creases in the renal blood flow and pressure. It is noteworthy that haemorrhage
is also a powerful stimulant for release of aldosterone and renin. Renin and the
plasma 'converting' enzyme activate angiotensin II which acts on the zona glomerulosa to initiate release of aldosterone and some corticosterone. In higher concentrations angiotensin II will also release glucocorticoids. These physiological
events result in sodium retention, restoration of the blood volume and inhibition
of further renin release (Fig. 2.9). Renin is bioassayed by measuring its effects on
the blood pressure in mammals and its activity has been identified in non-mammalian vertebrates including kidneys of freshwater and marine teleost fishes, reptiles and amphibians (FRIEDMAN, KAPLAN, and WILLIAMS, 1942; MIZOGAMI et al.,
1968; CAPELLI, WESSON, and ApONTE, 1970). The effects of angiotensin on corticosteroid levels in fishes has not yet been described but the levels of renin are
greater in fish kept in fresh water and this could reflect a role in sodium conservation.
DENTON (1965) has suggested that the mechanism controlling secretion of mineralocorticoids has been subject to evolution, and that the earliest method may be
represented by the direct effects of plasma sodium and potassium concentrations
on the gland. The renin-angiotensin system could be a more recent and sophisticated product that is sensitive to volume, and hence total sodium levels in the body.
Renal
Anterior pituitary
/
Corticotrophin
inhibitio~
~
Co...,::~;.,",
Angiotensin II ---) Adrenal /
Cortex ~
i
Aldosterone
Decrease plasma Na+
1
inc. plasma K+
1'",
, ,
",
Tubules
iohib;'':'''''"",l
,
Na" retention
inc. blood vol.
and pressure
Juxta+
glomerular
Angiotensinogen
Cell
,...,
, , , , , , , , , , , ,
inhibition "
, , , , , , , ,
'" Inc. renal arterial
E-(--Press. and/or blood flow
Decreased renal arterial
pressure and/or blood flow
(Na" depletion or mechanical
factors)
1
Kidney ~ Renin ~
-------~. inhibition
Fig.2.9 Factors controlling release of aldosterone (adapted from Davis et al. 1967).
77
is also a powerful stimulant for release of aldosterone and renin. Renin and the
plasma 'converting' enzyme activate angiotensin II which acts on the zona glomerulosa to initiate release of aldosterone and some corticosterone. In higher concentrations angiotensin II will also release glucocorticoids. These physiological
events result in sodium retention, restoration of the blood volume and inhibition
of further renin release (Fig. 2.9). Renin is bioassayed by measuring its effects on
the blood pressure in mammals and its activity has been identified in non-mammalian vertebrates including kidneys of freshwater and marine teleost fishes, reptiles and amphibians (FRIEDMAN, KAPLAN, and WILLIAMS, 1942; MIZOGAMI et al.,
1968; CAPELLI, WESSON, and ApONTE, 1970). The effects of angiotensin on corticosteroid levels in fishes has not yet been described but the levels of renin are
greater in fish kept in fresh water and this could reflect a role in sodium conservation.
DENTON (1965) has suggested that the mechanism controlling secretion of mineralocorticoids has been subject to evolution, and that the earliest method may be
represented by the direct effects of plasma sodium and potassium concentrations
on the gland. The renin-angiotensin system could be a more recent and sophisticated product that is sensitive to volume, and hence total sodium levels in the body.
Renal
Anterior pituitary
/
Corticotrophin
inhibitio~
~
Co...,::~;.,",
Angiotensin II ---) Adrenal /
Cortex ~
i
Aldosterone
Decrease plasma Na+
1
inc. plasma K+
1'",
, ,
",
Tubules
iohib;'':'''''"",l
,
Na" retention
inc. blood vol.
and pressure
Juxta+
glomerular
Angiotensinogen
Cell
,...,
, , , , , , , , , , , ,
inhibition "
, , , , , , , ,
'" Inc. renal arterial
E-(--Press. and/or blood flow
Decreased renal arterial
pressure and/or blood flow
(Na" depletion or mechanical
factors)
1
Kidney ~ Renin ~
-------~. inhibition
Fig.2.9 Factors controlling release of aldosterone (adapted from Davis et al. 1967).
77