European Journal

of Pharmacology,

195 (1991)

217-224

217

Elsevier Science Publishers B.V. 0014.2999/91/$0X50 ADONIS 001429999100241U 0 1991

EJP 51784

peripheral effects of prostagla secretion in

ic aci

Denis Sautereau, Maria Chicau-Chovet, Annick Tsocas and Claude ROB? insert

U239,

Far&6

X.

Bichat, 16 Rue H. Huchard, 75018 Paris* Frame

Received 14 November 1990, revised MS received 19 December 199tI. accepted 8 January 1991

The intracerebroventricular (i.c.v.) administration of prostaglandin E, (PGE,) inhibits gastric secretion at doses that are inactive by i-v. administration in the rat. The present study was undertaken to examine the central and peripheral effects of enprostil. a potent synthetic PGE, analogue, on gastric acid secretion as compared to those of PGE,. We used conscious rats equipped with a chronic gastric fistula and a cannula to allow injection into the third ventricle of the brain. Gastric acid output was measured under basal interdigestive conditions and during stimulation with submaximal doses of pentagastrin and histamine. Total inhibition of basal and stimulated gastric acid output was obtained after i.c.v. PGE, and after i.c.v. or i.v. enprostil administration. After i.v. PGE,, the maximal observed inhibition was not greater than 50%. The ratio of Ef& values for i.v. administered to i.c.v. administered PGE, was 64 or more, whereas the ratio of EDs, values for i.v. enprostil to i.c.v. enprostil was 9 to 13. Under all conditions studied, enprostil was more potent than PGE, and this greater potency was more prominent after i-v. administration (ratio 250 to 2500) than after i.c.v. administration (ratio 10 to 400). The blockade of a,-adrenoceptors by idazoxan did not suppress the i~bition of gastric secretion produced by i.c.v. PGE, or enprostii. It is concluded that low doses of PGE, inhibit gastric acid output mainly through a central mechanism, whereas low doses of enprostil potently inhibit gastric acid output through both a central and a peripheral mechanism. o12-Adrenoceptors are not essential for the effect of i.c.v. prostanoids on gastric acid secretion.

Gastric secretion;

Prostagiandins;

Prostaglandin

E,; Enprostil;

Adrenoceptors;

(Injections:

1. introduction

2. Materials and metbds

The central effects of prostaglandins on the gastrointestinal tract include inhibition of gastric secretion, post-prandial restoration of motor migrating complexes, stimulation of intestinal secretion of water and electrolytes {for review, see Sautereau and Roz~, 1988), and inhibition of pancreatic secretion (Sautereau et al., 1989). The purpose of the present study was to measure the central and peripheral effects of enprostil, a potent synthetic derivative of prostaglandin E, (PGE,) with anti-ulcer therapeutic properties (Roszkowski et al., 1986; Goa and Monk, 1987), on gastric secretion in the rat in compa~son with those of PGE,, whose central gastric in~bitory properties are well established (Puurunen, 1983a-d).

2.1. Animal rechniques

Correspondence LO:C. Roz& INSERM U239, Facult&X. Bichat. 16 Rue H. Huchard, F75018 Paris, France.

intracerebroventricular)

Male Wistar rats (Iffa Credo, 69210, St. Gertnain I’Arbresle, France), weighing 250-350 g, were equipped with a Thomas cannula inserted into the rum&al portion of the stomach (Borella and Herr, 1971) and with a chronic stainless steel cannula implanted into the third ventricle of the brain (Rozt et al., 1980) to permit i.c.v. injections. The correct position of the cannula was confirmed by the ascension of cerebrospinal fluid in the cannula. A period of at least two weeks was allowed for the rats to recover from surgery. Experiments \:ere carried out, 1~0 more than once a week, on conscious rats that were kept in Bollman-type cages during the collection of gastric juice. In these restraint cages, the size of which was matched to each animal, the rats stand on their paws and can move back and forth to a limited extent. When trained, the animals enter the cage spontaneously.

animais were deprived nf food 18 h before each inacnt. but water was available ad libitum. The opened. the stomach was rinsed two 4 ml tap water. and a stabilization of f b teas aHowed_ Gastric juice was then ed as XI-min fractions. The volume was measured ml. and the acid output was measured t~t~ti~~ the samples to pH 7 with 0.01 N NaOH etrobm autotitrator). The basal level was taken as the Gwen of tbe two first 20-min samples. The conditions of st~rn~~atio~ were: (I) none (basal secretion); (2) pentagastrin. 1 .ug/kg per h (a dose causing about 70% of the

maximal acid response), infused i.v. into a tail vein for 3 h; (3) histamine, 1 mg/kg per h i-v. for 3 h (a dose causing about 70% of the maximal acid response). PGE, or enprostil (i.v. or i.c.v.) was injected after the response to the stimulation had reached a plateau (40 min after beginning the stimulation). In antagonist experiments, idazoxan or prazosin was injected S.C. 10 min before PGE, or enprostil. Prostanoids were injected i.v. into a tail vein in a volume of 1 ml/kg. For i.c.v. administration. 2.5 ~1 was injected, followed by 4 ~1 saline to compensate for the dead space of the cannula. 2.3. Drugs PGE, was purchased from Sigma Chemicals (St. Ilouis, MO, USA), enprostil was a gift from Syntex France (Puteaux, France), idazoxan was a gift from Reckitt and Colman (~ngston-upon-Hull, England), and prazosin was a gift from Pfizer (Brussels, Belgium). 2.4. Statistics

-20

0 +

g

20

40

60

80 100 120 140 160 180 Minutes h

39Ppnc9

150

8 0 =

g

lQ43

i

2

50

s

2

0 -20

0

20

40

60

The results are given in the figures as the means + S.E.M. for groups of 6-10 animals. Since the molecular weight of enprostil (400) is relatively close to that of PGE, (352), the doses of prostanoids are expressed as weight units, which are generally used in animal and therapeutic studies on prostanoids. Statistical significance was taken as P c 0.05 after ANOVA and Dunnett’s test, or after computing a regression line. ED5, values were calculated from the regression line corresponding to the approximately linear portion of the response versus log(dose) curve (Tallarida and Murray, 1984).

80 100 120 140 160 100 Minutes

3. Results 3.1. Effect of prostanoids on basal interdigestive gastric acid seer&m

.-I-

.Ol

. . . . ..

,I

1

.

F

100 10 PGE2 (&g:kg)

Fig. 1. Effect of i.c_v. (a) and i.v. (b) injections of PGEz (arrow) on basal gastric acid output in conscious chronic fist&a rats. The results, expressed as percent of the basal level, are means of six to nine animals per group. To simplify the graphs, the S.E.M. are indicated only for the control groups (i.c.v. or i.v. injection of vehicle alone), and three doses only are shown in (a). The dase-response curves of(c) were drawn from the peak ~bitions observed in each group. In addition. the peak inhibition observed in another group receiving a venous infusion of 3 &kg PGE, over 20 rnin (v. inf.) is drawn on graph (cl. The overall average basal acid oatput in all 12 groups was 36.fQ3.5 pm01/20 min. * P < 0.05, * * P c 0.01, * * * P c 0.001 for the global difference between groupsat each time (ANOVA).

3. I. I. PGE, The i.c.v. injection of PGE, (0.03-10 pg/kg) produced a decrease in gastric acid output, which peaked after 40 min and returned towards the preinjection values after 60 min (fig. la). However, after the highest doses used, the acid output remained lower than in controls for 2-3 h. The maximal observed inhibition was near total (92 + 0.9% after 10 pg/kg); the log(dose) response curve was approximately linear over the range 0.03-l p&/kg (fig. lc), and the calculated EDm was 0.14 pgfkg (table 1). The i.v. injection of PGE, (3-30 pg,/kg) produced a decrease in gastric acid output, which peaked after 40 min (fig. lb). The maximal observed inhibition was 68 f 4.1% after 10 pg/kg. The calculated EDs0 was 8.9

219 TABLE 1

EDsa values of PGE, and enprostil injected i.c.v. or i.v. on basal, pentagastrin-stimulated and histamine-stimulated gastric acid output. The ED5, values were calculated by linear regression analysis of the log dose-response curves. -.%, J’cE, &g/k@ I.C.V. I.V.

I.v. ED&.c.v. EDso I.C.V.

EDs,,

I.v. EDSBfi.c.v. ED,,

I.V.

Pentagastrin

0.14 8.9 64

0.072 ZlOO ZlOOO

1OOngikg 10 no/ka

Histamine 0.33 95 290

120

0

20

40

60

80

100 120

enprostil&g/kg)

I.V.

Molar I.C.V.

Basal

-0.

*

raho

PGE*

0.00033 0.0038 11

EDs,/enprosril 482 2660

0.0055 0.05 9

0.029 0.38 13

* +

140 160 Ia0 Minutes

1 )rW lOrlaikt7

..*

b

EDso

15 z2200

13 283

pg/kg, and the ratio i.v. ED,,/i.c.v. ED,, was 64. However, the lo&dose) response curve was poor, and the peak inhibition obtainable with i-v. PGE, seemed limited to about one half of the basal acid output. To exclude the possibility that the greater in~bition found after i.c.v. injection of PGE, might be caused by the slow leakage of PGE, from the cerebrospinal fluid into the general circulation, a venous infusion of 3 pg,/kg PGE, was given over 20 min. Under these conditions, the observed peak i~~bition was much lower than that observed after i.c.v. injection of 3 ag/kg PGE, and was comparable to the inhibition observed after bolus injection Of the same dose of PGE, (see ‘ v.inf.’ in fig. lc). 3. i.2. Enpros~i~ The i.c.v. injection of enprostil (0.001-0.1 pg/kg) produced a dose-related and long-lasting inhibition of gastric acid output, which reached 97 f 1.1% after 0.1 pg/kg and was still observed after 180 min (fig. 2a). The EDso was calculated as 0.33 ng/kg from the doseresponse curve (fig. 2~). The i.v. injection of enprostil (0.0001-l pg/kg) produced dose-related and durable decreases in gastric acid output, which lasted for more than 180 min after 1 gg/kg of i.v. enprostil (fig. 2b). The maximum decrease was 99 f 0.9% after 1 pg/kg. The calculated ED,, was 3.8 ng/kg. and the ratio i.v. E4Ji.c.v. ED5,, was 11 (table 1). 3.2. Ejfect of prostanoids gastric acid secrehon

120

0

20

40

.OOOl ,001

60

-01

80 100 120140

,l

160180 Minutes

100 1 10 Enprostil(fig/kg)

Fig. 2. Effect of i.c.v. (a) and i.v. (b) injections of enprostil (arrow) on basal gastric acid output in conscious chronic fistula rats. Presentation of results and abbreviations as in fig. 1. Means of seven to nine rats per group, The overall average basal acid output in all 10 groups WAS 35.2 f 2.0 ~mol/ZO min.

gastric acid output was largely suppressed by the dose of 0.3 yg/kg, whereas the higher dose of 1 pg/kg totally suppressed pentagastrin-stimulated secretion and also reduced basal secretion (fig. 3a). The calculated EDS0 was 0.07 pg/kg (table 1). The i-v. injection of PGE, (lo-100 pg/kg) induced only a short-lasting just significant reduction in gastric acid output, which peaked at 32.5 + 10.0% after 100 pg/kg i.v. PGE,. The ETP,, could not be calculated, but was estimated to be greater than 100 lug/kg (fig. 3). The ratio of the i.c.v./i.v. potencies was greater than 1000 (table 1).

on pentagastrin-stimulated

3.2.1. PGE, The i.c.v. injection of PGEz (0.01-l pg/kg) induced a dose-related decrease in pentagastrin-stimulated gastric acid output (fig. 3a). Pentagastrin-stimulated

3.2 2. Enprostil The i.c.v. injection of enprostil (0.001-l pg/kg) induced a dose-related, large and long-lasting inhibition of pentagastrin-stimulated gastric acid output (fig. 4a). After 0.1 @/kg of i.c.v. enprostil, the pentagastrinstimulated increase in gastric acid output over the basal

3.3. Effect of prostanoids on histamine-stimulated gastric acid output

&;; -20

0

20

40

80

100 120 140 160 180

80

100‘120 140 160180

60

I."FGEP

40

3.3.1. PGE, The i.c.v. injection of PGE, (0.01-l pgjkg) decreased histamine-stimulated gastric acid output, with a smaller efficacy and potency than it inhibited pentagastrin-stimulated gastric acid output (fig. 5a). The calculated EDso was 0.33 pg/kg (table 1). The i.v. injection of PGE, had little effect on histamine-stimulated gastric acid output, and it was necessary to increase the dose of PGE, to 100 pg/kg to cause a significant and short-lasting inhibition of histamineinduced gastric acid output (fig. 5b). The log(dose)-response curve was steep, and the EDS0 was approximately 95 pg/kg (fig. 5).

20

od..-... -20

0

20

40

‘.‘-I

60

*-I.’

’

Minutes

Basal -20 .OOOl

,001

.Ol

.l

1

10

0

20

40

60

80 103120140

100

Minutes

PGE2(@kg) Fig. 3. Effect of i.c.v. (a) and i.v. (b) injections of PGE, (arrow) on pentagastxin-stimulated gastric acid output in conscious chronic fistula rats. Pentagastrin (PG. 1 pg/kg per h) was infused i.v. for 3 h. The results, expressed as percent of the response to PG measured between 20 and 40 min of infusion, are meaus of seven to eight animals per group. To simplify the graphs, the S.E.M. are indicated only for the groups receiving PG ‘alone’ (i.e.+ an i.c.v. or i.v. injection of the prostanoid vehicle), and three doses only are shown in graph (a). The dose-response curves (c) were drawn from the peak inhibitions observed in each group. The overall average PG-stimulated gastric acid output in all 11 groups was 108.3+5.6 pmol/20 min. The average observed basal acid output is indicated on graph (c) (31.2& 1.2%). * PCO.05, ** P