Channels, pumps, and exchangers in the gill and kidney of freshwater fishes: Their role in ionic and acid‐base regulation

165

SummaryTransepithelial transport is one of the major processes involved in the mechanism of homeostasis of body fluids in vertebrates including fish. The current models of ion regulation in fish gill ionocytes have been proposed mainly based on studies in traditional model species like salmon, trout, tilapia, eel and killifish, but the mechanisms are still being debated due to the lack of convincing molecular physiological evidence. Taking advantage of plentiful genetic databases for zebrafish, we studied the molecular/cellular mechanisms of ion regulation in fish skin/gills. In our recently proposed model, there are at least three subtypes of ionocytes in zebrafish skin/gills: Na + -K + -ATPase-rich (NaR), Na + -Cl -cotransporter (NCC) and H + -ATPase-rich (HR) cells. Specific isoforms of transporters and enzymes have been identified as being expressed by these ionocytes: zECaC, zPMCA2 and zNCX1b by NaR cells; zNCC gill form by NCC cells; and zH + -ATPase, zNHE3b, zCA2-like a and zCA15a by HR cells. Serial molecular physiological experiments demonstrated the distinct roles of these ionocytes in the transport of various ions: HR, NaR and NCC cells are respectively responsible for acid secretion/Na + uptake, Ca 2+ uptake and Cl -uptake. The expression, regulation and function of transporters in HR and NaR cells are much better understood than those in NCC cells. The basolateral transport pathways in HR and NCC cells are still unclear, and the driving forces for the operations of apical NHE and NCC are another unresolved issue. Studies on zebrafish skin/gill ionocytes are providing new insights into fish ion-regulatory mechanisms, but the zebrafish model cannot simply be applied to other species because of species differences and a lack of sufficient molecular physiological evidence in other species.

Section: H + -Atpase Rich (Hr) Cellsmentioning

confidence: 99%

Ion uptake and acid secretion in zebrafish (Danio rerio)

Hwang

2009

165

“…A plethora of evidence has clearly established the gills of teleost fish as the predominant site of acid-base relevant ion transfer, the transfer of H + and/or HCO 3 -, for the maintenance of systemic pH (reviewed by Perry and Laurent, 1990;Goss et al, 1992Goss et al, , 1995Perry, 1997;Claiborne et al, 2002;Perry et al, 2003;Evans et al, 2005). Current models (e.g.…”

Section: Introductionmentioning

confidence: 99%

“…Current models (e.g. Claiborne et al, 2002;Perry et al, 2003;Evans et al, 2005) postulate that CO 2 entering the gill epithelium is hydrated to HCO 3 -and H + in the presence of the enzyme carbonic anhydrase (CA). Regulation of systemic pH is then achieved by adjusting the rates of acid and/or base excretion, which in turn are linked to ion uptake through the involvement of a Na -ATPase) and a Cl -/HCO 3 -exchange mechanism.…”

Section: Introductionmentioning

confidence: 99%

The role of branchial carbonic anhydrase in acid-base regulation in rainbow trout (Oncorhynchus mykiss)

Georgalis

Perry

Gilmour

2006

Self Cite

SUMMARY The objective of the present study was to examine the branchial distribution of the recently identified rainbow trout cytoplasmic carbonic anhydrase isoform (tCAc) and to investigate its role in the regulation of acid-base disturbances in rainbow trout (Oncorhynchus mykiss). In situ hybridization using an oligonucleotide probe specific to tCAc revealed tCAc mRNA expression in both pavement cells and mitochondria-rich cells (chloride cells). Similarly, using a homologous polyclonal antibody,tCAc immunoreactivity was localized to pavement cells and mitochondria-rich cells in the interlamellar region and along the lamellae of the gills. Exposure of rainbow trout to hypercarbia (∼0.8% CO2) for 24 h resulted in significant increases in tCAc mRNA expression (∼20-fold;quantified by real-time PCR) and protein levels (∼1.3-fold; quantified by western analysis) but not enzyme activity (assessed on crude gill homogenates using the delta-pH CA assay). Inhibition of branchial CA activity in vivo using acetazolamide reduced branchial net acid excretion significantly by 20%. This effect was enhanced to a 36% reduction in branchial net acid excretion by subjecting the trout to hypercarbia (∼0.8%CO2) for 10 h prior to acetazolamide injection, an exposure that significantly increased branchial net acid excretion. The results of the present study support the widely held premise that branchial intracellular CA activity (tCAc) plays a key role in regulating acid-base balance in freshwater teleost fish.

“…An electrochemical gradient for Ca 2+ , established by basolateral Ca 2+ -ATPase and Na + /K + -ATPase activity, and further facilitated and maintained by an apical, outwardly directed H + pump provides the necessary gradient to allow for Ca 2+ uptake under these conditions (Perry et al, 2003). Hydrogen ions supplied to this pump are mostly from hydration of CO 2 .…”

Section: Introductionmentioning

confidence: 99%

Characterization of mechanisms for Ca2+ and HCO3–/CO32– acquisition for shell formation in embryos of the freshwater common pond snail Lymnaea stagnalis

Ebanks

O’Donnell

Grosell

2010

SUMMARY The freshwater common pond snail Lymnaea stagnalis produces embryos that complete direct development, hatching as shell-bearing individuals within 10 days despite relatively low ambient calcium and carbonate availability. This development is impaired by removal of ambient total calcium but not by removal of bicarbonate and/or carbonate. In this study we utilized pharmacological agents to target possible acquisition pathways for both Ca2+ and accumulation of carbonate in post-metamorphic, shell-laying embryos. Using whole egg mass flux measurements and ion-specific microelectrode analytical techniques, we have demonstrated that carbonic anhydrase-catalyzed hydration of CO2 is central in the acquisition of both shell-forming ions because it provides the hydrogen ions for an electrogenic vacuolar-type H+-ATPase that fuels the uptake of Ca2+via voltage-dependent Ca2+ channels and possibly an electrogenic Ca2+/1H+ exchanger. Additionally, CO2 hydration provides an endogenous source of HCO –3. Thus, hydration of endogenous CO2 forms HCO –3 for calcification while hydrogen ions are excreted, contributing to continued Ca2+ uptake, as well as creating favorable alkaline internal conditions for calcification. The connections between Ca2+ and HCO –3 acquisition mechanisms that we describe here provide new insight into this efficient, embryonic calcification in freshwater.