674 SECTION VIII Renal Physiology
Angiotensin II feeds back to inhibit renin secretion by a
direct action on the JG cells. Vasopressin also inhibits renin
secretion in vitro and in vivo, although there is some debate
about whether its in vivo effect is direct or indirect.
Finally, increased activity of the sympathetic nervous sys-
tem increases renin secretion. The increase is mediated both
by increased circulating catecholamines and by norepineph-
rine secreted by postganglionic renal sympathetic nerves. The
catecholamines act mainly on β 1 -adrenergic receptors on the
JG cells and renin release is mediated by an increase in intra-
cellular cAMP.
The principal conditions that increase renin secretion in
humans are listed in Table 39–3. Most of the listed conditions
decrease central venous pressure, which triggers an increase
in sympathetic activity, and some also decrease renal arteri-
olar pressure (see Clinical Box 39–3). Renal artery constric-
tion and constriction of the aorta proximal to the renal
arteries produces a decrease in renal arteriolar pressure. Psy-
chologic stimuli increase the activity of the renal nerves.
HORMONES OF THE HEART &
OTHER NATRIURETIC FACTORS
STRUCTURE
The existence of various natriuretic hormones has been postu-
lated for some time. Two of these are secreted by the heart. The
muscle cells in the atria and, to a much lesser extent in the ven-
tricles, contain secretory granules (Figure 39–10). The granules
increase in number when NaCl intake is increased and ECF ex-
panded, and extracts of atrial tissue cause natriuresis.
The first natriuretic hormone isolated from the heart was
atrial natriuretic peptide (ANP), a polypeptide with a char-
acteristic 17-amino-acid ring formed by a disulfide bond
between two cysteines. The circulating form of this polypep-
tide has 28 amino acid residues (Figure 39–11). It is formed
from a large precursor molecule that contains 151 amino acid
residues, including a 24-amino-acid signal peptide. ANP was
subsequently isolated from other tissues, including the brain,
where it exists in two forms that are smaller than circulating
ANP. A second natriuretic polypeptide was isolated from por-
cine brain and named brain natriuretic peptide (BNP; also
known as B-type natriuretic peptide). It is also present in the
brain in humans, but more is present in the human heart,
including the ventricles. The circulating form of this hormone
contains 32 amino acid residues. It has the same 17-member
ring as ANP, though some of the amino acid residues in the
ring are different (Figure 39–11). A third member of thisTABLE 39–2 Factors that affect renin secretion.
Stimulatory
Increased sympathetic activity via renal nerves
Increased circulating catecholamines
Prostaglandins
Inhibitory
Increased Na+ and Cl– reabsorption across macula densa
Increased afferent arteriolar pressure
Angiotensin II
VasopressinTABLE 39–3 Conditions that increase renin secretion.
Na+ depletion
Diuretics
Hypotension
Hemorrhage
Upright posture
Dehydration
Cardiac failure
Cirrhosis
Constriction of renal artery or aorta
Various psychologic stimuliCLINICAL BOX 39–3
Role of Renin in Clinical Hypertension
Constriction of one renal artery causes a prompt increase in
renin secretion and the development of sustained hyper-
tension (renal or Goldblatt hypertension). Removal of the
ischemic kidney or the arterial constriction cures the hyper-
tension if it has not persisted too long. In general, the hy-
pertension produced by constricting one renal artery with
the other kidney intact (one-clip, two-kidney Goldblatt hy-
pertension) is associated with increased circulating renin.
The clinical counterpart of this condition is renal hyper-
tension due to atheromatous narrowing of one renal artery
or other abnormalities of the renal circulation. However,
plasma renin activity is usually normal in one-clip one-
kidney Goldblatt hypertension. The explanation of the hy-
pertension in this situation is unsettled. However, many pa-
tients with hypertension respond to treatment with ACE in-
hibitors or losartan even when their renal circulation
appears to be normal and they have normal or even low
plasma renin activity.