Abstract
Marine teleosts have extracellular fluids less concentrated than their environment, resulting in continual water loss, which is compensated for by drinking, with intestinal water absorption driven by NaCl uptake. Absorption of Cl(-) occurs in part by apical Cl(-)/HCO(3)(-) exchange, with HCO(3)(-) provided by transepithelial transport and/or by carbonic anhydrase-mediated hydration of endogenous epithelial CO(2). Hydration of CO(2) also liberates H(+), which is transported across the basolateral membrane. In this study, gulf toadfish (Opsanus beta) were acclimated to 9, 35 and 50 ppt. Intestinal HCO(3)(-) secretion, water and salt absorption, and the ensuing effects on acid-base balance were examined. Rectal fluid excretion greatly increased with increasing salinity from 0.17+/-0.05 ml kg(-1) h(-1) in 9 ppt to 0.70+/-0.19 ml kg(-1) h(-1) in 35 ppt and 1.46+/-0.22 ml kg(-1) h(-1) in 50 ppt. Rectal fluid composition and excretion rates allowed for estimation of drinking rates, which increased with salinity from 1.38+/-0.30 to 2.60+/-0.92 and 3.82+/-0.58 ml kg(-1) h(-1) in 9, 35 and 50 ppt, respectively. By contrast, the fraction of imbibed water absorbed decreased from 85.9+/-3.8% in 9 ppt to 68.8+/-3.2% in 35 ppt and 61.4+/-1.0% in 50 ppt. Despite large changes in rectal base excretion from 9.3+/-2.7 to 68.2+/-20.4 and 193.2+/-64.9 mumol kg(-1) h(-1) in 9, 35 and 50 ppt, respectively, acute or prolonged exposure to altered salinities was associated with only modest acid-base balance disturbances. Extra-intestinal, presumably branchial, net acid excretion increased with salinity (62.0+/-21.0, 229.7+/-38.5 and 403.1+/-32.9 mumol kg(-1) h(-1) at 9, 35 and 50 ppt, respectively), demonstrating a compensatory response to altered intestinal base secretion associated with osmoregulatory demand.
Highlights
Marine teleosts maintain the osmotic pressure of their extracellular fluids at ~300–350 mosmol l–1, approximately one-third that of seawater (~1000 mosmol l–1) (Shehadeh and Gordon, 1969)
The contribution of HCO3– derived from each of these two sources varies among species (Grosell, 2006), but each source accounts for approximately 50% of intestinal HCO3– secretion in the gulf toadfish, Opsanus beta (Grosell and Genz, 2006)
The total fluid volume excreted from the rectum during the 48 h of experimentation was 0.17±0.05 ml kg–1 h–1 in 9 ppt, 0.70±0.19 ml kg–1 h–1 in 35 ppt and 1.46±0.22 ml kg–1 h–1 in 50 ppt
Summary
Marine teleosts maintain the osmotic pressure of their extracellular fluids at ~300–350 mosmol l–1, approximately one-third that of seawater (~1000 mosmol l–1) (Shehadeh and Gordon, 1969) This results in a constant diffusive salt gain from the surrounding environment, as well as osmotic water loss, for which fish compensate by drinking seawater. In O. beta, the H+ liberated by intracellular carbonic anhydrase-mediated CO2 hydration is transported across the basolateral membrane into the extracellular fluids (Grosell et al, 2001) in exchange for Na+ (Grosell and Genz, 2006) and results in net acid absorption (Grosell and Taylor, 2007) proportional to the HCO3– secretion driving intestinal fluid absorption
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