corticosteroid's action. Synthetic analogues of corticosteroid hormones, such as
fluorocortisol and prednisolone, also increase sodium transport across the bladder.
In contrast to the neurohypophysial hormones, whose effects can be seen in less
than a minute, the increase in sodium transfer that follows exposure of the serosal
side of the bladder to aldosterone has a latent period of about 60 to 90 min. This
could be due to a delayed rate of access to receptor sites, or to a prolonged series
of 'secondary reactions.' As the delay is independent of the hormone's concentration, CRABBE (1963) suggested the latter explanation.
The receptor sites for aldosterone in the toad bladder appear to be situated intracellularly, probably in the nucleus. EDELMAN, BOGOROCH, and PORTER (1963)
demonstrated by radioautography, that 3H -aldosterone was accumulated preferentially to progesterone in the nucleus. The nuclei of these cells have since been
separated by centrifugation procedures, and have been shown directly to bind aldosterone in preference to other steroids. The relative abilities of different steroids
to displace bound aldosterone were found to correspond roughly with their activities in increasing sodium transport (AUSIELLO and SHARP, 1968). These results are
interpreted with some caution, as it is difficult to be sure that the binding sites are
those specifically related to the physiological response. FANESTIL and EDELMAN
(1966) found comparable binding sites for aldosterone in the nuclei of rat kidneys,
and concluded that such sites are probably proteinaceous, as only proteolytic enzymes could increase the rate of release of aldosterone from such loci.
In an effort to unravel the nature of the secondary series of reactions involved
in the effect of aldosterone, EDELMAN et at. (1963) and CRABBE and DE WEER(1964)
tested the effects of actinomycin D and puromycin on the bladder. Actinomycin
D prevents transcription of nuclear DNA to messenger RNA, while puromycin
inhibits the subsequent ribosomal protein synthesis. Both these agents inhibited
the effects of aldosterone on sodium transport. Aldosterone was also shown to increase the rate of incorporation of 3H-uridine into RNA (PORTER, BOGOROCH,
and EDELMAN, 1964) in the toad bladder. The increase in RNA formation is independent of changes in intracellular sodium levels, so that the action of the hormone
is probably direct and not mediated, at this stage, by ionic changes (DE WEERand
CRABBE, 1968). The results all strongly suggest that aldosterone changes the activity
of certain genes according to the theory of JACOB and MONOD (see MONOD, 1966).
Thus, aldosterone may interact in the nucleus with a repressor, thereby facilitating
the expression of an operator gene to allow the operon to transcribe messenger
RNA that initiates protein synthesis.
The final effect of aldosterone, increased sodium transport, appears to
be mediated by the action of a protein whose formation is induced by the hormone. Such a protein could conceivably act in several ways: 1. It could increase
the action of the 'sodium pump'. This could be done either directly, as by increasing
the activity of Na-K activated ATPase or indirectly by furnishing it with additional
energy substrate (ATP). 2. Outflux (or back diffusion) of sodium from the serosal
to mucosal surface could be reduced with a resulting increase in the net flux. 3.
If more sodium were admitted to the cell across its mucosal border, then the
'sodium pump' may be expected to respond by increasing the rate at which it extrudes sodium from the cell.
86
fluorocortisol and prednisolone, also increase sodium transport across the bladder.
In contrast to the neurohypophysial hormones, whose effects can be seen in less
than a minute, the increase in sodium transfer that follows exposure of the serosal
side of the bladder to aldosterone has a latent period of about 60 to 90 min. This
could be due to a delayed rate of access to receptor sites, or to a prolonged series
of 'secondary reactions.' As the delay is independent of the hormone's concentration, CRABBE (1963) suggested the latter explanation.
The receptor sites for aldosterone in the toad bladder appear to be situated intracellularly, probably in the nucleus. EDELMAN, BOGOROCH, and PORTER (1963)
demonstrated by radioautography, that 3H -aldosterone was accumulated preferentially to progesterone in the nucleus. The nuclei of these cells have since been
separated by centrifugation procedures, and have been shown directly to bind aldosterone in preference to other steroids. The relative abilities of different steroids
to displace bound aldosterone were found to correspond roughly with their activities in increasing sodium transport (AUSIELLO and SHARP, 1968). These results are
interpreted with some caution, as it is difficult to be sure that the binding sites are
those specifically related to the physiological response. FANESTIL and EDELMAN
(1966) found comparable binding sites for aldosterone in the nuclei of rat kidneys,
and concluded that such sites are probably proteinaceous, as only proteolytic enzymes could increase the rate of release of aldosterone from such loci.
In an effort to unravel the nature of the secondary series of reactions involved
in the effect of aldosterone, EDELMAN et at. (1963) and CRABBE and DE WEER(1964)
tested the effects of actinomycin D and puromycin on the bladder. Actinomycin
D prevents transcription of nuclear DNA to messenger RNA, while puromycin
inhibits the subsequent ribosomal protein synthesis. Both these agents inhibited
the effects of aldosterone on sodium transport. Aldosterone was also shown to increase the rate of incorporation of 3H-uridine into RNA (PORTER, BOGOROCH,
and EDELMAN, 1964) in the toad bladder. The increase in RNA formation is independent of changes in intracellular sodium levels, so that the action of the hormone
is probably direct and not mediated, at this stage, by ionic changes (DE WEERand
CRABBE, 1968). The results all strongly suggest that aldosterone changes the activity
of certain genes according to the theory of JACOB and MONOD (see MONOD, 1966).
Thus, aldosterone may interact in the nucleus with a repressor, thereby facilitating
the expression of an operator gene to allow the operon to transcribe messenger
RNA that initiates protein synthesis.
The final effect of aldosterone, increased sodium transport, appears to
be mediated by the action of a protein whose formation is induced by the hormone. Such a protein could conceivably act in several ways: 1. It could increase
the action of the 'sodium pump'. This could be done either directly, as by increasing
the activity of Na-K activated ATPase or indirectly by furnishing it with additional
energy substrate (ATP). 2. Outflux (or back diffusion) of sodium from the serosal
to mucosal surface could be reduced with a resulting increase in the net flux. 3.
If more sodium were admitted to the cell across its mucosal border, then the
'sodium pump' may be expected to respond by increasing the rate at which it extrudes sodium from the cell.
86