Localization of Na+/K+-ATPase in silkworm brain: A possible mechanism for protection of Na+/K+-ATPase from Ca2+

In mammalian blood, the Na+ concentration is higher than the K+ concentration, whereas in hemolymph of lepidopterous insects, the K+ concentration is higher than the Na+ concentration. Na+/K+-ATPase regulates Na+ and K+ concentrations in mammalian blood. Therefore, the absence of Na+/K+-ATPase in lepidopterous insects might be expected. However, we have observed that Na+/K+-ATPase is abundant in nerve tissues of larvae of silkworm, a lepidopterous insect. Furthermore, we found that silkworm Na+/K+-ATPase was completely inhibited by 3 mM Ca2+in vitro (Homareda, 2010), although the Ca2+ concentration is very high (30–50 mM) in the hemolymph of silkworm larvae. To investigate the reason why silkworm Na+/K+-ATPase is not inhibited by Ca2+in vivo, we observed the localization of Na+/K+-ATPase in nerve tissues using immunohistochemical techniques. Na+/K+-ATPase was distributed in the cortex and neuropile but not in the perineurium of the silkworm brain, while plasma membrane Ca2+-ATPase appeared to distribute in the perineurium as well as in the cortex and neuropile. These results support a possibility that neuronal Na+/K+-ATPase is protected from a high Ca2+ concentration by the blood–brain barrier consisting of perineurial glial cells with plasma membrane Ca2+-ATPase.