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Extrarenal Mechanisms in Hydromineral and Acid‐Base Regulation in Aquatic Vertebrates

Leonard B. Kirschner

10.1002/cphy.cp130109

Source: Supplement 30: Handbook of Physiology, Comparative Physiology

Originally published: 1997

Published online: January 2011

Full Article on Wiley Online Library



Abstract

The sections in this article are:

1 Physical Background
1.1 Dissipative Flows
2 Hyperosmotic Regulation
2.1 Reducing Dissipative Flows
2.2 Compensatory Flows: Water
2.3 Compensatory Flows: Active Ion Absorption
2.4 The Frog Skin Model: Symmetric Solutions and Voltage Clamp
2.5 A Revised Model: Open‐Circuit and Dilute Solutions
2.6 Ionic Regulation in Other Aquatic Vertebrates
2.7 Apical Membrane Behavior: Fluxes and Electrical
2.8 Apical Membrane Mechanisms and the Frog Skin Model
2.9 The Energetic Cost of Osmotic Regulation in Freshwater Vertebrates
3 Acid‐Base Regulation and Epithelial Ion Transfer
4 Hypoosmotic‐Hypoionic Regulation
4.1 Reducing Dissipative Flows
4.2 Compensatory Flows: Water
4.3 Compensatory Flows: Ions
4.4 Model for Salt Extrusion by Gills
4.5 Remaining Questions about Hyporegulator Gill Function
4.6 Reptilian Salt Gland
5 Acid‐Base Balance in Marine Fish
6 Isosmotic, Hypoionic Regulation
6.1 Diffusion Gradients
6.2 Evasive Mechanisms: Urea/TMAO
6.3 Compensatory Mechanisms: Urea/TMAO
6.4 Evasive Mechanisms: Water and Electrolytes
6.5 Compensatory Flows: Ions
6.6 Intracellular Osmotic Regulation
6.7 Crab‐Eating Frog and Coelacanth
6.8 The Energetic Cost of Osmotic Regulation in SW
7 Marine Mammals
8 Summary
8.1 Osmotic Regulation: FW
8.2 Osmotic Regulation: SW
8.3 Acid‐Base Balance
Figure 1. Figure 1.

KU model for NaCl absorption across frog skin to about 1980. Skin is shown bathed on both sides by Ringer's solution since that was how most experiments were conducted. Dashed lines, diffusion; solid lines, active transport. Channels are shown as cylinders, pumps as circles. Chloride movement is depicted as diffusive through a paracellular pathway. However, it was recognized at about this time that the Cl− pathway was through the mr cells 207,345. The polarity of Vap was shown to be cell‐negative to outside solution 136,250, though originally thought to be positive (cf. Fig. 2 and text).
Figure 2. Figure 2.

Equivalent circuit for the original KU model. Vap is cell‐positive to outside bathing solution. The pathway for diffusion of Cl− (and other ions) is shown as a shunt in parallel with the Na+ transport pathway. The force acting on Na+ at the apical membrane is an inward‐oriented electrochemical gradient (ΔμNa). At the basolateral membrane, it is the electrochemical potential of the Na+‐K+ pump (Vpump).
Figure 3. Figure 3.

NaCl absorption by frog skin from a dilute external solution (2 mM). The upper is a principal cell with an apical Na+ channel and basolateral Na+‐K+ pump. The lower is an mr cell with carbonic anhydrase (C.A.), an apical proton pump, and a Cl−‐HCO3− exchanger. The apical membrane also contains the concentration‐dependent Cl− channel shown closed (constricted) in dilute solution. These cells also transport a small fraction of the total Na+ absorbed. The mechanism appears to be the same as in the principal cells.
Figure 4. Figure 4.

Data generated by the mathematical model based on Figure 3. Open circles, Na+ influx (Jin); solid circles, apical membrane potential (Vap), both over the “physiological” range of Na+ concentrations. Inset shows Vap over a much extended concentration range.

compare with data in ref. 251
Figure 5. Figure 5.

Model accounting for Cl− absorption by frog skin from Na+‐free solution. The proton pump is shown on the basolateral membrane.
Figure 6. Figure 6.

Correlation between Jin(Na) and Jeff(NH4) in rainbow trout (squares; 172) and goldfish (circles; 219). Solid line is a regression of data from another experiment on rainbow trout 237.
Figure 7. Figure 7.

Effect of environmental hyperoxia on blood gases and acid‐base status in rainbow trout 141. Fish were initially exposed to air bubbled through their aquaria. From 0.5 to 72 h the gas was oxygen, and at 72 h it was changed back to air. Upper panel shows ambient (P1O2) and arterial (P4O2) oxygen concentrations. Middle and lower panels show arterial Pco2, pH, and [HCO3−]. Compensation for initial acidosis was complete by 72 hr. Subsequent alkalosis was almost completely reversed in 24 h.

Reprinted from reference 140) by permission of Elsevier Science Publishers BV, Academic Publishing Division
Figure 8. Figure 8.

Compensation for hypercapnic acidosis in rainbow trout. The aeration gas was 1% CO2 in air. Ambient [Na+] was either 40 μM (open circles) or 3 m M (closed circles). The line is drawn from data in reference 267.
Figure 9. Figure 9.

Relation between net SID fluxes (ordinate) and acid equivalent fluxes (abscissa) in rainbow trout exposed sequentially to normoxia, hyperoxia, and normoxia as in Figure 7.

Reprinted from reference 361 by permission of Elsevier Science Publishers BV, Academic Publishing Division
Figure 10. Figure 10.

Fluid absorption from gut lumen to blood in a marine teleost. The apical cotransporter permits entry of Na+ (diffusive), K+ (active), and 2Cl− (active) into the cell. Sodium is pumped actively into the paracellular channel, with K+ and Cl− following passively. Entry of these ions raises paracellular osmotic concentration above luminal and permits osmotic water flow across the intracellular junction into the channel. Since there is little impediment at the basal end, convective flow will carry the fluid (water and ions) into the ECF. Some water may also enter the channel from the cells as shown. Half of the Na+ absorbed flows through the cation‐selective paracellular pathway, the other half through the cells (cotransporter plus Na+ −K+ pump).
Figure 11. Figure 11.

NaCl extrusion from blood to SW or salt gland lumen in marine fish. The NaK2Cl cotransporter is shown on the basolateral membrane in this model. Chloride, but not Na+, channels are shown in the apical membrane. With the [Cl−]cell shown, there is a small (2–5 mV) gradient favoring Cl− diffusion from cell to external medium. Sodium transfer is shown as passive through the paracellular pathway. TEP is characteristic of the elasmobranch rectal gland. In teleost epithelia, it would be 20–30 mV. Putative apical K+ channels, suggested in the text, are not shown.
Figure 12. Figure 12.

Excretion of solutes by the dolphin after feeding. A female Tursiops truncatus (10 kg) was fed 4.7 kg of mackerel and 2 h later give 1 l SW by stomach tube (shown as hour 0). The first urine sample was taken 1 h after the SW, but the urine was forming throughout this period and the value is placed midway between 0 and 1 h. Urine was collected periodically and assayed for volume (○), osmolality (X), and urea (•) concentrations. Values obtained for [Na+] and [Cl−] are not shown. The lower dashed line represents mean urine volume during fasting, and the upper dashed line is the average osmolality during fasting.

Data are from reference 284, and the figure is reprinted from reference 184 by permission of John Wiley & Sons, Inc


Figure 1.

KU model for NaCl absorption across frog skin to about 1980. Skin is shown bathed on both sides by Ringer's solution since that was how most experiments were conducted. Dashed lines, diffusion; solid lines, active transport. Channels are shown as cylinders, pumps as circles. Chloride movement is depicted as diffusive through a paracellular pathway. However, it was recognized at about this time that the Cl− pathway was through the mr cells 207,345. The polarity of Vap was shown to be cell‐negative to outside solution 136,250, though originally thought to be positive (cf. Fig. 2 and text).



Figure 2.

Equivalent circuit for the original KU model. Vap is cell‐positive to outside bathing solution. The pathway for diffusion of Cl− (and other ions) is shown as a shunt in parallel with the Na+ transport pathway. The force acting on Na+ at the apical membrane is an inward‐oriented electrochemical gradient (ΔμNa). At the basolateral membrane, it is the electrochemical potential of the Na+‐K+ pump (Vpump).



Figure 3.

NaCl absorption by frog skin from a dilute external solution (2 mM). The upper is a principal cell with an apical Na+ channel and basolateral Na+‐K+ pump. The lower is an mr cell with carbonic anhydrase (C.A.), an apical proton pump, and a Cl−‐HCO3− exchanger. The apical membrane also contains the concentration‐dependent Cl− channel shown closed (constricted) in dilute solution. These cells also transport a small fraction of the total Na+ absorbed. The mechanism appears to be the same as in the principal cells.



Figure 4.

Data generated by the mathematical model based on Figure 3. Open circles, Na+ influx (Jin); solid circles, apical membrane potential (Vap), both over the “physiological” range of Na+ concentrations. Inset shows Vap over a much extended concentration range.

compare with data in ref. 251


Figure 5.

Model accounting for Cl− absorption by frog skin from Na+‐free solution. The proton pump is shown on the basolateral membrane.



Figure 6.

Correlation between Jin(Na) and Jeff(NH4) in rainbow trout (squares; 172) and goldfish (circles; 219). Solid line is a regression of data from another experiment on rainbow trout 237.



Figure 7.

Effect of environmental hyperoxia on blood gases and acid‐base status in rainbow trout 141. Fish were initially exposed to air bubbled through their aquaria. From 0.5 to 72 h the gas was oxygen, and at 72 h it was changed back to air. Upper panel shows ambient (P1O2) and arterial (P4O2) oxygen concentrations. Middle and lower panels show arterial Pco2, pH, and [HCO3−]. Compensation for initial acidosis was complete by 72 hr. Subsequent alkalosis was almost completely reversed in 24 h.

Reprinted from reference 140) by permission of Elsevier Science Publishers BV, Academic Publishing Division


Figure 8.

Compensation for hypercapnic acidosis in rainbow trout. The aeration gas was 1% CO2 in air. Ambient [Na+] was either 40 μM (open circles) or 3 m M (closed circles). The line is drawn from data in reference 267.



Figure 9.

Relation between net SID fluxes (ordinate) and acid equivalent fluxes (abscissa) in rainbow trout exposed sequentially to normoxia, hyperoxia, and normoxia as in Figure 7.

Reprinted from reference 361 by permission of Elsevier Science Publishers BV, Academic Publishing Division


Figure 10.

Fluid absorption from gut lumen to blood in a marine teleost. The apical cotransporter permits entry of Na+ (diffusive), K+ (active), and 2Cl− (active) into the cell. Sodium is pumped actively into the paracellular channel, with K+ and Cl− following passively. Entry of these ions raises paracellular osmotic concentration above luminal and permits osmotic water flow across the intracellular junction into the channel. Since there is little impediment at the basal end, convective flow will carry the fluid (water and ions) into the ECF. Some water may also enter the channel from the cells as shown. Half of the Na+ absorbed flows through the cation‐selective paracellular pathway, the other half through the cells (cotransporter plus Na+ −K+ pump).



Figure 11.

NaCl extrusion from blood to SW or salt gland lumen in marine fish. The NaK2Cl cotransporter is shown on the basolateral membrane in this model. Chloride, but not Na+, channels are shown in the apical membrane. With the [Cl−]cell shown, there is a small (2–5 mV) gradient favoring Cl− diffusion from cell to external medium. Sodium transfer is shown as passive through the paracellular pathway. TEP is characteristic of the elasmobranch rectal gland. In teleost epithelia, it would be 20–30 mV. Putative apical K+ channels, suggested in the text, are not shown.



Figure 12.

Excretion of solutes by the dolphin after feeding. A female Tursiops truncatus (10 kg) was fed 4.7 kg of mackerel and 2 h later give 1 l SW by stomach tube (shown as hour 0). The first urine sample was taken 1 h after the SW, but the urine was forming throughout this period and the value is placed midway between 0 and 1 h. Urine was collected periodically and assayed for volume (○), osmolality (X), and urea (•) concentrations. Values obtained for [Na+] and [Cl−] are not shown. The lower dashed line represents mean urine volume during fasting, and the upper dashed line is the average osmolality during fasting.

Data are from reference 284, and the figure is reprinted from reference 184 by permission of John Wiley & Sons, Inc
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Article Title: Extrarenal Mechanisms in Hydromineral and Acid‐Base Regulation in Aquatic Vertebrates
 

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Leonard B. Kirschner. Extrarenal Mechanisms in Hydromineral and Acid‐Base Regulation in Aquatic Vertebrates. Compr Physiol 2011, Supplement 30: Handbook of Physiology, Comparative Physiology: 577-622. First published in print 1997. doi: 10.1002/cphy.cp130109