [Ca2+] in the extracellular fluid during calcemic challenges

The elasmoid scales of fish represent a significant internal reservoir of calcium ions (Ca2+), but little is known about the contribution of these scales to the short-term regulation of Ca2+ homeostasis in the extracellular fluid (ECF). This gap in our knowledge is partly due to the technical challenges involved in measuring small Ca2+ fluxes around the scales of live fish in real time. Here, we describe a technique for exfoliating, mounting, and culturing intact living zebrafish Danio rerio scales, then subjecting them to examination using an extracellular, non-invasive, surface-scanning ion-selective electrode technique (SIET). In a Ca2+-sensitive configuration, the SIET can resolve Ca2+ flux values in the low-to-sub picomole/square centimeter/second range, with a spatial resolution of approximately 5 μm. We quantified the Ca2+ fluxes into and out of scales under different extracellular calcemic challenges set to mimic a variety of Ca2+ concentrations in the ECF and showed that the results were similar to those previously reported from isolated mouse metatarsal bone. Our new data extend our current understanding of the role played by fish scales in the short-term homeostatic regulation of Ca2+ concentration in the ECF. They also support the suggestion that scales might provide an inexpensive and complementary model for studying the fundamentals of bone-mediated homeostatic Ca2+ regulation and the diseases that result from its dysregulation