is also an energy substrate, or if there are interactions with parts of the cell indirectly
concerned with the transport process.
A very elegant, yet simple, demonstration of active sodium transport was made
by USSING and ZERAHN in 1951 using the frog skin. These experiments have provided both a standard method and experimental design for the analysis of such processes. It has been known for 100 years that frog skin in vitro displays an electrical
p. d. with the corium (inside) positive, relative to the solution on the outside (DU
BOIS REYMOND, 1848). In 1937 KROGHshowed that frogs can take up sodium and
chloride through their skin against a chemical concentration gradient, from solutions as dilute as 1O-5M. The experiment of USSING and ZERAHN allowed a precise
analysis of these processes to be made in this tissue. Pieces of the ventral skin of
frogs were clamped between two lucite chambers each containing Ringer solution
of identical composition. The electrical p . d. between the two sides was measured
and found to be 40 to 50 mV, inside (corium) positive. An external current was
applied from a battery in the opposite direction to the biological current, so as to
reduce the p . d . to zero, thus effectively short-circuiting the skin. Under these conditions no electrical or chemical concentration difference existed across the membrane. Sodium transfer in either direction was measured with the aid of 22Na(influx)
and 24Na(outflux) and it was found that the influx exceeded the outflux, clearly
demonstrating the active transfer of sodium. In addition it was found that when
the current used to short-circuit the skin (the short-circuit current, SCC), and the
net sodium transfer, were both expressed as millicoulombs they were nearly equal.
This rules out the active transfer of chloride, which in such a system carries the
current through the external circuit. These experiments not only afford a clear demonstration of active sodium transport, but the methods have in addition been used
for the analysis of the ion permeability of other membranes such as the intestine
and anuran urinary bladder. Vertebrate hormones can alter sodium transport in
such in vitro systems and have been widely used to examine the actions of such
hormones more closely.
When more than one substance is moved simultaneously, arguments as to the
prime mover are often of semantic as well as biological interest. Such co-transport,
linked to active sodium transport, influences the movement of many sugars and
amino acids across the plasma membrane as well as across epithelial membranes
(transepithelial). Whether or not sodium transfer itself is an example of such cotransport linked to the primary movement of a metabolite in the cell is not clear,
but affords an example of the difficulties in such a classification. Active sodium
transport would appear to take place transepithelially in the gut, amphibian skin
and urinary bladder, fish gills and in the tubular systems of the kidney and glands
like the 'salt glands' of birds, reptiles and chondrichthyeans. Active chloride transport may also occur across some of these membranes but while the evidence for
this is accumulating, its unequivocal elevation to such a status seems to be lagging
among the more conservative 'transport workers'. For the purposes of physiology
it is nevertheless important.
Transport of sodium across the plasma membrane itself is ultimately responsible for both rransepithelial transfer and the normal functioning of the cell. Active
extrusion of sodium, usually coupled to active accumulation of potassium, has been
demonstrated in many tissues including the erythrocyte, muscle and nerve cells.
12
concerned with the transport process.
A very elegant, yet simple, demonstration of active sodium transport was made
by USSING and ZERAHN in 1951 using the frog skin. These experiments have provided both a standard method and experimental design for the analysis of such processes. It has been known for 100 years that frog skin in vitro displays an electrical
p. d. with the corium (inside) positive, relative to the solution on the outside (DU
BOIS REYMOND, 1848). In 1937 KROGHshowed that frogs can take up sodium and
chloride through their skin against a chemical concentration gradient, from solutions as dilute as 1O-5M. The experiment of USSING and ZERAHN allowed a precise
analysis of these processes to be made in this tissue. Pieces of the ventral skin of
frogs were clamped between two lucite chambers each containing Ringer solution
of identical composition. The electrical p . d. between the two sides was measured
and found to be 40 to 50 mV, inside (corium) positive. An external current was
applied from a battery in the opposite direction to the biological current, so as to
reduce the p . d . to zero, thus effectively short-circuiting the skin. Under these conditions no electrical or chemical concentration difference existed across the membrane. Sodium transfer in either direction was measured with the aid of 22Na(influx)
and 24Na(outflux) and it was found that the influx exceeded the outflux, clearly
demonstrating the active transfer of sodium. In addition it was found that when
the current used to short-circuit the skin (the short-circuit current, SCC), and the
net sodium transfer, were both expressed as millicoulombs they were nearly equal.
This rules out the active transfer of chloride, which in such a system carries the
current through the external circuit. These experiments not only afford a clear demonstration of active sodium transport, but the methods have in addition been used
for the analysis of the ion permeability of other membranes such as the intestine
and anuran urinary bladder. Vertebrate hormones can alter sodium transport in
such in vitro systems and have been widely used to examine the actions of such
hormones more closely.
When more than one substance is moved simultaneously, arguments as to the
prime mover are often of semantic as well as biological interest. Such co-transport,
linked to active sodium transport, influences the movement of many sugars and
amino acids across the plasma membrane as well as across epithelial membranes
(transepithelial). Whether or not sodium transfer itself is an example of such cotransport linked to the primary movement of a metabolite in the cell is not clear,
but affords an example of the difficulties in such a classification. Active sodium
transport would appear to take place transepithelially in the gut, amphibian skin
and urinary bladder, fish gills and in the tubular systems of the kidney and glands
like the 'salt glands' of birds, reptiles and chondrichthyeans. Active chloride transport may also occur across some of these membranes but while the evidence for
this is accumulating, its unequivocal elevation to such a status seems to be lagging
among the more conservative 'transport workers'. For the purposes of physiology
it is nevertheless important.
Transport of sodium across the plasma membrane itself is ultimately responsible for both rransepithelial transfer and the normal functioning of the cell. Active
extrusion of sodium, usually coupled to active accumulation of potassium, has been
demonstrated in many tissues including the erythrocyte, muscle and nerve cells.
12