Jinae N. Roa and Martin Tresguerres

Na+/K+-ATPase (NKA)- and vacuolar H+-ATPase (VHA)-rich cells in shark gills respectively secrete excess acid and base to seawater to maintain blood acid-base homeostasis. Both cell types are rich in mitochondria indicating high ATP demand; however, their metabolic fuel is unknown. Here we report that NKA- and VHA-rich cells have large glycogen stores. Glycogen abundance in NKA-rich cells was lower in starved sharks compared to 24h post-fed sharks, reflecting higher energy demand for acid secretion during normal activity and glycogen replenishment during the post-feeding period. Conversely, glycogen abundance in VHA-rich cells was high in starved sharks and it became depleted post-feeding. Furthermore, inactive cells with cytoplasmic VHA had large glycogen stores and active cells with basolateral VHA had depleted glycogen stores. These results indicate glycogen is a main energy source in both NKA- and VHA-rich cells, and point to differential energy use associated with net acid and net base secretion, respectively.

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