Opioid Modulation of Fetal Glucose Homeostasis

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*

±


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 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


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


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



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-



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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Send reprint ogy, Cornell

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and Fetal