β-Adrenergic regulation of Na⁺uptake by larval zebrafishDanio rerioin acidic and ion-poor environments

Abstract

Kumai Y, Ward MA, Perry SF. ␤-Adrenergic regulation of Na ϩ uptake by larval zebrafish Danio rerio in acidic and ion-poor environments. Am J Physiol Regul Integr Comp Physiol 303: R1031-R1041, 2012. First published September 26, 2012 doi:10.1152/ajpregu.00307.2012.-The potential role of adrenergic systems in regulating Na ϩ uptake in zebrafish (Danio rerio) larvae was investigated. Treatment with isoproterenol (a generic ␤-adrenergic receptor agonist) stimulated Na ϩ uptake, whereas treatment with phenylephrine (an ␣1-adrenergic receptor agonist) as well as clonidine (an ␣2-adrenergic receptor agonist) significantly reduced Na ϩ uptake, suggesting opposing roles of ␣-and ␤-adrenergic receptors in Na ϩ uptake regulation. The increase in Na ϩ uptake associated with exposure to acidic water (pH ϭ 4.0) was attenuated in the presence of the nonselective ␤-receptor antagonist propranolol or the ␤1-receptor blocker atenolol; the ␤2-receptor antagonist ICI-118551 was without effect. The stimulation of Na ϩ uptake associated with ion-poor water (32-fold dilution of Ottawa tapwater) was unaffected by ␤-receptor blockade. Translational gene knockdown of ␤-receptors using antisense oligonucleotide morpholinos was used as a second method to assess the role of adrenergic systems in the regulation of Na ϩ uptake. Whereas ␤1-or ␤2B-receptor knockdown led to significant decreases in Na ϩ uptake during exposure to acidic water, only ␤2A-receptor morphants failed to increase Na ϩ uptake in response to ion-poor water. In support of the pharmacology and knockdown experiments that demonstrated an involvement of ␤-adrenergic systems in the control of Na ϩ uptake, we showed that the H ϩ -ATPase-rich (HR) cell, a subtype of ionocyte known to be a site of Na ϩ uptake, is innervated and appears to express ␤-adrenergic receptors (propranolol binding sites) at 4 days postfertilization. These data indicate an important role of adrenergic systems in regulating Na ϩ uptake in developing zebrafish.␤-adrenergic receptor; zebrafish; low pH; ion-poor water; knockdown; ionocyte; H ϩ -ATPase-rich cell IN FRESHWATER (FW) teleosts, ionic constancy of the body fluids in the face of continuous passive ion loss is achieved by the active absorption of ions from the environment. Ion uptake occurs via a specialized subset of epithelial cells, termed ionocytes or mitochondrion-rich cells (MRCs), distributed in the adult gill and on the skin (predominantly on the yolk sac) of larvae. The molecular mechanisms underlying the active uptake of the major ions Na ϩ , Cl Ϫ , and Ca 2ϩ have received considerable attention over the past 80 years. However, there are still several unresolved questions regarding the nature of ion uptake by FW fish (for recent reviews on FW fish osmoregulation, see Refs. 10,[14][15][16].In response to changes in the environment, FW teleosts modulate their ion transport mechanisms to maintain overall ion homeostasis. In particular, endocrine factors, including cortisol and prolactin, have received significant attention in the modulation o...

Zebrafish as an animal model to study ion homeostasis

“…This reverse or biphasic control of α- and β-adrenergic receptors in fish ion regulation was re-confirmed in zebrafish through pharmacological and loss-of-function approaches [69]. Pharmacological screening was performed to identify α- and β-adrenergic receptors with inhibitory and stimulatory roles, respectively, on Na + uptake in zebrafish embryos [69]. β receptor loss of function impaired Na + uptake only under acidic or ion poor environment, suggesting that Na + uptake in zebrafish is regulated via adrenergic receptors [69].…”

“…In additional to their role in vasoactive regulation, epinephrine/norepinephrine is known to directly affect gill ionic transport via inhibition by α-adrenergic receptors and stimulation by β receptors (for a detailed review, see [28]). This reverse or biphasic control of α- and β-adrenergic receptors in fish ion regulation was re-confirmed in zebrafish through pharmacological and loss-of-function approaches [69]. Pharmacological screening was performed to identify α- and β-adrenergic receptors with inhibitory and stimulatory roles, respectively, on Na + uptake in zebrafish embryos [69].…”

“…Pharmacological screening was performed to identify α- and β-adrenergic receptors with inhibitory and stimulatory roles, respectively, on Na + uptake in zebrafish embryos [69]. β receptor loss of function impaired Na + uptake only under acidic or ion poor environment, suggesting that Na + uptake in zebrafish is regulated via adrenergic receptors [69]. In support of this hypothesis, it was found that HR cells can be labeled with fluorescent propranolol (a β receptor antagonist) and zn-12 (a genetic neuronal marker) [69].…”

“…β receptor loss of function impaired Na + uptake only under acidic or ion poor environment, suggesting that Na + uptake in zebrafish is regulated via adrenergic receptors [69]. In support of this hypothesis, it was found that HR cells can be labeled with fluorescent propranolol (a β receptor antagonist) and zn-12 (a genetic neuronal marker) [69]. These findings further argue for β-adrenergic regulation of ion transport function in fish gill ionocytes but also raise several questions.…”

“…These findings further argue for β-adrenergic regulation of ion transport function in fish gill ionocytes but also raise several questions. Treatment of zebrafish embryos with exogenous epinephrine was reported to suppress Na + uptake, while norepinephrine treatment stimulated Na + uptake; this surprising result was attributed to different affinities to the receptors [69]. As in mammals, epinephrine and norepinephrine reach their target tissues (including the gills and skin) in teleosts via both neurotransmission from local sympathetic innervation and by systematic stimulation of chromaffin cells [28].…”