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Institut für Vegetative Physiologie und Pathophysiologie, Universität Hamburg, D-20246 Hamburg, Germany
IN THE
1980S A REVOLUTION took place in cardiovascular
physiology unnoticed by many researchers working in the field. At that time, the rat and the dog were the prime animal models to study integrative mechanisms involved in cardiovascular regulation. The
notorious but true saying that cells do not have a blood pressure implies that in vivo experiments will always be critical in the process
of discovering new principles of circulatory control. Studies in rats
and dogs have helped to unravel many basic principles of cardiovascular
physiology, such as autoregulation of blood flow (11, 28),
reflex control of blood pressure (3), or blood volume
regulation through signals originating from the heart (2),
to name just a few. Nevertheless, the lack of techniques to directly
link the activity of specific genes to physiological functions Today, more than 3,000 different knockout mice have been
constructed. Many of these mutations affect cardiovascular function, and some of them have helped to solve long-standing open questions in
cardiovascular regulation. For example, it has been known for long that
the tubuloglomerular feedback (TGF) is a key mechanism of
autoregulation of renal blood flow and glomerular filtration and is
intimately involved in blood volume homeostasis. In the kidney, the
proximal part of the distal tubule gets in close contact with the
afferent arteriole to form the juxtaglomerular apparatus. Within this
region, the TGF communicates changes in proximal tubular flow rate to
afferent arteriolar smooth muscle and renin-secreting cells, causing a
decrease in vascular tone and an increase in renin secretion rate if
tubular flow rate falls and vice versa. Twenty years ago it was
postulated that changes in local adenosine concentrations contribute to
the signaling mechanism underlying the TGF response; however,
participation of adenosine has remained controversial. Now two
independent groups of investigators reported that mice carrying a
targeted deletion of the A1-adenosine receptor, which is
expressed at a high level in the juxtaglomerular apparatus, completely
lack a TGF response (5, 34). These studies clearly demonstrate that adenosine, the major agonist of
A1-adenosine receptors, plays an essential role in the
signaling cascade mediating the TGF. Future studies will have to show
whether this role is that of a mediator or a modulator (25,
29).
This example illustrates the major strength of the genetic approach
compared with the more classical pharmacological and correlative approaches. Although the latter are inherently indirect,
pharmacological studies often suffer from uncertain pharmacokinetics
and a lack of specificity. Accordingly, the observation of a sevenfold
higher salt intake after an overnight fluid restriction in mice
carrying a targeted deletion of the oxytocin gene constitutes strong
direct evidence for the concept derived from infusion studies of
agonists and antagonists that sodium appetite is negatively regulated
by central oxytocin (1). The finding that mice lacking
tumor necrosis factor- In accordance with its increasing importance in cardiovascular
research, the basic physiology of the murine circulatory system has
been intensively studied recently. Most relevant techniques and
experimental protocols to assess cardiovascular function have now been
adapted to the small size of the mouse. These include chronic
measurements of blood pressure (6, 14, 21, 23) and cardiac
output (16), the analysis of the different components of
the baroreceptor reflex (22), and renal function and
balance studies (21). Many aspects of the murine
cardiovascular system Nevertheless, there are also important peculiarities specific to the
mouse. For example, studies using autonomic blocking agents indicate
that mice generate a high resting cardiac sympathetic tone (14,
17). Mice have a relatively labile blood pressure and are
extremely sensitive to stressful conditions such as anesthesia (21) and changes in ambient temperature (37).
Sympathetic neurotransmitter release is also enhanced by stressful
housing conditions, such as social deprivation and exposure to novel
odors (9). A negative energy balance can cause severe
hypotension and bradycardia in mice, possibly resulting in
torporlike states (37). The development of hypotension is
accelerated if mice are housed at an ambient temperature of 23°C,
which is the normal room temperature in most laboratories. Dietary
preferences (31) as well as metabolic responses to leptin
(13) differ between strains and can confound
cardiovascular measurements. The metabolism of melatonin is also strain
dependent (18), which may impact on circadian
cardiovascular rhythms. These factors need to be considered when
designing experiments in mice.
It is very likely that targeted deletions will be introduced into most
of the mouse's ~30,000 genes within the next few years. Keeping in
mind the murine peculiarities listed above, the combination of mouse
genetics and physiology seems to have a bright future. There are,
however, several caveats. Most importantly, the vast majority of
targeted deletions inactivate the target gene in each cell from
conception onward. Such a lifelong deletion of gene function cannot
only severely alter developmental processes, but might also disturb the
normal homeostatic balance, causing secondary and/or compensatory up-
and downregulation at the level of the genome as well as the level of
the organism. Thus more subtle genetic constructs will be required,
such as conditional targeted deletions and combinations of gene
inactivations with the introduction of transgenes that restrict the
expression of a specific gene to a limited area (e.g., in the central
nervous system) or time (e.g., during development) (10,
20). Furthermore, although gene targeting is a very efficient
way to investigate the function of a certain gene, it is considerably
less powerful to uncover the genetic pathways underlying a given
physiological process, particularly if the biology of this process is
only poorly understood. In this latter situation, mutagenesis screens
are much more likely to yield new insights. Such screens, however,
require huge numbers of animals to be generated, housed, and
phenotyped. Even though mutagenesis screens are already performed in
mice, they can be realized much easier in the zebrafish (4,
26). And, finally, despite all the similarities between mice,
rat, and humans, we will always need research that translates the
results obtained in mice to the human physiology and pathophysiology
(12).
ARTICLE
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REFERENCES
which
was then possible already in cellular systemsremained a major
drawback. Thus the development of experimental procedures to induce
mutations in single genes and the eventual successful generation of
mice carrying a targeted mutation in 1989 (7, 19) marked
the beginning of a new era area also in cardiovascular physiology.
(TNF-) display threefold elevated renal
renin mRNA levels under control conditions as well as after salt
depletion provides the first in vivo evidence for a possible
physiological function of TNF- as a negative regulator of renin
synthesis (35). Studies in TNF- 
/ mice also
indicate that a strong increase in the local production of TNF-
initiates intimal hyperplasia after vascular injury (38).
Further examples for important advances in our understanding of
cardiovascular regulation obtained by the use of gene-targeted mice are
the discovery of a major role of the K+ channel 
-subunit
KCNE1 in K+ and renal fluid homeostasis (36),
the unmasking of an enhanced central response to dehydration in
angiotensin AT1A / mice (24), the
identification of multiple physiological functions of
2-adrenergic receptors (27),and
the demonstration that the vagal effects on atrial rate are independent
from M3-muscarinic receptor signaling (33).
anatomic and physiologicalare identical to
larger mammals. Nearly all values of cardiovascular parameters obtained
in mice are well within the range predicted by allometric scaling
equations from rats and humans (15). The baroreceptor
reflex (22) and the regulation of fluid and electrolyte
homeostasis by the renin-angiotensin system (8), two of
the major blood pressure regulating systems (30, 32), seem
to function in a very similar fashion as in larger mammals.
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FOOTNOTES |
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Address for reprint requests and other correspondence: H. Ehmke, Institut für Vegetative Physiologie und Pathophysiologie, Universität Hamburg, Martinistrasse 52, D-20246 Hamburg, Germany (E-mail: ehmke{at}uke.uni-hamburg.de).
The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
10.1152/ajpregu.00531.2002
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