Analysis of the excitatory motor response evoked by nicotinic and muscarinic blockade of ovine small bowel

Analysis of the excitatory motor response evoked

by nicotinic and muscarinic blockade of ovine

small bowel

Krzysztof W. Romañski

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Abstract:

It has been reported that the administration of anticholinergic drugs evokes inhibitory and excitatory responses, but the precise char-

acter of the latter has not yet been defined. This study was thus devoted to analyzing its occurrence following various doses of hexa-

methonium (Hx) and atropine (At) administration in the course of different phases of the small-intestinal migrating motor complex

(MMC) in fasted and non-fasted sheep and to further characterize the excitatory responses in comparison with individual phases of

the MMC. Two basic types of excitatory response were found. In the course of chronic experiments, various doses of Hx and At

evoked rebound excitation (RE, i.e., irregular contractions or spike bursts evoked in response to the anticholinergic drug) alternating

with phase 3-like activity (not the organized phase 3 of the MMC or its parts). The intensity of these changes varied and was related

to the drug dose. Thus intense and non-intense RE activity were distinguished. In non-fasted sheep, these alterations were slightly

less pronounced than in fasted animals. When the drug was given during phase 1 of the MMC, RE did not occur or was greatly re-

duced and its arrival was delayed. Hx triggered RE mostly in the duodenum, while the action of At was most effective in the jejunum.

It is concluded that Hx and At initially hamper small-intestinal motility and just after that evoke a secondary stimulatory response,

i.e., phase 3-like activity and RE of different intensity, duration, and repeatability in fasted and non-fasted sheep. These stimulatory

effects may resemble unorganized phases of the MMC.

Key words:

sheep, duodenum, jejunum, anticholinergic drugs, rebound excitation, phase 3-like activity, migrating myoelectric complex

Introduction

The cholinergic system undeniably plays a leading

role in the regulation of gastrointestinal motility [9]. It

participates in the control of peristaltic contractions

and motor patterns, including the well-described mo-

tility pattern termed the migrating myoelectric or mo-

tor complex (MMC). This pattern exhibits cyclical

character and comprises three or four consecutive

phases. During phase 1 of the MMC, no contractions

can usually be observed. Phase 2 of the MMC is char-

acterized by a gradually increasing incidence of spike

bursts or contractions and is terminated by the most

intense and propagated spiking or contractile activity,

i.e., phase 3. Phase 4 of the MMC is not always re-

corded and afterwards phase 1 of the subsequent

MMC episode arrives [11]. The MMC cycle is in-

duced principally by intramural neurohormonal

mechanisms [19] and it is possible that the initiation

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of its most characteristic phase 3 is responsible for the

occurrence of the whole MMC cycle [41]. In rumi-

nants, unlike monogastrics, the MMC cycle is ob-

served not only during the interdigestive state, but

also in non-fasted animals and even after feeding

[36]. The mechanisms controlling the arrival of the

MMC are less known in these species, although the

role of the cholinergic system also seems to be cru-

cial. It was found that the administration of anticho-

linergic drugs inhibits ovine small-intestinal motility

and, after the inhibitory period, rebound excitation

(RE) can occur [8, 33, 45]. Its character has not yet

been entirely defined. It was also found that blockade

of the cholinergic receptors might evoke a premature

phase 3 of the MMC, as observed in dogs following

local administration of hexamethonium (Hx) and atro-

pine (At) [47]. Therefore, it is possible that the intra-

venous administration of the principal anticholinergic

drugs may induce phase 3 or phase 3-like activity also

in sheep. Thus the initiation of RE by anticholinergic

drugs requires further investigation to characterize

precisely the excitatory myoelectric and motor re-

sponse to cholinergic blockade and also to analyze

and compare the unspecific RE and phase 3 of the

MMC or phase 3-like activity evoked by anticholiner-

gic drug administration. This was the aim of the pres-

ent experiments.

Materials and Methods

Surgical preparation of sheep

Eight healthy adult rams of the Polish Merino breed

each weighing 38–44 kg were used. The rams were

fed with good quality hay, 1000 g daily, and a special

grain mixture (CJ mixture, Dolpasz, Wroc³aw), 200 g

daily, and then fasted for 22–24 h before surgery.

Drinking water was not limited. The animals were

prepared for the surgery by slow intravenous admini-

stration of 25% ethyl alcohol at a dose of 3.5–4.0 ml kg–1

body mass preceded by 2.5 ml of Combelen (Bayer)

given intramuscularly [31]. Fifty to sixty ml of 1%

Polocainum (Polfa) was injected locally, around the

incision area. After the general and local anesthesia,

right lateral laparotomy was performed under aseptic

conditions and four Teflon-coated bipolar platinum

electrodes were implanted at the serosal side to the

duodenal bulb, 6 cm distally to the pyloric ring (one

electrode), to the distal duodenum, 50 cm below the

bulbar electrode (one electrode), and to the jejunum,

200 and 300 cm from the duodenal electrode (two

electrodes). Furthermore, in four of these rams, strain

gauge force transducers, calibrated before implanta-

tion, were sewn just near the duodenal electrode.

Other details of this procedure have been described

elsewhere [31, 34]. Marked electrode wires were ex-

teriorized over the skin through the stab incision, sol-

dered to a plug, and fixed to the skin. The animals

gradually returned to normal feeding within 2–3 days.

The skin sutures were removed after 8–10 days fol-

lowing the surgery.

Experiments

A total of 480 experiments lasting 5–8 h each were

conducted strictly according to the schedule approved

by the Local Ethics Committee. In the course of each

experiment, the animals were deprived of food and

drinking water. Myoelectric and motor activities were

continuously recorded using a multichannel electro-

encephalograph (Reega Duplex TR XVI, Alvar Elec-

tronics, Montreuil) adapted for motor and myoelectric

recordings. Before the experiments on fasted animals,

the rams were deprived of food for 48 h. In the course

of the experiments on non-fasted animals, food was

removed from the cage 12 h before the experiment.

At least two consecutive phases 3 of the MMC

were recorded each time in the course of control ex-

periments and before anticholinergic drug administra-

tion. The experiment was performed only when motil-

ity during the preceding control recording was nor-

mal. During control recordings, 5 ml of 0.15 M NaCl

were injected for 30 s into the jugular vein through

a thin polyethylene catheter introduced before each

experiment. The saline injections were performed in

the course of phase 2 (2b) of the MMC about 5 min

after its start in the duodenum. In the course of the ba-

sic experiments, slow intravenous injections of Hx

(bromide salt; Sigma, St. Louis, MO, USA) at small,

moderate, and high doses, i.e., 1, 2, or 5 mg kg–1, At

(sulfate salt; Polfa, Warszawa) at doses of 0.02, 0.1,

0.5, and 1.5 mg kg–1 (i.e., small, moderate, high, and

the highest dose), and three combinations of these

drugs (Hx 1 mg kg–1 plus At 0.02 mg kg–1, Hx 2 mg

kg–1 plus At 0.1 mg kg–1, and Hx 2 mg kg–1 plus At

0.5 mg kg–1 of body weight) were applied. The high-

est doses of the single drugs and drug combinations

were administered for 60 s and the remaining doses of

�� 293

Cholinergic blockade and excitatory motor response��

the drugs for 30 s. Both at the beginning of drug ad-

ministration and at the end of the injections the rates

were gradually increased and decreased, respectively.

The type and dose of the individual drug or saline

injection was designed in random order. The injec-

tions of the drugs were started at the same MMC peri-

ods as the infusions of saline. No more than two experi-

ments were performed weekly in one sheep. Neither food

nor water was offered in the course of the experiment.

After cessation of all the experiments, the animals were

sacrificed and the positions of the electrodes and the

strain gauge force transducer were further confirmed.

Data analysis

The MMC cycles and their individual phases were

identified in the duodenum according to the criteria

proposed by Code and Marlett [11] with some own

modifications [30, 31]. The most characteristic phase

3 of the MMC was identified as an uninterrupted

spike burst series of maximal or submaximal ampli-

tude, lasting at least two min in the duodenal bulb and

at least three min in lower small-intestinal segments.

They were preceded by phase 2 and followed by

phase 4 of the same MMC cycle or directly followed

by phase 1 of the next MMC cycle. The normal phase

3 of the MMC was well organized and exhibited a mi-

gratory character. Since phase 2 is usually longer in

sheep than phase 1, the division of phase 2 into sub-

phases 2a and 2b of the MMC, suggested earlier by

Dent et al. [15], was performed according to more de-

tailed criteria [31].

The myoelectric recordings were analyzed by vis-

ual inspection and the character of the myoelectric re-

sponse to the anticholinergic drugs was evaluated. In

the tracings, RE, as interrupted or dispersed spike

burst series, was distinguished from phase 3-like ac-

tivity (Figs. 1–3). Two basic types of RE were also

distinguished. An uninterrupted strong (maximal or

submaximal spike burst amplitude and duration) spike

burst series, each lasting at least one minute, was

qualified as phase 3-like activity. Excitatory activity

with lower spike burst incidence occurring after anti-

cholinergic drug administration and before the arrival

of the normal phase 2 and phase 3 of the MMC was

included formerly in the RE. Maximal or submaximal

amplitude and duration of the spike bursts forming

phase 3 of the MMC during the control period were

considered as the greatest amplitude and duration ob-

served in the course of a given experiment. A lack of

RE or phase 3-like activity within 30 min after ad-

ministration of the anticholinergic drug was qualified

as absence of an excitatory response. When the nor-

mal phase 2 of the MMC, the minute rhythm, or nor-

mal phase 3 of the MMC arrived within 30 min fol-

lowing the cholinergic blockade, it was also inter-

preted as a lack of excitatory response to Hx or At.

Summed durations of spike burst series forming

the different types of excitatory response to choliner-

gic blockade were calculated. Furthermore, the maxi-

mal amplitudes of the spike bursts (mean amplitude of

five spike bursts exhibiting the highest amplitude dur-

ing the relevant period) forming the spontaneous

phases 2a, 2b, and 3 of the MMC before cholinergic

blockade and the spike bursts forming phase 3-like

activity and intense and non-intense RE triggered by

the administration of anticholinergic drugs, were cal-

culated according to the method proposed previously

[28]. Maximal durations of the spike bursts (mean du-

ration of the five spike bursts of longest duration in

the course of the relevant period) forming the sponta-

neous phases 2a, 2b, and 3 of the MMC before cho-

linergic blockade and the spike bursts forming phase

3-like activity and intense and non-intense RE (also

called RE), were also calculated. The maximal ampli-

tudes and durations of duodenal phasic contractions

of the same events indentified as previously described

[32] were calculated as well. Furthermore, the dura-

tions of the first representative uninterrupted spike

burst and contraction series forming MMC phases 2a

and 2b, the entire phase 3, and observed excitatory

events were calculated.

Statistical analysis

All the values were tabulated and presented as the

means and standard deviations. Statistical signifi-

cance, considered significant when p < 0.05, p < 0.01,

or p < 0.001, were calculated using Student’s t-test for

paired values and were preceded by one-way analysis

of variance [44].

Results

The administration of 0.15 M NaCl in the course of

phases 1, 2a, or 2b of the MMC in fasted and non-

fasted sheep evoked no effect and these results are not

294 ��

shown. The motor recordings obtained from the duo-

denal strain gauge force transducer were analogous to

the duodenal myoelectric recordings (Figs. 1–3).

Slow injections of all the doses of both anticho-

linergic drugs inhibited spiking activity for a couple

of minutes and the inhibitory period was followed by

periods of phase 3-like activity and/or by intense or

non-intense RE (Figs. 1–3). These effects arrived as

single or repeated series of spike bursts or contrac-

tions and as many as 15 recurring patterns were ob-

served, maximally after the highest doses of Hx and

At or following the combination of their highest

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doses. After administration of the anticholinergic

drugs, the postinhibitory excitation was often com-

plex and not uniform since phase 3-like activity could

be distinguished from RE (Figs. 1–3). While phase 3-

like activity resembled the spontaneous phase 3 of the

MMC, the spike burst series forming the intense RE

roughly resembled phase 2b of the MMC and those

forming the non-intense RE were not much different

from phase 2a of the MMC (Figs. 4–7). However, in

the duodenal bulb, phase 3 of the MMC was often not

fully developed and was therefore often different

from phase 3-like activity in this intestinal segment

(Figs. 6, 7). In the duodeno-jejunum, phase 3-like ac-

tivity, contrary to the spontaneous phase 3 of the

MMC, was often interrupted (Fig. 3).

In non-fasted animals, administration of the lowest

dose of Hx induced phase 3-like activity and RE of

the shortest duration, while the two highest doses of

the drug were most effective (data not shown). The

summed (total) durations of the given excitatory

changes, i.e., the sum of all repetitions following sin-

gle drug administration (or drug combination), are

also presented. A similar dose-effect relationship was

observed following At administration, although the

changes evoked by At were less pronounced than

those following Hx. Combinations of At plus Hx

(data not shown) triggered a greater effect than the

same doses of the drugs when given alone. The effects

evoked by At administration and observed in the duo-

denal bulb and duodenum were usually less pro-

nounced than those observed in the jejunum, while

these relationships following Hx administration were

the opposite (Fig. 7). The summed durations of the

phase 3-like activity and RE (intense and non-intense

RE) induced by Hx and At administration in non-

fasted sheep were usually slightly shorter than those

observed in fasted animals, although the dose-

response character of this stimulatory effect was not

abolished (data not shown).

When the effect of anticholinergic drugs on the

summed duration of intense RE events was analyzed,

the effect of At administration in the duodenal bulb

and duodenum was not much less pronounced than

that of Hx (Fig. 7). The differences in the duration of

intense RE (the first representative spike burst and

contraction series, Fig. 6) obtained following anticho-

linergic drug administration in the course of various

phases of the MMC were not as evident as those in the

duration of the whole (summed) RE period (Fig. 7).

The maximal amplitudes of the spike bursts form-

ing phase 3-like activity and intense and non-intense

RE were usually higher than those of the spike bursts

forming the spontaneous phases 3, 2b, and 2a, respec-

tively (Fig. 4). Furthermore, these values were greater

following Hx than At administration. There were also

significant differences in the spike burst amplitudes of

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phase 3-like activity (highest spike burst amplitudes),

intense RE (moderate spike burst amplitudes), and

non-intense RE (lowest spike burst amplitudes) fol-

lowing a given dose of the anticholinergic drug. Simi-

lar differences in phasic contraction amplitudes were

observed in the mechanical activity recordings (Fig.

4).

The maximal durations of the spike bursts and pha-

sic contractions were lowest during the spontaneous

phase 2a of the MMC and during non-intense RE

(Fig. 5). The durations of the spike bursts and phasic

contractions forming phase 3-like activity were

slightly longer than those forming the spontaneous

phase 3 of the MMC. There were similar differences

in phase 2b vs. intense RE and phase 2a of the MMC

vs. non-intense RE. There were no marked differences

between the results of At and Hx administration at the

moderate doses regarding these parameters (Fig. 5).

The effects of the remaining doses of anticholinergic

drugs on spike burst and phasic contraction amplitude

and duration are not shown here.

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Discussion

The results indicate that the character of the postin-

hibitory excitatory response to the slow injection of

Hx and At can be complex. The almost regular series

of spike bursts of maximal or submaximal intensity,

lasting about one minute or more, can be interpreted

as phase 3-like activity, while the remaining, less

regular and intense spiking activity of lower spike

burst frequency can be called RE. However, it re-

mains uncertain whether the same rule can be applied

in relation to the duodenal bulb and duodeno-

jejunum. In sheep, phase 3 of the MMC mostly origi-

nates from the duodenal bulb and the duodenum, but

not from the stomach [37]. Thus the duration of phase

3 of the MMC starting from the duodenal bulb is often

shorter than one minute and occurs when phase 3 in

this region is practically not fully developed. Phase 3

should usually last not less than two minutes [25].

The myoelectric response of the duodenal bulb to Hx

and At administration differed from that in the

duodeno-jejunum. In the ovine duodenal bulb the

slow waves are not always present and the frequency

of the spike bursts is lower than in the remaining part

of the duodenum. The spike burst amplitude is usually

greater than in the lower parts of the small bowel.

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However, in the present study there were no marked

differences in excitatory response to the anticholiner-

gic drugs between the duodenal bulb and lower small

intestine since this response was not spontaneous.

In dogs, administration of At inhibited small bowel

motility apparently without RE, while after Hx the

relatively short inhibition was followed by an excita-

tory response [18]. In rabbits, At was able to evoke

the rebound excitation [3]. The occurrence of RE after

the administration of anticholinergic drugs in sheep

was not always reported explicitly [7, 38], but some

other studies further clarified this problem [8, 33]. It

was also reported that the administration of anticho-

linergic drugs may abolish phase 3 of the MMC and

lower the site of its origin [35]. The principal anticho-

linergic substances can thus induce inhibitory and ex-

citatory changes in gastrointestinal motility, at least in

sheep. The present study indicates that the excitatory

changes comprise either a not well-organized phases

3 of the MMC, called phase 3-like activity, or a less

regular excitatory response called the RE. Further

analysis led to differentiation of the RE into intense

RE and non-intense RE. The amplitudes and dura-

tions of the spike bursts forming the RE were not

much different from those of the normal phase 3 of

the MMC [29], since in this study, although the whole

period of postinhibitory spiking activity was ana-

lyzed, phase 3-like activity was never distinguished

from RE. As observed here, the phase 3-like activity

closely resembled the non-migrating phase 3 of the

MMC. While an intense RE may resemble phase 2b

and a non-intense RE may resemble phase 2a and the

inhibitory period is similar to phase 1 of the MMC,

changes evoked by Hx and At could also be inter-

preted as disorganized or even mixed phases of the

MMC cycle. Generally, not all the series of spike

bursts can be interpreted as phase 3, phase 3-like ac-

tivity, or intense RE. During vomiting and in some

other situations, the duration of these spike bursts

(called intense spike bursts) is longer than during

phase 3 of the MMC and are thus not linked with the

slow waves [23]. This was not the case here.

The mechanism evoking postinhibitory excitation

in response to anticholinergic drug administration re-

mains unknown. Considering certain similarities be-

tween the myoelectric changes evoked by cholinergic

blockade and the regular phases of the MMC, it can

be speculated that anticholinergic drugs disorganize

rather than inhibit the already existing MMC cycle. It

is less probable that they evoke a disorganized MMC

cycle. It is believed that the role of the enteric nervous

system in the control of small-intestinal MMC is

greater than that of the vagus nerve. Intramural neu-

rons are primarily responsible for the generation and

migration of the spontaneous MMC [40]. While the

M3 receptor subtype, located principally on smooth

muscles, is responsible for triggering the spiking ac-

tivity, the M1 receptor subtype may stimulate the

propagated contractions and is responsible for the ini-

tiation of the migrating phase 3 of the MMC [42]. The

available data suggest that the M1 receptor subtype

are autoreceptors located presynaptically on choliner-

gic neurons in the enteric nervous system along with

the M2 and M4 receptor subtypes [10, 24]. That is

why pirenzepine and telenzepine, drugs exhibiting the

greatest affinity to the M1 receptor subtype, may in-

duce a premature (also migrating as a normal phase 3)

phase 3 of the MMC [26, 42]. Since At blocks the

muscarinic M1 receptors (but also other muscarinic

receptor subtypes), it could mimic this action of

pirenzepine and telenzepine, although not completely

because its affinity to the muscarinic receptor sub-

types is different. Nevertheless, Tohara et al. [47]

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found in dogs that Hx and At also evoke the migratory

phase 3 of the MMC; it is thus possible that the cho-

linergic M1 receptor subtype participates in the exci-

tatory motor response to both these drugs. However,

the mechanism of action of Hx seems to be somehow

different from that of At. Hx hampers stimulatory in-

put through a particular unit of the neuronal nicotinic

postsynaptic ganglionic receptor, but it also acts on

the presynaptic nicotinic autoreceptor located on my-

enteric ganglia [24]. Thus the administration of anti-

cholinergic drugs seems to act locally, disorganize the

MMC phase, and dissociate the coordination between

phase 2 and phase 3 of the cycle. The precise roles

and locations of the cholinergic receptors in the ovine

small bowel are unknown, but it can be assumed that

they are similar to those in monogastric species.

RE is frequently observed in various physiological

studies and occurs as soon as the action of the physio-

logical stimulus is hampered and an inhibitory re-

sponse then develops [2]. This can be called the pri-

mary response, while the secondary response is

stimulatory. The occurrence of RE is thus interpreted

as a biphasic response, and sometimes even a tripha-

sic response is observed [3, 20]. In the present study,

the higher doses of Hx and At evoked inhibitory re-

sponses alternating with the stimulation of myoelec-

tric activity; this response therefore often exhibited

a multiphasic character. The precise mechanism respon-

sible for the postinhibitory excitation evoked after Hx

and At administration is still highly speculative, al-

though there are several regulatory pathways which

may participate in the generation of such response. It

is possible that Hx and At can act peripherally via dif-

ferent acetylcholine receptors or via the same receptor

in different types of neurons (Fig. 8). The myoelectric

response to the stimulation of one type or subtype of

cholinergic receptor can be inhibitory and of another

type or subtype stimulatory. It is well known that Hx

acts principally through nicotinic receptors present

within the autonomic ganglia [43], but it has been re-

ported that Hx may also exhibit antimuscarinic activ-

ity [17, 49]. In turn, it is not clear whether At exhibits

any affinity to nicotinic receptors, but it has been re-

ported that there are some functional links between

nicotinic and muscarinic receptors [6]. Both an inhibi-

tory and a subsequent stimulatory response can also

be evoked by the action of the anticholinergic drug on

different types of neurons or by simultaneous action

on presynaptic and postsynaptic receptors [4, 16]. As

mentioned, Hx can block both nicotinic hetero- and

autoreceptors and thus evoke inhibitory and stimula-

tory responses. The inhibitory (primary) response pre-

cedes the stimulatory (secondary) response apparently

due to the faster and stronger inhibition of postsynap-

tic nicotinic receptors than of presynaptic nicotinic re-

ceptors. Then, the secondary response develops and

can last even longer than the primary response. The

action of At can be similar and can occur at a lower

(deeper) level of the enteric nervous system, namely

the postganglionic. Inhibition of the cholinergic M3

receptor subtype, located primarily on smooth muscle

cells and possibly also on myenteric neurons, first

evokes an inhibitory effect. Since, as stated above, At

also inhibits muscarinic autoreceptors, a stimulatory

effect can occur thereafter. These mechanisms might

thus explain the similarities between the biphasic ac-

tions of Hx and At on small-intestinal motility medi-

ated by different cholinergic receptors. However, the

contribution of other mechanisms cannot be excluded.

The inhibition of cholinergic receptors by the admini-

stration of the anticholinergic substances may stimu-

late noradrenergic inhibitory nerves [13]. These and

other observations suggest that substantial regional

differences in the distribution of cholinergic receptors

exist in the gastrointestinal tract.

Another possibility of evoking RE is that Hx and,

especially, At can act both centrally and peripherally

and that these responses are usually opposite. It is

known that gastrointestinal motility is regulated cen-

trally and this also concerns the MMC pattern [21].

However, central regulation of the MMC pattern ap-

pears to be of secondary importance. The role of the

vagus nerve seems to be significant in the control of

gastrointestinal spontaneous contractions and of the

fed pattern [12, 14]. However, it does not have

a marked role in the control of the MMC, at least in

dogs and sheep [1, 36]. The involvement of neuroen-

docrine reactions in the myoelectric small-intestinal

response to anticholinergic drugs represents the next

possibility of RE induction [48]. Several stimulatory

and inhibitory neuromediators and neuromodulators

have been identified to exert a regulatory effect on

gastrointestinal motility, also comprising the MMC

cycle, and their action was invariably mediated by

cholinergic neurons. Stimulation or inhibition of cho-

linergic receptors located pre- or postsynaptically

may affect the release of these substances both from

neurons and from endocrine cells in the gastrointesti-

nal tract and may modulate the motor response. At ad-

ministration can thus block the muscarinic receptors

300 ��

located on endocrine cells in the gastrointestinal mu-

cosa, thereby affecting gut hormone release and possi-

bly inducing the motility alterations. At also exhibits

a central effect and might thus contribute to retarda-

tion of phase 3 of the MMC [21]. Therefore, there are

several possible mechanisms evoking the RE and it

can be concluded that the regulatory mechanisms are

programmed to evoke the opposite type of response

when the preceding response is prolonged. There are

also functional links between neural and neuroendo-

crine mechanisms controlling gastrointestinal motil-

ity, also in ruminant species. They appear to belong to

a group of intestinal adaptation mechanisms prevent-

ing overexertion of the smooth muscles.

Excitatory response to the administration of the an-

ticholinergic drugs was observed not only when the

drugs were given during phase 2, but also during

phase 1 of the MMC. This response was thus delayed

and slightly reduced. Therefore, the refractory period

[39] did not influence it considerably. It was postu-

lated that At reduces the duration of the refractory pe-

riod; the same can be true for Hx and facilitate the ar-

rival of RE. Simultaneous administration of Hx and

At evoked an additive effect. A similar effect was ob-

tained previously [22, 34]. Since nicotinic receptors

are located on intramural ganglia [43] and muscarinic

receptors are located on postganglionic neurons (as

pre- and postsynaptic receptors) or on smooth mus-

cles [10], during blockade of nicotinic receptors only

pre- and postsynaptic neuronal receptors are blocked

at the same time. The addition of At may further

block the smooth muscle receptors and this hypothe-

sis may explain why the effects of both these drugs

are additive, although some other explanations cannot

be excluded.

Evident but not pronounced differences were ob-

served in the results of experiments performed on

fasted and non-fasted animals. Therefore, they do not

seem to be very important. The most probable expla-

nation for these differences is that digestive hormones

(gastrin, cholecystokinin, secretin, glucagon, neuro-

tensin) may hamper the myoelectric response to anti-

cholinergic drug administration, as it was reported in

monogastric animal species [5, 27] as well as in sheep

[46]. In non-fasted sheep there is almost a constant

flow of digesta from the rumen towards the small in-

testine [36] and there is practically no interdigestive

period. Thus the gastrointestinal endocrine cells are

continuously stimulated by digesta and gut hormones

are permanently released into the blood stream and lu-

men. They might affect not only secretory processes,

but also gastrointestinal motility.

Finally, it can be summarized that Hx and At exert

inhibitory and excitatory myoelectric and motor re-

sponses in the ovine small bowel. The excitatory re-

sponse was not uniform and phase 3-like activity and

intense and non-intense RE, resembling unorganized

phases of the normal MMC cycle, could be distin-

guished. There were substantial differences in the

character of this response in the duodenal bulb, duo-

denum, and jejunum. The type of drug, drug dose,

MMC phase, and feeding conditions were the impor-

tant factors determining the character and strength of

this postinhibitory stimulatory response. These results

might thus help in increasing our understanding of the

generation of the MMC and can serve as a model for

the study of small-intestinal motility in sheep.

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Analysis of the excitatory motor response evoked by nicotinic and muscarinic blockade of ovine small bowel