Jun 9, 1995 - (1 23-i 42 days) (term on plasma .... lateral cerebroventricle for the infusion ofthe opioid peptides. (Szeto et al., 1990). ...... H. H., Zsu, Y. S., Us,.
0022-3565/95/2751-0334$03.OO/O
Jouiui. OF Copyright © 1995 ,JPET 275:334-339, THE
m
PHARMACOLOGY
Expiui.tzrriu. Tiizpw’ris for Pharmacology
by The American
and
Society
Experimental
Vol. Printed
Therapeutics
Opioid Modulation of Fetal Glucose Receptor Subtypes1 HAZEL
Yl SOONG,
H. SZETO,2
of Pharmacology,
Department
Accepted
275,
in
No. 1 U.S.A.
1995
for publication
DUN-LI
Cornell
WU
University
and
PETER
Medical
Homeostasis:
Role of
V. CHENG
College,
New
York, New
York
June 9, 1995
ABSTRACT
cemia
and
indicating
The
hyperglycemia
both were
were
mediated
has
(Radosevich non, 1991;
been
found
to
increase
et al. , 1984; May et al. Johansen et al. , 1992).
sponse to morphine can be suggesting that it is mediated
et al.
(Johansen various response
1993).
of the
hyperglycemic
phin
also
this
effect
it may
not
to have produced could
This
levels
blocked by by opioid
the
use
Hanre-
naloxone, receptors
involvement
in this
significant
be blocked
be an opioid and other
whereas
on glucose
of the
of selective
the
mu,
hyperglycemia,
opiate
for publication September work was supported, in part,
23, 1994. by Grant
recepDynor-
delta
although suggesting
alkaloids
are
suggest
homeostasis
that
widely
1
This
have rate
2Recipientofa
National
Institute
ABBREVIATIONS: Dynprphin
334
Research on Drug
Scientist Abuse.
DAMGO,
A(1-1 3); ANOVA,
DevelopmentAward
from
the
(DAOO100)from
National
analysis
of variance.
opioids
on
fetal
glucose
and receptor-selectivity.
et al.,
fetal oxygenation, movements and
1982;
,1988). However,
homeostasis
transfer
the
has
DPDPE,
across
not
the
direct actions by administration
#{176}urresults
glucose [d-A1a2,N-Me-Phe4,Gly-o-enkephalin;
of
on dose
Hodgkinson the effect been
decrease neurobehavioral
heart
and Farkhanda, ofopioids on fetal
investigated.
Studies
in
adult animals would suggest that intrauterine exposure to opioids may result in fetal hyperglycemia. Knowledge of the effects of opioids on fetal glucose metabolism is important as fetaj hyperglycemia has been reported to result in fetal hypoxemia and affect cerebral metabolism, electrocortical activity and breathing movements (Philipps et al. , 1982, 1984; Richardson et al. , 1982; Fowden et al. , 1992; Rosenkrantz et al. , 1993). Opioids may directly affect fetal glucose homeostasis as a result of placental drug transfer, or indirectly by altering
meostasis the
effects
been reported to alter variability, breathing
glucose
med DA02475
the
used as obstetrical analgesics. These opioid compounds are readily distributed across the placenta to the fetus (Szeto et al. , 1978, 1982; Kuhnert et al. , 1979; Golub et al. , 1986), and
glucose Received
that
are dependent
activity (Epstein 1982; Ban et al.
hyperglycemic
homeostasis.
by naloxone,
action. related
Morphine
glucose Bossone and hyperglycemic
Possible
response,
no effect a
not
been mor-
agonists (Gunion et al. , 1991). The mu receptor is critical to the media-
kappa opioid showed that the
appears
homeostasis has out in animals, plasma
with
results
The
, 1988;
receptors
investigated
naloxone,
receptors.
completely specifically
of opioid
and
delta results
tor
, 1992,
subtypes has been
by i.v.
opioid
effects of morphine on glucose for decades. In studies carried
known phine
tion
antagonized
by specific
mu-selective agonist, [D-Ala2,N-Me-Phe4,Gly-ol]-enkephaIin (1 00 pg/hr i.c.v., n = 6), resulted in a significant increase in both plasma glucose (F3,20 = 1 1 .50; P = .001) and lactate (F3,20 = 3.77; P = .007) concentrations. In contrast, the delta-selective agonists, [D-Pen2,D-Pen5]-enkephalin (30 and 1 00 pg/hr i.c.v.) and [D-Ala-deItorphin I (0.3 and 1 .0 pg/hr i.c.v.) had no effect on plasma glucose or lactate levels. Similarly, Dynorphin A(11 3) (1 60 and 480 pg/hr i.c.v.) and U50,488H {trans-(±)-3,4dichloro-N-methyl-[2-(1 -pyrrolidinyl)-cyclohexyl]benzeneacetamide} (200 p.g/hr i.c.v.) also had no effect. The effects of morphine and [D-Ala2,N-Me-Phe4,Gly5-ol]-enkephalin on fetal plasma glucose and lactate levels cannot be accounted for by changes in maternal plasma glucose and lactate levels. These
show homeostasis,
that
placenta.
In this
study,
we
exam-
of morphine on fetal glucose hoof morphine to the fetal lamb.
morphine exerts with hypoglycemia
[D-Pen2,D-Pen5]-enkephalin;
DELT,
dual at
action on fetal low doses and
[D-A1a9-deltorphin
I; DYN,
Downloaded from jpet.aspetjournals.org at ASPET Journals on July 29, 2015
Opioids have long been known to influence glucose homeostasis in the adult. However, their role in modulating glucose regulation in the fetus is not known. The objectives of this study were to determine the effects of morphine on fetal plasma glucose levels and to ascertain the role of opioid receptor subtypes in fetal glucose homeostasis. The studies were carned out in 38 unanesthetized fetal sheep (1 23-i 42 days) (term being -1 45 days). Intravenous infusion of morphine to the fetus resulted in dual actions on fetal plasma glucose, with hypoglycemia after 1 .2 mg/hr (F3,16 = 6.02; P = .006; n = 5) and hyperglycemia after 5.0 mg/hr (F316 = 5.58; P = .008; n = 5). Significant increase in plasma lactate concentration also was found after 5.0 mg/hr (F316 = 5.25; P = .01 0). Both hypogly-
1995
Oplold
hyperglycemia effects fetal
at high
of selective glucose
the rationale
in
doses.
mu,
In addition,
and
delta
homeostasis.
Information
hyperglycemic response for the development
pounds
as obstetrical
we
kappa
compared
opioid on receptor
in the of more
the
agonists
Animal
selectivity
fetus may provide a selective opioid com-
Quantitation
analgesics.
preparation. fetal
These
sheep
with
studies
were
gestational
carried
ages
from
123
to
142
design.
ing or lying quietly access to food and
The studies
were carried
out with
from
the
and
mother
at the
end
infusion,
dose
Biochemicals i.c.v.
International,
at
a rate
(6 mg/hr)
naloxone
TABLE
1
Effects
of morphine
on fetal
Natick,
of 0.3 was
mI/hr. infused
plasma
MA) were
and lactate
glucose
Dose (iv.)
n
1.2 0.8 1.0 0.9 0.8
polyethylene
and the plasma
glucose
mg/hr
(ANOVA,
F316 (ANOVA,
different
ues,
glucose and
are
the
samples
6.02, F316
=
with
P
were
=
.006) 5.58, P
fetal
on fetal
hyperglycemia
observed significant
To illustrate
the
normalized in
plasma
changes
in
to control
val-
glucose
change either
after
1.2
after
.008).
=
morphine,
lowest plasma
actions
and
1. No significant
was However,
1). The on
dual
lactate. on either
hypoglycemia
of change
in table
effect
=
were
percentage
(table no
to
concentration
concentration of morphine.
glucose and had no effect
exerted
doses,
responses
summarized
lactate mg/hr
had
Morphine at higher
5.0 mg/hr two
mg/hr,
increases
levels
in plasma 0.6 or 1.2 in plasma
lactate concentration were found after 2.5 mg/hr (ANOVA, F3,16 3.82, P = .050) and 5.0 mg/hr (ANOVA, F3,16 = 5.25, P = .010) (table 1). Blockade of morphine’s actions by naloxone. Naloxone was used to determine if both hypoglycemic and hyperglycemic actions of morphine on fetal plasma glucose were mediated either
by opioid plasma
receptors.
glucose
or
Naloxone lactate
alone
had
concentrations
no effect
on
(table
2).
Naloxone
completely blocked both the hypoglycemic response of morphine and the hyperglycemic response to 5.0 mg/hr of morphine (table 2). The increase in plasma lactate with 5.0 mg/hr of morphine also was abolished (table 2). Effects of selective opioid peptides on fetal glucose and lactate. All opioid peptides were infused into the fetal to 1.2
infused
mg/hr
Plasma Lactate
% Change from control
mg/dl 16.4 ± 15.1 ± 18.2 ± 17.7 ± 16.7 ±
chilled
concentrations
% Change from control Control
1 hr
6 5 5 5 5
concentrations.
into
concentration
0.6
concentration.
Control
Saline 1.0 mI/hr Morphine 0.6 mg/hr Morphine 1.2 mg/hr Morphine 2.5 mg/hr Morphine 5.0 mg/hr * P < 0.05 compared to
lactate
on plasma of saline alone
Plasma Glucose Drug
and collected
centrifuged
or lactate
plasma
plasma
In a separate series of i.v. to the fetus starting
glucose
of morphine infusion
glucose
after
and
all
EDTA,
of morphine,
the
thchloro-N-methyl-[2-(1-pyrrolidinyl)-cyclohexyl]benzeneacetamide}
(Research
containing
plasma
at 1 and 3 hr after drug infusion for determination ofplasma glucose and lactate concentrations. Each animal received only one drug on any given day, and at least 2 days were allowed between studies. Drugs and chemicals. Morphine (0.6, 1.2, 2.5 and 5.0 mg/hr) was infused i.v. to the fetus at a rate of 1.0 mI/hr. DAMGO (10 and 100 Lg/hr; Sigma Chemical Co., St. Louis, MO), DPDPE (30 and 100 pg/hr; Cambridge Research Biochemical Inc., Wilmington, DE), DELT (0.3 and 1.0 gIhr; a gift from Dr. Peter Schiller, Montreal, Canada), DYN (160 and 480 pg/hr; supplied by the National Institute on Drug Abuse, Rockville, MD) and U50,488H ftrans-(±)-3,4-
to the fetus experiments,
tubes
glucose
(1.0 ml) were
samples
Effects
the ewe stand-
of drug
blood
Intravenous
water
fetus
ofplasma
Fetal
Results
in a small experimental cart. The ewe had free throughout the study period. The ewe was allowed a period of 3 hr to acclimate to the study conditions before drug infusion. Toward the end of the 3rd Kr, blood samples were obtained for determination of arterial blood gases and pH (Radiometer ABL3O) and control plasma glucose and lactate concentrations. Animals were included in the study only if fetal P02 > 16 mm Hg, PC02 < 60 mm Hg and pH > 7.30. Drugs were infused to the fetus either i.v. (1.0 ml/hr) or i.c.v. (0.3 mI/h.r) for 1 hr. Blood samples were collected
335
6.1 3.5 -12.0 2.1 12.6
2 hr
±
4.2
7.9
±
±
2.2
7.2
±
±
0.9* 6.0 4.2
± ±
±
22.6 27.4
con trol levels (Dun nett’s post hoc test).
± ±
1 hr
4 hr
4.1 8.4 5.1 5.5 6.8*
6.6 4.1 -14.1 4.0 13.1
±
5.5 1.8 2.5* 6.8
mg/dl 22.8 ± 24.5 ± 19.3 ± 18.2 ±
±
2.9
20.5
± ± ±
±
2.5 2.6 1.2 1.1 1.0
-0.9 -7.6 -6.0 17.6 13.3
± ± ± ± ±
2 hr 2.4 3.9 4.3 14.7 8.1
-5.6 -6.6 7.4 53.7 93.5
± ± ± ± ±
2.5 4.9 4.7 15.5* 31.2*
4 hr -2.2 0.7 8.0 16.4 70.4
±
3.3
± ±
6.9 1.7 6.3
±
13.8*
±
Downloaded from jpet.aspetjournals.org at ASPET Journals on July 29, 2015
days (term being approximately 145 days). Five or more days before the study, fetal sheep were surgically instrumented for implantation ofchronic indwelling catheters, in accordance with guidelines approved by the Institution for the Care and Use of Animals. Details of the surgical procedure have been described previously (Szeto, 1983). Polyvinyl catheters were placed in the distal aorta for blood sampling and in the inferior vena cava for drug infusion. In some animals, two catheters were placed in the inferior vena cava, one for infusion ofthe opioid agonist and the other for infusion of the opioid antagonist. An additional polyvinyl catheter was placed in the lateral cerebroventricle for the infusion ofthe opioid peptides (Szeto et al., 1990). All catheters were tunneled s.c. to the maternal flank and stored in a pouch. In addition, a polyvinyl catheter was placed in the maternal femoral artery. Intraoperatively, 2 g of ampicilhin was placed in the amniotic cavity and 1 g of ampicillin in the peritoneal cavity of the ewe.
Study
Glucose
Fetal
either assayed immediately or frozen at -70#{176}Cuntil later analysis. Glucose and lactate concentrations were determined enzymatically, in triplicate, using a glucose and lactate analyzer (model 23L; Yellow Springs Instrument Co., Yellow Springs, OH). Statistical analysis. All data are presented as mean ± S.E.M. A single-factor ANOVA with repeated measure (factor = time) was used to examine the effects of all drugs and vehicle controls on fetal plasma glucose and lactate concentrations. The Dunnett’s test was used for post hoc comparison of each time point to the predrug control. Differences were considered significant when P < .05.
out in 38 un-
ranging
and
1 hr before morphine or DAMGO infusion, and was maintained throughout the morphine (1.2 or 5.0 mg/hr i.v.) or DAMGO (100 .tg/hr i.c.v.) infusion and for another 3 hr afterward. Control groups included saline (1.0 mI/hr i.v. and 0.3 mI/hr i.c.v.) and naloxone (6 mg/hr i.v.).
on
Methods anesthetized
Receptors
336
Szetoetal.
TABLE
Vol.275
2
Blockade
of morphine’s
on fetal
effects
plasma
glucose
and
lactate
concentrations
by naloxone
Plasma Glucose Morphine Dose
Naloxone Dose
#{176}
%
Control
Plasma Lactate
Change from control
1 hr
4 hr
1 hr
mg/dl
0 1.2 mg/hr 1.2 mg/hr
6 mg/hr
5
17.8
±
0
5
18.2
±
5
18.7
±
6 mg/hr
0.7 1.0 1.9
1.7 -12.0 7.4
± ± ±
4.3 -5.0 #{216}9* 5.5 -1.4 4.2 27.4
3.8
±
5j
-4.3 -14.1
4.9 6.8*
±
5.0 mg/hr 0 5 16.7 ± 0.8 12.6 ± 5.0 mg/hr 6 mg/hr 5 16.8 ± 2.5 2.2 ± 4.9 -0.2 a Same data as shown in table 1. * P < 0.05 compared to control levels (Dunnett’s post hoc test).
±
± 2.2
N-methyl-[2-(1-pyrrolidinyl)-cyclohexyl]benzeneacetamide). Neither 160 nor 480 .tg/hr of DYN
had any effect on fetal plasma glucose or lactate concentrations (table 3). The more selective kappa agonist, U50,488H, also had no effect on either plasma glucose or lactate levels (table 3). Effects of morphine and DAMGO on maternal plasma glucose and lactate. Intravenous infusion ofsaline to the fetus had no effect on maternal glucose but caused a significant though small (12%) increase in maternal lactate levels at 2 hr (table 5). A similar increase in plasma lactate
opiold
agonists
on fetal
plasma
glucose
±
4.3
±
2.5* 7.8 2.9 3.9
-2.2 13.1
±
5.5
±
±
mg/dl 15.9 ± 0.5 19.3 ± 1.2 19.1 ± 2.4 20.5 ± 1.0 16.6 ± 1.0
was observed DAMGO had levels.
-2.7 -6.0 -2.0 13.3 -3.1
Dose (icy.)
n
0.3 mI/hr
10 pg/hr 100 pg/hr 30 gThr 100 pg/hr 0.3 p.g/hr
5 5 6 6 5 5 5 5 5
±
mg/dl 16.3 ± 19.5 ± 18.8 ± 20.6 ± 19.6 ± 18.2 ± 18.6 ± 16.3 ± 15.1 ±
1.0 1.4 1.6 1.0 0.8 1.6 DELT 1.0 pg/hr 1.9 DYN 160 pg/hr 1.2 DYN 480 pg/hr 1.1 U50,488H 200 gig/hr 5 15.9 ± 1.0 * P < 0.05 compared to control (Dunnett’s post
± ± ± ± ±
3.0 4.7 6.0 31.2* 2.8
-1.4 8.0 -2.7 70.4 -1.2
± ± ± ± ±
3.0 1.7 7.7 13.8* 2.8
ofmorphine to the fetus. plasma glucose or lactate
The effects of morphine on fetal plasma glucose regulation were complicated, resulting in hypoglycemia at low doses and hyperglycemia at higher doses. In most studies carried out in adult animals, morphine has been found to cause a dosedependent increase in glucose levels which can be blocked by naloxone pretreatment (Radosevich et al. , 1984; Feldberg and Wei, 1986; May et al. , 1988; Bossone and Hannon, 1991; Johansen et al. , 1992). This increase in glucose levels has been attributed to a centrally mediated stimulation of catecholamine release by opioids (Feldberg and Gupta, 1974; van Loon et al. , 1981; Radosevich et al. , 1984; May et al. , 1988; Bossone and Hannon, 1991). Others have suggested that opioids can cause hyperglycemia by direct stimulation of pancreatic secretion of glucagon (Ipp et al., 1978, 1980; Radosevich et al., 1984; Johansen et al., 1992). However, in addition to the increased production of glucose, insulin resistance also may contribute to the opioid-induced hyperglycemia.
A concurrent
increase
in plasma
insulin
levels
has
been
reported by some investigators (van Loon and Appel, 1983; Johansen et al., 1992, 1993). The mechanism by which morphine causes hyperglycemia in the fetal sheep has yet to be determined. Our findings show that it is not due to changes in maternal plasma glucose level as a result of transplacental distribution of morphine to the mother. Previous studies
and lactate
concentrations Plasma Lactate
% Change from control
% Change from control Control
1 hr
DAMGO DAMGO DPDPE DPDPE DELT
±
-2.6 7.4 0.4 93.5 -2.0
4 hr
Discussion
Control
Saline
±
2.6 4.3 6.1 8.1 2.1
at 2 hr after 5.0 mg/hr no effect on maternal
Plasma Glucose Drug
± ±
2 hr
-3.8 ± 3.1 -5.9 ± 5.9 5.7 ± 3.7 -3.3 ± 2.3 -4.5 ± 4.1 8.7 ± 2.6 0.5 ± 3.4 3.9 ± 2.2 -8.4 ± 3.0 -7.8 ± 3.4 hoc test).
2 hr 6.0 -7.6 28.0
±
-3.5
±
-2.6
±
4 hr
6.1
±
6.7 5.1 6.9 5.1 5.7 3.6
3.1
±
2.8
0.3 -7.9
±
-5.6
±
3.5 5.5 4.4
± ±
±
1 hr
±
5.9 6.4 8.4* 8.7 5.5 4.4
mg/dl 18.8 ± 23.3 ± 25.4 ± 22.5 ± 16.8 ± 20.0 ±
±
3.3
20.3
±
2.7 ± 4.4 -6.1 ± 6.2 -5.3 ± 5.3
15.6 17.3 14.5
±
2.4 -7.1 46.1 2.2 5.5 1.5
±
0.9
± ± ± ±
± ±
1.9 2.9 3.1 2.7 0.9 2.5 3.3 1.7 1.9 3.6
-5.2 -3.0 3.7 -6.9 -7.4 -6.4 -2.9 -2.8 -2.5 2.2
± ± ± ± ± ± ± ± ± ±
2 hr 1.9 4.3 4.1 1.3 3.0 4.5 3.8 3.3 5.9 3.4
-0.6 1.1 79.5 -4.6 -3.3 -5.1 1.9 2.3 8.0 2.0
± ± ± ± ± ± ± ± ± ±
4 hr 2.5 3.7 37.8 1.3 2.7 3.5 2.5 5.4 2.2 2.3
-6.4 3.4 169 0.1 -6.0 -5.4 5.9 -1.3 -2.3 2.2
± ± ± ± ± ± ± ± ± ±
4.8 3.6 65.3* 2.4 2.9 5.3 4.1 3.9 3.1 5.1
Downloaded from jpet.aspetjournals.org at ASPET Journals on July 29, 2015
lateral ventricle as their ability to cross the blood-brain barrier is uncertain. The infusion of saline alone into the lateral ventricle had no effect on either glucose or lactate levels (table 3). DAMGO was used to ascertain the role of the mu opioid receptor in modulating fetal plasma glucose and lactate levels. DAMGO (10 pg/hr) had no significant effect on either plasma glucose or lactate concentrations (table 3). However, 100 tg/hr resulted in a significant increase in both plasma glucose (ANOVA, F320 = 11.50, P = .001) and lactate (ANOVA, F3,20 = 3.77, P = .027) concentrations 3 hr after the termination of DAMGO infusion (table 3). These effects of DAMGO were completely blocked by naloxone (table 4). The role of delta opioid receptors in modulating fetal glucose and lactate levels was examined using two highly selective delta agonists, DPDPE and DELT. Both peptides produced no discernible effects on either fetal plasma glucose or lactate concentrations (table 3). The potential role ofkappa opioid receptors on glucose and lactate homeostasis was studied using two kappa-selective agonists, DYN and U50,488H (trans-(±)-3,4-dichloro-
TABLE 3 Effects of selective
% Change from control
Control
2 hr
1995
Receptors
Opiold
TABLE
and Fetal
4
Blockade
of the effects
of DAMGO
on fetal
plasma
glucose
and lactate
concentrations
by naloxone
Plasma Glucose DAMGO
Naloxone
Plasma Lactate
% Change from control
% Change from control
Control
Control 2 hr
1 hr
mg/dl 0 6.0 mg/hr 5 17.8 ± 0.7 1.7 ± 4.3 -5.0 100 Ihg/hrb 0 6 18.8 ± 1.6 5.7 ± 3.7 28.0 100 pg/hr 6.0 mg/hr 4 15.5 ± 0.8 -12.4 ± 7.8 -13.2 a Same data as shown in Table 2. b Same data as shown in Table 3. * P < 0.05 compared to control levels (Dunnett’s post hoc test). TABLE
4 hr
± 3.8 6.9 ± 3.5
±
-4.3 46.1 -9.8
± ± ±
I hr mg/d! 15.9 ± 0.5 25.4 ± 3.1 17.3 ± 1.3
4.3 8.4* 3.0
2 hr
-2.7 ± 2.6 3.7 ± 4.1 1.8 ± 4.9
-2.6 79.5 -2.1
4 hr
± 3.0
-1.4
3.0 65.3* 1.8
±
±
37.8
169
±
±
4.7
0.5
±
5
Effects
of fetal opiold
on maternal
administration
plasma
glucose
and lactate
concentrations
Plasma Glucose Drug
Dose
n
Plasma Lactate
% Change from control Control 1 hr
2 hr
mg/dl Saline 1.0 mVhr 6 57.7 ± 4.4 -7.4 ± 2.7 Morphine 1.2 mg/hr 5 54.2 ± 1.6 -5.0 ± 2.5 Morphine 5.0 mg/hr 5 48.0 ± 2.6 3.4 ± 2.9 DAMGO 100 p.g/hr 5 59.0 ± 4.9 7.5 ± 3.5 * < 0.05 compared to control (Dunnett’s post hoc test).
from
our
laboratory
have
shown
that
maternal
-3.3 -2.3
±
-1.3
±
±
5.9
±
plasma
4 hr 3.5 2.7 1.7 3.1
mor-
phine concentration only reaches = 10% of fetal plasma concentration at steady state after fetal administration of morphine (Szeto et al., 1982). Thus, morphine appears to have a direct action on regulation offetal plasma glucose levels. It is known
that
sheep
glycogen
liver
is stored
et al.
(Barnes
conditions,
glucose
this
is most
likely
and
glucagon.
is not due
in large
, 1977).
Under
released
to low
However,
amounts
in the
normal
physiological
from
the
circulating
fetal
levels
infusions
fetal
liver,
and
of epinephrine
of either
epinephrine
or
1.2 0.5 4.8 6.7
± ± ± ±
1 hr mgldl 6.4 ± 5.8 ± 5.3 ± 7.1 ±
3.6 0.3 4.5 2.9
glucose
and
0.8 0.3 0.3 0.4
lactate
hypermetabolic
0.2 -8.7 -7.5 -9.9
levels state
in
2 hr 12.0 -16.7 13.1
±
2.8*
12.8
±
3.2
-7.4
±
±
6.3 4.9 5.0
±
4.9
-9.4
±
-6.8 -2.6
±
±
0.7* 4.5
5.9 8.8 5.0
±
2.2
± ±
may the
be associated fetus.
We
with
the
adult,
from
the
vein liver,
can
significantly
and
result
increase in
an
glu-
increase
in
plasma
the
effect
of morphine
on
fetal
plasma
epinephrine
or
glucagon is not known. However, these doses of morphine have been reported to significantly increase fetal heart rate, and this tachycardia can be abolished by propranolol (Zhu and Szeto, 1989). It is therefore quite likely that morphine
plasma
and
fetus,
glucose
Hannon
(1991)
reported
to the
output
portal
in both
Bossone
time,
the
previously
that these doses of morphine result in an excitatory state in the fetal lamb, with EEG desynchronization, increased skeletal muscle activity, breathing activity and tachycardia (Szeto, 1983, 1991; Szeto et al. ,1988a,b, 1991; Zhu and Szeto, 1989). Part of the increase in plasma lactate levels may be due to lactate production by skeletal muscle. Although changes in plasma lactate levels have rarely been studied in
glucose and lactate concentrations (Apatu and Barnes, 1991). Epinephrine was found to be less effective than glucagon in promoting glucose release. At the present
into
±
an overall
reported
lactate levels morphine administration. These metabolic accompanied by behavioral excitation and and shell temperatures. Although maternal levels increased significantly at 2 hr after
glucagon
4 hr
this
and
increase
does
not
an
increase
in adult pigs after changes also were a rise in both core plasma lactate morphine infusion
appear
to account
for
the
increase in fetal plasma lactate levels, because the change was very small (-12%) and a comparable increase was observed with saline vehicle alone. Hypoglycemia is rarely seen after morphine administradramatic
increases epinephrine release in the fetal sheep, and stimulates hepatic glycogenolysis via a beta-2 mechanism. Alternately, glucocorticoids also may play a role in the elevation of
tion in the ministration
fetal
that this hypoglycemic effect of morphine is stereospecific, is elicited only by the intrathecal route ofadmiistration and is caused only by supra-analgesic doses (Brase et al. ,1990). It is antagonized, at least in part, by naloxone and other opioid
doses levels
plasma
glucose
as we have
of morphine significantly (Taylor et al. , 1994).
A dose-dependent found after morphine least
in
utilization glucose increase
recently
part, has levels in
to been (Hay plasma
increase
increase
in
infusion an
increase shown
reported
plasma which
in
fetal
fetal
However,
plasma glucose levels, suggesting may contribute to the increase in from our laboratory suggest that
plasma
lactate was
to be directly
et al., 1983). lactate far
that
exceeded
cortisol
levels
probably
glycolysis related
the the
that other lactate pool. this increase
these
was
due, as
at
glucose
to fetal
antagonists. hypoglycemic
blood
percentage
of
increase
in
mechanisms Other data in plasma
anisms
adult.
It has
of morphine
involved
are
Results effect
not
been
reported
(Brase
et al.
understood.
from
a recent
may
be
the
after
, 1990),
intrathecal
but
However,
study result
suggested of increased
ad-
the
mech-
it is known
that
this
uptake
and metabolism of glucose by skeletal muscle (White et al., 1993). Increased skeletal muscle metabolism may certainly explain the hypoglycemia observed after 1.2 mg/hr of morphine to the fetus. At low morphine concentrations, glucose utilization may exceed glucose production, thereby resulting in hypoglycemia. With increasing morphine levels, this in-
Downloaded from jpet.aspetjournals.org at ASPET Journals on July 29, 2015
% Change from control
Control
cose
337
Glucose
338
Szeto
et al.
Vol. 275
tion
of doses
used
in the
present
study
was
based
on results
of previous investigations with DAMGO in the fetal lamb. The dose of 100 pg/hr has been shown to cause maximal increase in fetal heart rate (Szeto et al. ,1990) and decrease in fetal breathing and other body movements (unpublished data), whereas doses higher than 100 pg/hr have been found to have nonselective effects. In addition, preliminary data show that this dose of DAMGO causes a significant increase in fetal plasma cortisol at 4 hr (Taylor et al. , 1994). These findings
suggest
that
the
increase
in
plasma
glucose
and
lactate levels may be secondary to a glucocorticoid response to DAMGO, which is consistent with the findings in the adult rat, where the increase in plasma glucose was accompanied by an increase in plasma corticosterone levels (Gunion et al., 1991). The slow onset of drug action may be related to the rate of diffusion of this peptide from the lateral ventricle to its
site
of action.
The
increase
in plasma
glucose
and
lactate
Acknowledgments
was apparent by 2 hr after the onset ofDAMGO infusion, but did not reach statistical significance until 4 hr, because of large interindividual variation in the rate of increase. When DAMGO was administered as a bolus into the third ventricle of adult rats, peak effects on plasma glucose was observed between 1 to 1.5 hi, and the increase was still significant at 2 hr (Gunion et al. , 1991). In the present study, the rate of diffusion of DAMGO may be even slower due to a smaller concentration gradient, because the peptide was administered as a constant rate infusion rather than as a bolus. This also may account for the apparent long duration of action of DAMGO. In contrast, two highly selective delta agonists, DPDPE and DELT, had no effect on fetal plasma glucose or lactate levels when administered i.c.v. This lack of effect suggest that either delta receptors are not functionally mature in the late-term fetal lamb, or that delta receptors do not have any modulatory action on glucose homeostasis. The first possibility
is unlikely
DPDPE crease
inasmuch
as we have
found
that
these
doses
istered into the third ventricle (Gunion et al. , 1991). We have similarly found a lack of effect of DPDPE on fetal heart rate over a wide range of doses (Szeto et al. , 1990), and these same doses ofDPDPE and DELT did not alter fetal plasma cortisol levels (Taylor et al. , 1994). Administration of DYN into the third ventricle has been reported to increase serum glucose levels in adult rats, although the effect was only transient and rather modest compared to DAMGO (Gunion et al. , 1991). In contrast, we found that i.c.v. DYN had no effect on fetal plasma glucose levels, even when the dose was increased to 960 pg/hr. Interestingly, the modest hyperglycemia to DYN in the adult was not sensitive to naloxone, suggesting that it may be mediated by nonopioid mechanisms (Gunion et al. , 1991). Unfortunately, the mechanisms behind this DYN action have not been investigated. The lack of a comparable response in the fetus would suggest that these nonopioid mechanisms are not important in fetal glucose homeostasis. Although DYN is considered a putative kappa agonist, in vitro binding studies have shown that DYN also binds with high affinity to mu and delta receptors (Garzon et al. , 1984). We have therefore also used a more selective kappa agonist in these studies, U50,488H. Our results showed that central administration of U50,488H also had no effect on fetal plasma glucose or lactate levels, suggesting that kappa receptors do not play a role in modulating fetal carbohydrate metabolism. The effects of these kappa agonists on other fetal physiological systems are not known. In summary, the present results suggest that the effects of opioids on fetal plasma glucose and lactate levels are dependent on dose and receptor-selectivity, and that only the mu opioid receptor appears to be involved in the central regulation of fetal carbohydrate metabolism.
of
(100 pg/hr) and DELT (1 pg/hr) significantly inbreathing movements in the fetal lamb, and these effects were antagonized by naltrindole (Cheng et al. , 1992, 1993). It is therefore more likely that delta receptors do not play a role in glucose homeostasis in the fetus. This is consistent with the finding that DSLET ([D-A1a2, Leu5, Thr6Ienkephalin), another highly selective delta agonist, also had no effect on serum glucose levels in adult rats when admin-
The supply
authors
supply
of DYN.
of DELT
wish
to
thank
Dr.
and the National
Peter
Schiller
Institute
for
on Drug
his
Abuse
generous
for the
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and Fetal