Endocrinology Vol. 140, No. 3 1036-1043
Copyright © 1999 by The Endocrine Society
Differential Effects of Superior and Inferior Spermatic Nerves on Testosterone Secretion and Spermatic Blood Flow in Cats1
S. R. Chiocchio,
A. M. Suburo,
E. Vladucic,
B. C. Zhu2,
E. Charreau,
E. E. Décima3 and
J. H. Tramezzani
Instituto de Neurobiología, Serrano 669 (1414); Facultad de
Ciencias Biomédicas, Universidad Austral (A.M.S.), Garay 125
(1063); and Instituto de Biologia y Medicina Experimental (E.V., E.C.),
Obligado 2490 (1428), Buenos Aires, Argentina
Address all correspondence and requests for reprints to: Dr. Sara R. Chiocchio, Instituto de Neurobiología, Serrano 669 (1414), Buenos Aires, Argentina. E-mail: fuacta{at}ssdnet.com.ar
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Abstract
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It has been postulated that testosterone secretion is partially
regulated by signals from the spermatic nerves. To further examine this
hypothesis in vivo, the superior (SSN) or the inferior
(ISN) spermatic nerves were stimulated electrically (varying intensity,
25 Hz, 0.2 msec, 10 min) in anesthetized cats, determining the
testosterone concentration and the blood flow in the spermatic vein. In
some additional experiments arterial blood was sampled, and
norepinephrine (NE) output was calculated. Stimulation of the SSN
(25–35 V) increased the testosterone concentration in spermatic vein
blood (P < 0.01 compared with prestimulation
levels). The response varied among animals, reaching a 50–100%
increase in some animals, whereas in others it ranged from almost
undetectable to more than 10 ng/100 g·min. Under the same
experimental conditions, the NE output increased from 135.4 ± 99
to 1614.2 ± 347 pg/ml (P < 0.01), and
spermatic blood flow decreased from 24.1 ± 1.42 to 20.2 ±
1.65 ml/min·100 g (P < 0.05) during nerve
stimulation. By contrast, stimulation of the ISN (25–35 V) modified
neither the testosterone concentration, the NE output, nor the blood
flow in the spermatic vein. High intensity stimulation (36–70 V) of
each spermatic nerve evoked different vascular and hormonal effects.
SSN activation induced a marked decrease in spermatic blood flow during
stimulation and an increase in the testosterone response, whereas ISN
activation resulted only in an enhanced spermatic blood flow. Our
results suggest that testosterone secretion, although mainly dependent
on gonadotropin secretion, could be further regulated by neural inputs
from the SSN acting directly or alternatively through changes in blood
flow. It would appear that the SSN mainly supplies the vasoconstrictor
fibers to the testis, whereas the ISN provides vasodilator fibers.
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Introduction
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THE MALE mammalian gonad receives both
sensory and efferent innervation from the spinal ganglia and
prevertebral plexuses. Fibers converge to the testis along two major
pathways, the superior (SSN) and the inferior (ISN) spermatic nerves.
The former runs from the mesenteric and renal plexuses alongside the
testicular artery, whereas the ISN, originating in the pelvic and
inferior mesenteric plexuses, accompanies the vas deferens and
penetrates the epididymis (1, 2). In the rat, the ISN enters the testis
through the inferior testicular ligament (3).
Since the initial description by Bell and McLean (4), it has been
known that most testicular nerves are adrenergic. In the rat,
catecholaminergic fibers have been described around capsular blood
vessels (4), but adrenergic innervation of the parenchyma has not been
detected in either this rodent (3) or the guinea pig (5). By contrast,
in the testis of man, cat, and monkey, adrenergic fibers have been
observed around intratesticular blood vessels, seminiferous tubules,
and Leydig cells (2).
Immunocytochemical studies have shown a great variety of peptidergic
neurotransmitters in the testis. Nerve fibers containing neuropeptide
Y, vasoactive intestinal peptide (VIP), calcitonin gene-related
peptide, and substance P, among others, have been described in the
testicular capsule and around blood vessels (2, 3, 6, 7). In the cat
testis, we recently observed numerous peptidergic fibers in close
association with Leydig cells (unpublished).
In vivo studies have shown that testicular nerves may
influence androgen secretion (8, 9, 10, 11). In the rat, bilateral section of
the SSNs blocks the acute stress-induced rise of plasma testosterone
(8), inhibits the hCG-stimulated androgen production, and decreases the
number of testicular LH receptors (11).
Those observations suggest that testosterone secretion, although
mainly dependent on gonadotropin secretion, could be further regulated
by signals coming from the spermatic nerves. The present experiments
examined this hypothesis in vivo in the cat, a species with
richly innervated testis. Our experimental design was based on the
simultaneous measurement of testosterone concentration and blood flow
in the spermatic vein after electrical stimulation of either the SSN or
ISN.
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Materials and Methods
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General
Experiments were performed on adult domestic male cats (2.5–4.0
kg). Animals were kept in individual cages, with food and water
ad libitum during 15 days before the experiments. Food was
withdrawn 12 h before surgery, but water was freely supplied. Cats
were anesthetized with sodium pentobarbitone (40 mg/kg, ip). A
tracheotomy was performed, and a tracheal tube was inserted. Anesthesia
was maintained with the same anesthetic (
4 mg/kg·h, iv) through a
cannula inserted in the right femoral vein. Anesthetic level was
monitored in spontaneously breathing animals by clinical signs and the
carbon dioxide content of expired air. The percentage of expired carbon
dioxide was kept constant (range, 0.3–0.7%), adjusting the anesthetic
infusion rate. Under these conditions, mechanical stimulation of the
skin did not elicit cardiovascular responses. The femoral cannula was
also used to replace lost fluids, using either saline or blood derived
from other cannulas in the same animal. Body temperature was kept at
37.5 ± 0.5 C by a heating pad controlled by a rectal temperature
sensor. The bladder was continuously drained.
The experiments were performed in accordance with ethical guidelines
for the care and use of animals for experiments, approved by the
Government of Buenos Aires City (Health Secretariat, Expedient
89784/92), Argentina.
Preparation
A catheter was inserted in the right carotid artery and was
connected to a blood pressure transducer (Statham Instruments, Oxnard,
CA) providing a continuous record of blood pressure, respiratory rate,
and heart rate. SILASTIC brand cannulas (Dow Corning, Midland, MI) were
placed in the main left internal spermatic vein and in the jugular vein
from the same side. Cats were then heparinized (Liquemine, Roche,
Buenos Aires, Argentina; 75 µg/kg), and an extracorporeal
shunt was established between those veins, using a three-way valve. The
circuit allowed a continuous assessment of blood flow and hormonal
concentrations in the spermatic vein. Blood was returned to the
circulation through the right femoral cannula or was collected for
hormonal assays (0.1–0.5 ml/sample) and packed cell volume controls.
No significant variations in packed cell volume were observed during
each experimental session. Blood samples were centrifuged at 1500 x
g and 4 C, and the supernatants were stored at -70 C until
assayed.
The animals were killed at the end of the experiments by the
administration of an overdose of sodium pentobarbitone. On postmortem
evaluation, the anatomical integrity of the testis was examined before
it was removed, weighed, and immersed in Bouin fixative for routine
histology. Only those animals without gross abnormalities of
microscopic structures were included in the study.
Spermatic blood flow measurements
Spermatic venous blood flow (milliliters per min) was measured
by recording the filling time of graduated pipettes (12) and was
adjusted to 100 g testis (milliliters per min/100 g). The flow
values of our cat population were well within the range reported for
other animals species with other techniques (2). The spermatic blood
flow does include both testicular and epididymal flow, as no ligation
of the epididymal vessels was performed. However, as the epididymal
contribution is minimal (13), we can assume that the measured values
mainly represent the testicular blood flow.
In preliminary experiments the blood flow filling time values were
identical to those measured by blood volume collected during a
specified time. Both procedures detected equally well the changes
induced by nerve stimulation (see Results).
Nerve stimulation
Bipolar electrodes, with an interelectrode distance of 10 mm,
were placed on SSN or ISN. The electrodes were made of silver wire (0.5
mm in diameter), attached to an acrylic frame, and were placed on one
of the spermatic nerves just before closing the extracorporeal
circulation shunt. The SSN electrode was set along the spermatic
pedicle, 3–4 cm below the entrance of the spermatic vein cannula.
Thus, this electrode was closer to the testis than was the cannula. The
ISN electrode was placed in the inguinal portion of the vas deferens.
Tissues and electrodes were isolated with cellophane paper (3.5 x
1.5 cm) and covered with liquid paraffin-impregnated cotton swabs to
avoid desiccation and mass stimulation through current spread. Nerves
were stimulated using an arrangement of Tektronix, Inc. (Beaverton,
OR) waveform and pulse generator units. Square pulses of
0.2-msec duration were delivered as a 25-Hz tetani through a constant
voltage isolation unit. Using this arrangement, the delivered current
was distributed among the nerve fibers (between electrode tips) and the
rest of the body (in parallel with the intact nerve). To block the
centripetal conduction of impulses produced by the electrical nerve
stimulation, the proximal region of each nerve was infiltrated by 1%
xylocaine solution (30–50 µl; Lidocaina, Astra, Buenos Aires,
Argentina). The SSN was infiltrated near the insertion point of
the spermatic vein cannula, whereas the ISN was infiltrated in its
abdominal portion. Local nerve anesthesia was applied 10–15 mm
cephalad to the electrode site and 10–15 min before electrical
stimulation.
Only one given stimulus intensity was used in each animal. As most
spermatic nerve fibers are unmyelinated (14), the experiments started
with high voltage stimuli (70 V) to ensure activation of all nerve
fibers innervating the testis. After preliminary experiments using high
voltage, the SSN were stimulated with voltages varying randomly between
15–70 V in a series of cats (n = 17). This broad range of
stimulus intensities was used to explore effects dependent on
differential fiber excitability (i.e. according to
diameter). Comparison of the hemodynamic and hormonal values obtained
in these animals allowed sorting of the experiments into three groups
according to the intensity of nerve stimulation: 1) low intensity
(15–24 V) in which there were no effects, 2) medium intensity (25–35
V) with clear hormonal effects, and 3) high intensity (36–70 V) with
both hormonal and blood flow effects. Then, another series of
experiments (n = 11) was performed stimulating the ISN using the
same voltage ranges described above. Sham-stimulated animals (n =
6) had SSN or ISN electrodes applied as described but received no
stimulation.
To determine whether the hormonal effects evoked by SSN stimulation
could be attributable to norepinephrine (NE) release from SSN nerve
terminals, a separate series of experiments was conducted, stimulating
the SSN or the ISN with medium intensities (25–35 V). Nine cats were
studied under baseline conditions of barbiturate anesthesia, stable
arterial pressure, and normoxia. The animals were equipped with iv
catheters, as described previously, plus a polyethylene catheter
inserted into the femoral artery with its tip lying close to the origin
of the spermatic artery.
Hormonal analysis
Testosterone assays were directly made in predetermined plasma
dilutions, using a solid phase RIA (Immunotech
International, Dianova GmBH, Hamburg, Germany). The sensitivity
of the method, defined as the minimal concentration of testosterone
significantly different from 0 with a 95% probability, was 0.025
ng/ml. The cross-reactivity of the testosterone antiserum bound to the
assay tubes was 0.01–1% for C19 steroids and less than
0.03% for C18 and C21 steroids.
Plasma testosterone concentrations were corrected by packed cell volume
to whole blood values. The product of testosterone concentration
(nanograms per ml) in the spermatic vein blood and the spermatic blood
flow at the time of collection was termed the corrected testosterone
concentration (CTC) and was expressed as nanograms per 100 g/min.
NE in arterial and spermatic vein plasma was measured by HPLC with
electrochemical detection after alumina extraction (15, 16). The assay
was performed using a C18 guard column and a
C18 Spherisorb reverse phase column (25 cm x 4.6 mm
id) with a mobile phase (0.039 M citric acid, 0.10
M sodium acetate, 0.40 mM EDTA, and 0.20
mM sodium l-octyl sulfate, pH 4.2, containing
4.0% methanol) pumped at a flow rate of 1 ml/min (1600 psi). Under
these conditions, the intraassay coefficient of variation for a
spermatic venous plasma pool with a NE concentration of 1 ng/ml was
10% (n = 9). All samples were determined in the same assay. NE
output was calculated as the venous-arterial blood concentration
difference, multiplied by the blood flow at the time of collection and
expressed as picograms per min.
Analysis of data
For each experimental animal, four consecutive observation
periods were defined: PRE, 15 min before the onset of stimulation; ST,
10 min during stimulation; and P-1 and P-2, two periods of 15 min each
after stimulation. In each experimental period, three to five samples
for testosterone assay were obtained. Although all of the respiratory
and cardiovascular parameters were measured continuously, only those
spermatic blood flow values corresponding to the blood sample obtained
for determination of testosterone were used for CTC calculation. All
available CTC readings and cardiovascular parameters from each period
were summarized as an arithmetic average.
For each intensity group, differences in CTC values from different
experimental periods were analyzed via the Friedman global procedure
for repeated measures, followed by Dunn’s test for the significance of
pairwise comparisons (17). These are nonparametric procedures based on
ranks, and therefore, results are independent of the actual variability
between experimental units. Each cat was regarded as one experimental
unit with four repeated observations (one per experimental period).
Results were deemed significant if P < 0.05.
Statistical analyses of spermatic blood flow and NE output responses to
SSN and ISN stimulation were performed using ANOVA followed by
Student-Newman-Keuls tests. Results are expressed as the mean and
SE.
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Results
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Basal values
Basal values of spermatic blood flow and testosterone
concentration in the spermatic vein are shown in Figs. 1
and 2,
respectively. Within the sample, spermatic blood flow ranged between
20–64 ml/100 g·min (Fig. 1). The CTC values exhibited large
individual variations, being as low as 0.013 ng/100 g·min in some
animals and as high as 8000 ng/100 g·min in others (Fig. 2).
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Figure 1. The spread of the basal values of spermatic blood
flow (SBF) within our cat population (n = 42) is shown in this box
plot. The line crossing the box represents the median,
whereas the 25th, 10th, and 5th percentiles are represented by the
lower box hinge, whisker, and
dot, respectively. The 75th, 90th, and 95th percentiles
are similarly represented in the upper part of the
graph.
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Figure 2. This box plot depicts the spread of basal values
of CTC in the spermatic veins of our cat population (n = 42). The
median and the percentiles are indicated as described in Fig. 1 . Notice
the exponential nature of the vertical axis.
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Sham-stimulated animals
Physiological parameters, spermatic blood flow, and CTC values
were studied in sham-stimulated preparations, in which electrodes were
placed but no stimuli were delivered. Heart rate, respiratory rate,
expiratory PCO2 and PO2, mean arterial
pressure, and spermatic blood flow remained steady throughout the
experiment (Table 1 and Fig. 3). Periodic oscillations of CTC values,
such as those shown in Fig. 3, were observed in all cats. Thus, for
analysis of testosterone responses, CTC values were averaged for each
of the consecutive experimental periods (see Materials and
Methods). Values from each sham-stimulated cat are shown in Table 2.
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Figure 3. A representative example of the simultaneous
recording of mean arterial pressure (MAP), spermatic blood flow (SBF),
and testosterone concentration corrected for blood flow (CTC) in a
sham-stimulated cat. Although MAP and SBF remained stable, the CTC
values fluctuated widely over the course of the experimental session.
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Table 2. Testosterone concentrations in the spermatic vein
corrected for blood flow (CTC) in individual sham-stimulated male cats
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Blood flow responses to spermatic nerve stimulation
SSN. Heart rate and mean arterial pressure did not change
significantly in response to nerve stimulation. As shown in Fig. 4A, spermatic blood flow did not change
after stimulation at low intensity. In contrast, statistically
significant reductions in blood flow were observed during medium and
high levels of SSN stimulation (Fig. 4, B and C). The decrease in
spermatic blood flow (P < 0.05) induced by medium
level stimulation persisted after the electrical activation had ceased
(Fig. 4B). The spermatic blood flow decreased markedly
(P < 0.01) during high intensity stimulation, but
recovered to near-basal values in the poststimulation periods (Fig. 4C).
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Figure 4. Spermatic blood flow (SBF) in SSN-stimulated
animals at low (A), medium (B), and high (C) intensities. In all of the
histograms, the bars represent the average ±
SEM of five or six animals at different periods: PRE, 15
min before stimulation; ST, 10 min during stimulation; and P-1 and P-2,
consecutive 15-min periods after stimulation. A, No significant changes
were detected after low intensity stimulation. B, There was a
significant difference in SBF (*, P < 0.05)
between PRE and any of the other periods. C, A significant decrease
(**, P < 0.01) in SBF occurred during the
stimulation period.
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ISN. Heart rate and mean arterial pressure remained unchanged
in all experiments involving stimulation of the ISN with various
intensities. Spermatic blood flow did not change after stimulation at
either low (Fig. 5A) or medium intensity
(PRE, 36.2 ± 8.1; ST, 37.5 ± 7.9; P-1, 33.9 ± 7.5;
P-2, 31.3 ± 6.9 ml/min·100 g; n = 3). This parameter,
however, increased significantly (P < 0.05) during
stimulation at high intensity, returning to basal levels in the
poststimulation periods (Fig. 5B).
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Figure 5. Spermatic blood flow (SBF) in ISN-stimulated
animals at low (A) and high (B) intensities. In all of the histograms,
the bars represent the average ± SEM
of three (low) or six (high) animals at different periods. A, No
significant changes were detected in animals receiving a low intensity
stimulation of the ISN. B, An increase in SBF was observed during
stimulation with high intensity stimuli. There was a significant
statistical difference in SBF between ST and any of the other periods
(*, P < 0.05).
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Testosterone responses to spermatic nerve stimulation
SSN. As shown in Table 3 no
changes in CTC values were detected when electrical stimuli ranged
between 15–24 V, (i.e. the low intensity range as defined
in Materials and Methods). In contrast, voltages between
25–35 V (medium intensity) increased the CTC immediately after the
onset of stimulation. Values during stimulation and subsequent periods
were higher than prestimulation values in all animals (Table 3). The
magnitude of the response showed large individual variations. In some
individuals, CTC increased from 50 to 100% over basal values (Fig. 6, A–C), whereas in others, it rose from
almost undetectable values to more than 10 ng/100 g·min (Fig. 6D). In
this stimulation group, median (range) values for each period were:
PRE, 36.8 (0.3–2117) ng/100 g·min; ST, 62.7 (0.83–2298) ng/100
g·min; P-1, 83.0 (4.8–2527) ng/100 g·min; and P-2, 81.0
(10.5–2318) ng/100 g·min. Significant statistical differences (by
Friedman and Dunn’s tests, P < 0.01) were detected
for the comparisons PRE vs. P-1 and PRE vs.
P-2.
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Table 3. Testosterone concentrations in the spermatic vein
corrected for blood flow in individual SSN-stimulated male cats
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Figure 6. Illustrations of the testosterone concentration
corrected for blood flow (CTC) in response to stimulation of the SSN at
medium (A–D) and high (E and F) intensities in individual cats. The
horizontal bar in the top scale indicates
the duration of stimulation.
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In the high intensity group, five of six animals showed a large
decrease in CTC values during stimulation (Table 3). However, all
animals showed a significant increase during the first period after
stimulation. Median (range) values of this group were: PRE, 215.7
(6.8–1848) ng/100 g·min; ST, 157.9 (9.6–1111) ng/100 g·min; P-1,
324.6 (28–4855) ng/100 g·min; and P-2, 196.6 (23–2770) ng/100
g·min. A statistically significant difference (by Friedman and
Dunn’s tests, P < 0.01) was found for the comparison
ST vs. P-1.
ISN. CTC values did not change significantly in response to
nerve stimulation (Table 4). In animals
receiving low voltage stimulation, the CTC values slightly decreased
during the second poststimulation period.
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Table 4. Testosterone concentrations in the spermatic vein
corrected for blood flow in individual ISN-stimulated male cats
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NE response to SSN stimulation
Because the bulk of the adrenergic innervation of the testis is
provided by the SSN in the rat (3) and in the cat (our
unpublished observations), and NE stimulates testosterone secretion
(see Discussion), additional experiments were performed
measuring NE output in the spermatic vein in response to SSN or ISN
stimulation (medium intensity). Electrical activation of the SSN
markedly increased NE output in the spermatic vein from 135.4 ±
99 to 1614.2 ± 347 pg/min (P < 0.01) during
stimulation (Fig. 7). By contrast,
stimulation of the ISN under similar experimental conditions did not
affect significantly NE output (PRE, 128.7 ± 105; ST, 176 ±
121; P-1, 135 ± 87 pg/min; n = 4).
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Figure 7. Effect of medium intensity stimulation of the SSN
on the output of norepinephrine (NE) in the spermatic vein. Blood
samples were collected and pooled before (PRE, 15 min), during (ST, 10
min), and after (P-1, 15 min) stimulation. Each column
represents a mean of five values SEM. *,
P < 0.05 vs. P-1; **,
P < 0.01 vs. PRE.
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Discussion
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Our results indicate that spermatic blood flow and testosterone
concentration in the spermatic vein can be differentially modified by
stimulation of either the SSN or the ISN. Medium intensity stimulation
of the SSN increased both testosterone concentration and NE output in
the spermatic vein and induced a slight reduction of spermatic blood
flow in sodium pentobarbitone-anesthetized cats. By contrast,
stimulation of the ISN under similar experimental conditions did not
affect testosterone concentration or spermatic blood flow.
Low to medium intensity stimuli have been frequently used for
stimulation of autonomic nerves. Higher intensity stimuli, which also
have been used in the present study, were very useful for confirmation
of the selective effects of each nerve. Although high voltage
stimulation within a volume conductor, as in our experimental
arrangement, could allow electrical current to spread beyond the
electrode sites, opposite effects were obtained by activation of either
the SSN or the ISN. These different results elicited by stimulation of
each nerve indicate that isolation of the electrodes was efficient in
avoiding the occurrence of virtual cathodes (in nerve fibers) in places
outside of electrode location.
In the present study we show that SSN stimulation results in an
increase in the testosterone concentration in the spermatic vein.
Testosterone levels in the spermatic vein represent around 40–60% of
the levels found in the testicular vein (18, 19). However, in a variety
of mammals the testosterone secretion profiles in both veins are
comparable (19).
It has been shown that under normal conditions, arterial-venous
anastomoses in the pampiniform plexus provide arterial blood to the
venous effluent of the testis diluting the testosterone concentration
in the spermatic vein (2, 19). It could then be speculated that the
rise in the concentration of testosterone is secondary to
vasoconstriction of the spermatic artery (20) and probably of such
anastomoses. However, a redistribution of blood flow in the spermatic
vein by some nerve-induced constrictor mechanism need not be invoked.
If this had occurred, it should have been observed as a drastic
decrease in venous outflow and a minor increase in testosterone
concentration during stimulation. In fact, there was only a minor drop
in venous outflow and great changes in steroid concentration in the
majority of the animals with medium intensity stimulation.
Increases in testosterone levels with SSN stimulation could also occur
through other mechanisms. One possible explanation might be that nerve
stimulation induces an increased access of LH to the testis by
increasing testicular blood flow and thus inducing enhanced steroid
secretion (2). This possibility seems unlikely, because in our
experimental conditions, SSN stimulation evokes a slight reduction of
the spermatic blood flow.
The increase in testosterone secretion could also be due to some other
SSN-dependent rise of LH. Direct nerve connections between the testis
and the hypothalamus have been described (21, 22, 23, 24), but in our
experiments centripetal conduction was blocked by xylocaine anesthesia.
On the other hand, preliminary LH determinations by RIA in cats (using
a serum against bovine LH) showed that spermatic nerve stimulation did
not induce changes in the plasma level of this gonadotropin.
The simultaneous increases in testosterone and NE output in the
spermatic vein in response to SSN stimulation suggest the participation
of the sympathetic innervation in the control of the endocrine function
of the testis. Most probably, the increase in testosterone levels is a
direct effect of NE release from SSN terminals on Leydig cells. Support
for this hypothesis comes from several sources: 1) in vitro
experiments demonstrate the ability of noradrenaline to enhance
testicular steroidogenesis (25, 26, 27, 28, 29); 2) section of the SSN results in a
decreased testosterone secretion in response to hCG (11); 3) similar
results were obtained when the animals were chronically treated with
guanethidine, an adrenergic blocking agent that causes a
pharmacological sympathetic denervation (10); and 4) morphological
evidence showing that the SSN provides the bulk of the adrenergic
innervation to the testis (3).
As is well known, Leydig cell functions are under a complex control
system including various endocrine, paracrine, and autocrine
interactions (30). Therefore, it is also possible that NE and/or other
neurotransmitters released from SSN terminals affect substances
contained in other testicular cell types, which, in turn, act on Leydig
cells to stimulate testosterone secretion. Whatever the exact mechanism
may be, the present data support the view that increased superior
spermatic neural input to the testis elevates testosterone levels in
the spermatic vein.
Stimulation of each spermatic nerve induced a characteristic
modification of the spermatic blood flow. This parameter decreased
after stimulation of the SSN and increased after stimulation of the
ISN. The extent of the spermatic blood flow modification depended on
the intensity of the stimulus. As changes in spermatic blood flow took
place under a steady mean arterial pressure, they probably represent
modifications in vascular diameters induced by released neuromediators.
NE is probably involved in the response to SSN stimulation, as it has
been shown that intraarterial infusion of NE elicits a testicular
vasoconstrictor response resembling the neurally induced response (31, 32). Moreover, lumbar sympathectomy causes a bilateral increase in
testicular blood flow (32).
By contrast, spermatic blood flow increased with high intensity
stimulation of the ISN. Several neurotransmitters and neuromodulators
found in the ISN and in intratesticular nerves in the cat, including
VIP, galanin, and calcitonin gene-related peptide (33), could be
involved in this effect, as these substances exhibit strong
vasodilatory properties. Our previous study in the rat demonstrated
that VIP nerves fibers were present in the ISN but not in the SSN (3),
and this distribution may also be present in the cat testis.
In conclusion, the present data demonstrate that each spermatic nerve
differentially affects the spermatic testosterone concentrations and
blood flow. The finding that SSN stimulation induces simultaneous rises
in the testosterone concentration and NE output in the spermatic vein
permits us to suggest that sympathetic inputs contribute to the control
of plasma testosterone levels.
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Acknowledgments
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It is a particular pleasure to acknowledge Dr. Armando Garsd for
kindly reviewing the manuscript, Dr. Mauricio Rivera for measurement of
norepinephrine, Med. Vet. Silvina Heisecke and Marcela Marquez and Mrs.
Marcela Huerta for their expert technical assistance and Mr. Ignacio
Fossati for preparation of the manuscript.
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Footnotes
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1 This work was supported by Consejo Nacional de Investigaciones
Científicas y Técnicas and Fundación Instituto de
Neurobiología (Buenos Aires, Argentina). 
2 Present address: East China Normal University, North Zhongshan Road
3662, Shangai, Peoples Republic of China 200062.
3 Present address: INSIBIO, Chacabuco 461, 4000 Tucuman,
Argentina.
Received June 23, 1998.
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Abstract
Introduction
