Activity profiles of dalargin and its analogues in mu-, delta- and kappa-opioid receptor selective bioassays.

1. To elucidate the structural features ensuring action of [D-Ala2, Leu5]-enkephalyl-Arg (dalargin), a series of dalargin analogues were tested for their effectiveness in depressing electrically-evoked contractions of the guinea-pig myenteric plexus-longitudinal muscle preparations (mu- and kappa-opioid receptors) and the vasa deferentia of the hamster (delta-opioid receptors), mouse (mu-, delta- and kappa-opioid receptors), rat (similar to mu-opioid receptors) and rabbit (kappa-opioid receptors). The naloxone KB values in the myenteric plexus were also obtained. 2. [L-Ala2]-dalargin was 19 times less potent than dalargin, and its pharmacological activity was peptidase-sensitive. The ratio of delta-activity to mu-activity for [L-Ala2]-dalargin was 6.78, and KB was 7.9 nM. This emphasizes the role that D-configuration of Ala2 plays in determining the active folding of dalargin molecule as well as in conferring resistance to peptidases. 3. [Met5]-dalargin was equipotent to dalargin in the myenteric plexus, but was more potent in the vasa deferentia of hamster and mouse (KB=5.5 nM). Leu5 and the interdependence of Leu5 and D-Ala2 are of importance for the selectivity of dalargin for mu-opioid receptors. 4. Dalarginamide was more potent and selective for mu-opioid receptors than dalargin, whilst dalarginethylamide, though equipotent to dalarginamide in the myenteric plexus, was more potent at delta-opioid receptors (KB=5.0 nM). [D-Phe4]-dalarginamide and N-Me-[D-Phe4]-dalarginamide were inactive indicating the contribution of L-configuration of Phe4 to the pharmacological potency of dalargin. 5. N-Me-[L-Phe4]-dalarginamide possessed the highest potency and selectivity for mu-opioid receptors (the ratio of delta-activity to mu-activity was 0.00053; KB=2.6 nM). The CONH2 terminus combined with the N-methylation of L-Phe4 increased the potency and selectivity of dalargin for mu-opioid receptors.

Comparison of gamma-aminobutyric acid effects in different parts of the cat ileum.

The effects of gamma-aminobutyric acid (GABA) and those of a GABA(A) (muscimol) and a GABA(B) (baclofen) receptor agonists were determined on the spontaneous activity of longitudinally or circularly oriented preparations (segments) isolated from terminal, proximal and distal parts of the cat ileum. GABA applied at 1 microM to 2 mM caused dose-dependent biphasic changes (relaxation and contraction) in spontaneous activity of the longitudinal and circular layers in the terminal and distal parts of the cat ileum and monophasic changes (contraction) in the proximal part. The potency of GABA to elicit relaxant and/or contractile effects in different parts of the ileum showed a proximal-to-terminal increasing pattern. In the longitudinal layer of the distal and terminal ileum, muscimol (100 microM) mimicked the relaxation phase of the GABA effect, while baclofen (100 microM) simulated the contractile phase. Bicuculline, atropine and tetrodotoxin abolished GABA- and muscimol-induced relaxation and suppressed, but failed to prevent GABA- and baclofen-induced contractions. In addition, 2-hydroxysaclofen antagonized the baclofen-induced contractile effect, reduced the GABA-induced contractile phase but failed to prevent GABA- and muscimol-induced relaxation. In the circular layer of the same regions, muscimol mimicked the biphasic GABA effects, while baclofen was without effect. Bicuculline, atropine and tetrodotoxin completely prevented the GABA- and muscimol effects, while 2-hydroxysaclofen failed to antagonize them. In the longitudinal and circular layers of the proximal ileum, muscimol (100 microM) exerted a 'GABA-like' transient contractile effect, while baclofen (100 microM) did not elicit any response. Bicuculline, atropine and tetrodotoxin antagonized the GABA- and muscimol-induced contractile responses of longitudinal and circular layers, while 2-hydroxysaclofen was ineffective. The results suggested that the inhibitory and/or excitatory action of GABA on cholinergic transmission in different regions of cat ileum varies along an increasing gradient towards the terminal ileum and is mediated by GABA(A) and GABA(B) receptors in the terminal and distal ileum and by GABA(A) receptors in the proximal ileum.

Pattern of nonadrenergic, noncholinergic responses during short- or long-lasting electrical stimulation in guinea-pig ileum.

1. The pattern of responses of longitudinally oriented guinea pig ileum organ bath preparations was studied during short- (1-5 sec) or long-lasting (20 sec) electrical field stimulation (EFS, 0.8 msec, 40 V, 1-20 Hz). 2. In the presence of phentolamine (5 microM), propranolol (5 microM), and atropine (3 microM), the EFS elicited nonadrenergic, noncholinergic (NANC), tetrodotoxin (0.3 microM)-sensitive responses. 3. The 1-sec EFS evoked relaxation. The response to 5-sec EFS consisted of relaxation followed by twitch, whereas relaxation, twitch and tonic contraction characterized the NANC response to 20-sec EFS. The maximum relaxation was observed at 10-Hz short- or long-lasting EFS. 4. Both N-G-nitro-L-arginine (L-NNA, 0.1-0.5 mM) and apamin (1-5 microM) concentration dependently inhibited the relaxation of the NANC response to 10-Hz 20-sec EFS. During L-NNA treatment, the twitch and the tonic contractions were increased. The inhibitory effect of L-NNA was reversed by L-arginine (0.1-0.5 mM) but not by D-arginine. Sodium nitroprusside (1-10 microM) was without effect. 5. AP 13.2 ACOH (0.1 microM), a blocker of Substance P receptors, inhibited the twitch and the tonic contractions. The contractions were decreased after desensitization of purinoceptors by ATP and in the presence of the GABA(A) receptor antagonist bicuculline (30 microM). 6. Depending on the EFS duration, a subsequent occurrence of relaxation and contractions characterized the NANC responses. It seems that relaxation is mediated by nitric oxide whereas Substance P and ATP are involved in the maintenance of the twitch and the tonic contractions. Nitric oxide appears to exert an inhibitory effect on the excitatory transmitters, whereas purinergic mechanism(s) could modulate the nitric oxide-dependent relaxation.

Dependence of gamma-aminobutyric acid modulation of cholinergic transmission on nitric oxide and purines in cat terminal ileum.

The possible involvement of purines and/or nitric oxide (NO) in the gamma-aminobutyric acid (GABA)A receptor-mediated effects on the spontaneous activity of isolated preparations from longitudinal and circular muscles of cat terminal ileum was investigated. GABA had biphasic effects, which were neurogenic and muscarinic. ATP and adenosine dose dependently inhibited the activity of the muscles. A contractile response evoked by the nucleotide only was also observed. The effects of the purines were equipotent and resistant to Nomega-nitro-L-arginine (L-NNA), tetrodotoxin and to desensitization by alpha,beta-methylene adenosine 5'-triphosphate (alpha,beta-meATP), except for the contractile effect of ATP, which was abolished by alpha,beta-meATP. Pretreatment of the preparations with ATP or adenosine produced: (i) desensitization to the effects of the respective purinoceptor agonist only; and (ii) suppression of the GABA-induced responses of longitudinal and circular muscles. Hemoglobin and L-NNA greatly reduced or completely blocked the GABA(A)-induced relaxation and decreased the GABA(A)-induced contraction. Our results indicate that purines and NO, to a different extent, mediate the relaxant phase of the GABA effects in both layers. Interactions between muscarinic cholinoceptors and GABA-nitrergic pathway and a concomitant activation of postjunctional P1 and P2y purinoceptors are suggested to explain the prejunctional biphasic effects of GABA.

[Cys(O2NH2)2]enkephalin analogues and dalargin: selectivity for delta-opioid receptors.

To investigate the structure-activity relationships for potent and selective action of enkephalins at the delta-opioid receptors, two newly synthesized analogues, [Cys(O2NH2)2,Leu5]enkephalin and [Cys(O2NH2)2, Met5] enkephalin and the hexapeptide [D-Ala2,Leu5]enkephalyl-Arg (dalargin) were tested and compared with [Leu5]enkephalin and [Met5]enkephalin, for their effectiveness to inhibit electrically evoked contractions of the mouse vas deferens (predominantly enkephalin-selective delta-opioid receptors) and the guinea pig ileum (mu- and kappa-opioid receptors). The mouse vas deferens assays included evaluation of the effects of opioid agonists on the first, purinergic, and the second, adrenergic, components of electrically evoked biphasic responses (10 Hz and 20 Hz) and on ATP- or noradrenaline-evoked, tetrodotoxin-resistant responses. The opioids tested inhibited in a similar manner: (i) the purinergic and the adrenergic components of the electrically evoked contractions; and (ii) the ATP- and noradrenaline-induced postjunctional responses of the mouse vas deferens. Extremely low IC50 values (of 2-5 orders) were found for [Cys(O2NH2)2,Leu5] enkephalin, whose relative potency was between 239 and 1316 times higher than that of [Leu5]enkephalin. The order of potency for the other peptides in this tissue was: [Cys(O2NH2)2,Met5]enkephalin > [Leu5]enkephalin > dalargin > [Met5]enkephalin. The highest IC50 values in the guinea pig ileum assays, indicating the lowest affinity for mu-/kappa-opioid receptors, were obtained for the cysteine sulfonamide analogues, while dalargin showed a potency four times higher than that of [Met5]enkephalin. The order of potency in this tissue was: dalargin > [Met5]enkephalin > [Leu5]enkephalin > [Cys(O2NH2)2,Met5]enkephalin > [Cys(O2NH2)2,Leu5]enkephalin. The ratio, IC50 in guinea pig ileum: IC50 in mouse vas deferens, indicating selectivity of the respective peptide for delta-opioid receptors, was extremely high for [Cys(O2NH2)2,Leu5]enkephalin and especially for the adrenergic component of the responses. This ratio for [Cys(O2NH2)2,Met5]enkephalin was higher than the ratios for dalargin, [Leu5]enkephalin and [Met5]enkephalin, which were about 3 orders of magnitude lower. The results suggest that incorporation of hydrophilic Cys(O2NH2) in the enkephalin molecule greatly increases the potency and selectivity of the analogues at delta-opioid receptors, while both D-Ala2 substitution and lengthening of the peptide chain by Arg6 in the molecule of [Leu5]enkephalin decrease them.

Two types of functionally different GABAA receptors mediate GABA modulation of cholinergic transmission in cat terminal ileum.

1. The effects of GABA (1 microM-2 mM) on longitudinally or circularly oriented organ bath preparations of cat terminal ileum consisted of a relaxation phase with an inhibition of the rhythmic spontaneous phasic contractions, followed by a phase of contractions characterized by an elevation in basal tone and an increase in amplitude of the spontaneous phasic contractions. 2. Muscimol (100 microM), but not baclofen (100 microM), mimicked the relaxation phase of the response to applied GABA (100 microM) in all tissue preparations. In addition, muscimol induced a phase of contractile activity in the circular muscle layer whilst baclofen exerted a 'GABA-like' contractile effect on the longitudinal muscle layer. Bicuculline (30 microM) or picrotoxinin (30 microM) antagonized the GABA- or muscimol-induced relaxations in all preparations and decreased the GABA- but not the baclofen-induced contractions of the longitudinal muscle layer. 3. Tetrodotoxin (0.5 microM) or atropine (0.1 microM) prevented the bicuculline-sensitive phases of the GABA or muscimol effects on both muscle layers but not the contractile effect of baclofen on the longitudinal muscle layer. 4. The bicuculline-sensitive phases of the GABA effect on both muscle layers were almost completely eliminated by 1 nM pirenzepine. At this concentration pirenzepine did not affect the electrically-evoked cholinergic twitch contractions or contractile responses to applied acetylcholine of both muscle layers. 5. During electrically-evoked cholinergic twitch contractions of both muscle layers, GABA (100 microM) had an inhibitory effect. The inhibition occurred in the presence of pirenzepine (1 nM) but not of bicuculline (30 microM). 6. It is suggested that two types of functionally different bicuculline-sensitive GABAA receptors mediate an exitatory presynaptic and an inhibitory prejunctional action of GABA on the cholinergic transmission in cat terminal ileum.

Dalargin and [Cys-(O2NH2)]2 analogues of enkephalins and their selectivity for mu opioid receptors.

1. Effects of the enkephalins Met-enk (M) and Leu-enk (L), of two newly synthesized analogues--[Cys-(O2NH2)]2-Met-enk (CM) and [Cys-(O2NH2)]2-Leu-enk (CL)--and of a hexapeptide--D-Ala2-Leu5-Arg6 (Dalargin; DL) on the spontaneous and electrically stimulated activity were examined with respect to their selectivity for the mu opioid receptors in the longitudinal layer of guinea pig ileum. 2. M and CM exerted relaxing and contractile effects on the spontaneous contractile activity while L, CL and DL produced only relaxation. The order of potency towards the relaxatory phase was DL > M > CM > L > CL and towards the contractile phase CM > M. 3. The effects of enkephalins on the spontaneous activity were naloxone and TTX sensitive except for the contractile phase of M and CM which persisted in the presence of TTX. NO was not involved in the neurotransmission of the relaxatory responses, while the blockade of alpha and beta adrenoceptors showed the participation of adrenergic mechanisms. Relaxation and contraction induced by enkephalins could not be directly attributed to cholinergic neurotransmission. 4. The naloxone-sensitive and concentration-dependent inhibitory effects of enkephalins and their analogues on the electrically stimulated cholinergic contractions were established. The order of the relative potency of opioids was: DL-3.8; M-1.0; L-0.4; CM-0.01; CL-0.005. 5. These data indicated that the D-Ala2 substitution and lengthening of the peptide chain by Arg6 in the molecule of L increased the potency at the mu opiate receptors, while the substitution in position 2 with Cys-(O2NH2) in the molecule of M and L yielded a less potent and selective mu agonists.

Changes in the contractile responses to carbachol and in the inhibitory effects of verapamil and nitrendipine on isolated smooth muscle preparations from rats subchronically exposed to Pb2+ and Zn2+.

Male Wistar rats were exposed to Pb2+ or Zn2+ and to Pb2+ + Zn2+, receiving Pb(CH3COO)2 or/and ZnSO4 with drinking water for 30 days. Cumulative concentration-effect curves for carbachol were obtained in ileum and trachea isolated from control and heavy metal-treated rats. The effect of the Ca2+ channel blockers on the carbachol-induced contractions was studied by addition of increasing concentrations of verapamil or nitrendipine to the bath solution 20 min. prior to carbachol. The results showed that exposure of rats to heavy metals in doses which did not change the body weight and behaviour, altered the contractile responses to carbachol. The sensitivity to carbachol was higher in preparations from the ileum of Zn(2+)-exposed rats as compared to controls, while a tendency towards decreasing this sensitivity was observed in ileal preparations from Pb(2+)-treated animals. The concentration-effect curves for carbachol in ileal preparations from Pb2+ + Zn(2+)-treated rats did not differ from those in the preparations from untreated rats. The inhibitory effect of the Ca2+ channel blockers on the contractility of ileal and tracheal preparations from treated rats was weaker as compared to that in controls.

Opioid effects of short enkephalin fragments containing the Gly-Phe sequence on contractile responses of guinea pig ileum.

1. Effects of the fragments H-Gly-Phe-OH, H-Gly-Phe-NH2 or H-Gly-Phe-OMe on the electrically stimulated cholinergic contractions of the longitudinal layer in isolated guinea pig ileum and on the Morphine-, Met-enkephalin- or Leu-enkephalin-induced inhibition of these contractions were analyzed for opioid activity in respect to Gly-Phe sequence. 2. H-Gly-Phe-OH or H-Gly-Phe-NH2 exerted no effects, while H-Gly-Phe-OMe applied cumulatively (1 pM-1 mM), concentration-dependently reduced the contractions to electrical stimulation, the IC50 value being 1.96 +/- 0.06 microM. Naloxone (1-5 microM) did not reverse the H-Gly-Phe-OMe effects. 3. H-Gly-Phe-OMe at single concentrations (1-10 microM) significantly decreased the maximum inhibition produced by cumulatively added (0.1 nM-100 microM) morphine, Met-enkephalin or Leu-enkephalin. The regression lines for the opioids were shifted to the right but not always in a parallel fashion; the IC50 values were higher as compared to the controls and lower as compared to the IC50 values after naloxone. 4. The pA2 value for H-Gly-Phe-OMe with respect to morphine (6.43 +/- 0.14) did not differ from that to Met-enkephalin (6.68 +/- 0.35) or Leu-enkephalin (9.06 +/- 0.98); the slope of the pA2 plot to morphine was near unity. 5. These data indicated that H-Gly-Phe-OMe exerted predominantly a potent non-competitive opioid antagonistic effect suggesting that short enkephalin fragments containing the Gly-Phe sequence might possess an opioid activity.

Biphasic GABA-A receptor-mediated effect on the spontaneous activity of the circular layer in cat terminal ileum.

1. The GABA and GABA-A receptor agonist muscimol changed the spontaneous mechanical activity of a circular layer isolated from cat terminal ileum, while the selective GABA-B receptor agonist (+/-)baclofen had no effect. 2. GABA at doses ranging from 1 microM to 2 mM elicited concentration-dependent biphasic responses which consisted of a relaxation followed by contraction, with a tonic and a phasic component. The EC50 values, calculated at 95% confidence limits (CL), were 94.9 microM (83.5-109.8 microM) and 66.0 microM (51.2-75.5 microM) for the relaxation and contractile phases, respectively. 3. The GABA-induced biphasic responses were sensitive to bicuculline and picrotoxinin and were entirely mimicked by muscimol. Bicuculline competitively antagonized the effects of GABA and gave closely similar pA2 values for both phases of these responses--inhibitory and stimulatory. Cross-desensitization occurred only between GABA and muscimol and not between (+/-)baclofen and GABA, or (+/-)baclofen and muscimol. 4. Both bicuculline-sensitive phases evoked by GABA and muscimol were abolished by tetrodotoxin or atropine, but were unaffected by guanethidine or naloxone. 5. The present results suggested that the biphasic GABA effect on the mechanical activity of the circular layer in cat terminal ileum was mediated by prejunctional GABA-A receptors, most probably through an action on the cholinergic pathway.

Changes in the contractile responses to carbachol and in the inhibitory effects of verapamil and nitrendipine on isolated smooth muscle preparations from rats subchronically exposed to Co2+ and Ni2+.

Male Wistar rats were exposed to subtoxic doses of Co2+ or Ni2+, receiving Co(NO3)2 or NiSO4 with drinking water for 30 days. No significant differences in the body weight and no visible changes in the behaviour of the controls and experimental animals were established. Cumulative concentration-effect curves for carbachol were obtained in ileum and trachea isolated from control and Co(2+)- or Ni(2+)-treated rats. The effect of the Ca2+ antagonists on the carbachol-induced contractions was studied by adding increasing concentrations of verapamil or nitrendipine to the bath solution 20 min prior to carbachol. The results showed that exposure of rats to subtoxic doses of Co(NO3)2 or NiSO4 altered the contractile responses to carbachol. The changes in the pD2 values and the shift to the left of the concentration-effect curves suggest a higher sensitivity to carbachol in preparations from the ileum of Co(2+)- or Ni(2+)-exposed rats. The tracheal strips isolated from control and heavy metal-treated rats showed a less potent sensitiveness to carbachol as compared to the ileal segments. An opposite tendency for decreased cholinergic reactivity was observed in tracheal strips from Co(2+)- and Ni(2+)-treated animals. The inhibitory effect of the Ca(2+)-antagonists on the contractility of ileal preparations from Co(2+)-treated rats increased at all concentrations of verapamil and at the highest concentration of nitrendipine, but decreased at lower concentrations of nitrendipine. The effect of verapamil on the preparations from Ni(2+)-exposed rats was unchanged or even decreased at higher verapamil concentrations.(ABSTRACT TRUNCATED AT 250 WORDS)

GABAA and GABAB receptor-mediated effects on the spontaneous activity of the longitudinal layer in cat terminal ileum.

1. GABA and GABAergic agonists-muscimol and (+/-)baclofen changed the spontaneous mechanical activity in isolated cat terminal ileum. 2. GABA at doses ranging from 5 microM to 2 mM produced concentration-dependent biphasic responses consisting of a transient relaxation followed by contractions with a tonic and a phasic components. 3. The GABA-induced relaxation was sensitive to bicuculline and picrotoxinin and was mimicked by muscimol, while the GABA-induced contractions were insensitive to bicuculline and picrotoxinin and were mimicked by (+/-)baclofen. Specific cross desensitization occurred between GABA and muscimol or GABA and (+/-)baclofen. 4. The bicuculline-sensitive relaxation induced by GABA and muscimol was abolished by atropine or tetrodotoxin (TTX), while the bicuculline-insensitive contractions induced by GABA and (+/-)baclofen were not antagonized by atropine or TTX, though they were slightly suppressed. 5. The GABA effects in the longitudinal layer of cat terminal ileum were mediated by the following receptors: -GABAA prejunctional receptors whose activation causes relaxation, probably through an inhibitory action on cholinergic neurons; -GABAB prejunctional receptors whose activation cause contractions; -GABAB postjunctional receptors located on the smooth muscle membrane whose activation induces tonic and phasic contractions.

Effects of (MET-5) enkephalin on the electrically-evoked mechanical responses in longitudinal and circular strips of the cat terminal ileum.

In longitudinal and circular strips from cat terminal ileum field electrical stimulation at a frequency of 2 Hz evoked contractile responses. Stimulation at frequencies of 10 or 30 Hz elicited contractions of the longitudinal muscle and relaxations of the circular strips. (Met-5) enkephalin (1 nM) naloxone-dependently reduced the contractile and increased the inhibitory responses. Atropine (3 microM) converted the contractile responses to slight relaxations and potentiated the inhibitory responses. After atropine (3 microM) and guanethidine (50 microM) both longitudinal and circular strips responded to electrical stimulation with relaxations. In atropine-pretreated strips (Met-5) enkephalin was effective only in the circular strips, increasing the inhibitory responses. In contrast, after atropine and guanethidine (Met-5) enkephalin decreased these inhibitory responses. In unstimulated strips (Met-5) enkephalin failed to change the responses to acetylcholine and noradrenaline. It is concluded that (Met-5) enkephalin reduces the excitatory cholinergic components of the electrically-evoked responses in both longitudinal and circular strips as well as the excitatory adrenergic and the inhibitory non-adrenergic, non-cholinergic components of the responses in the circular strips by acting presynaptically. Demonstration of (Met-5) enkephalin-like immunoreactivity showed immunostaining in nerves of the myenteric plexus and in nerve fibers between the smooth muscle cells suggesting that (Met-5) enkephalin effects could be also of physiological significance.

GABAB receptor-mediated contractile effects resistant to tetrodotoxin in isolated cat ileum.

The effects of gamma-aminobutyric acid (GABA) and GABAergic drugs were studied on longitudinal strips from cat terminal ileum prepared after removing the myenteric plexus. GABA and baclofen exerted concentration-dependent contractile effects. Muscimol was ineffective, and bicuculline did not antagonize the effect of GABA. The complete elimination of the neural input to the smooth muscle cells by tetrodotoxin failed to prevent the action of GABA and baclofen. Pharmacological analyses of the effects indicated the existence of GABAB receptors on the smooth muscle cells in the longitudinal layer of cat terminal ileum.

Effects of met-enkephalin on the mechanical activity and distribution of met-enkephalin-like immunoreactivity in the cat small intestine.

Naloxone-dependent effects of Met-enkephalin (10(-8) M) on the spontaneous and electrically induced mechanical activities were studied in longitudinal and circular preparations isolated from the cat duodenum, jejunum and ileum. Met-Enkephalin changed the spontaneous activity of all preparations tested with the exception of the circular preparations from the ileum. Met-Enkephalin-induced responses of the longitudinal preparations from the ileum were abolished by treatment with tetrodotoxin (10(-7) M), while the responses of both longitudinal and circular preparations from the duodenum and jejunum were only partially depressed, being resistant to tetrodotoxin components. The latter were most pronounced in the duodenum. The neurogenic electrically induced (0.5 msec, 5 Hz, 150 pulses) responses of all the preparations consisted mainly of contractile components which were significantly and naloxone-dependently reduced by Met-enkephalin (10(-8) M). The contractile components of the responses, which were reduced by Met-enkephalin, were entirely abolished by atropine (3 x 10(-6) M). Both Met-enkephalin and atropine inhibitory effects on the neurogenic responses were more pronounced in the ileum. Met-Enkephalin was found in nerve fibers of the myenteric plexus distributed mainly among the circular muscle. Single immunoreactive nerve fibers were observed in the longitudinal muscle layer of the duodenum but not in the jejunum and ileum. The distribution of Met-enkephalin-like immunoreactivity along the small intestine did not show significant differences among the three intestinal regions tested. The results obtained suggest that Met-enkephalin can modulate the mechanical activity of the cat small intestine, inhibiting cholinergic transmission and/or activating smooth muscle opioid receptors.(ABSTRACT TRUNCATED AT 250 WORDS)

Activation of delta-type opioid receptors modulates the responses of cat terminal ileum to field electrical stimulation.

1. The effects of (D-Ala2, D-Leu5) enkephalin amide (DADLE) on the responses of the cat terminal ileum to field electrical stimulation (pulse duration of 0.5 msec, train duration of 10 sec, 30 V) were evaluated by the changes in the contractile or the relaxatory responses of longitudinal and circular strips to electrical stimuli with a frequency of 2, 10 or 30 Hz. 2. Stimulation with a frequency of 2, 10 or 30 Hz elicited contractile responses from the longitudinal strips while in the circular strips 2 Hz stimulation induced contractions and 10 or 30 Hz stimulation caused relaxation. Tetrodotoxin (TTX) (0.1 mumol/l) abolished the electrically-induced responses in both longitudinal and circular strips. 3. DADLE (1 nmol/l) significantly inhibited the cholinergic contractile responses of the longitudinal strips to 2, 10 or 30 Hz stimulation and the contractile responses of the circular strips to 2 Hz stimulation. The relaxatory responses of the circular strips to 10 or 30 Hz stimulation were insignificantly increased by DADLE. 4. On the background of guanetidine (10 mumol/l) and atropine (3 mumol/l) DADLE significantly decreased the nonadrenergic, noncholinergic relaxatory responses of the circular strips to 2, 10 or 30 Hz stimulation. 5. DADLE did not change the maximum effects and the EC50 values of acetylcholine and noradrenaline in both longitudinal and circular strips. 6. It is suggested that in the cat terminal ileum activation of delta-type opioid receptors modulates the mechanical activity suppressing the cholinergic responses in the longitudinal and circular layers as well as the adrenergic and nonadrenergic, noncholinergic responses in the circular layer.

Effects of leu-enkephalin on the mechanical activity of longitudinal and circular muscles of the small intestine of the cat.

The effects of leu-enkephalin on the spontaneous and electrically-evoked activity were studied in the longitudinal and circular strips isolated from the duodenum, jejunum and ileum of the cat. Leu-enkephalin affected the spontaneous activity of both longitudinal and circular strips, with the exception of the circular strips from the ileum, in a naloxone-dependent manner. Elimination of the neural input to the smooth muscle cells with tetrodotoxin blocked the effects of leu-enkephalin in the longitudinal and circular strips from the jejunum and in the longitudinal strips from the ileum. In the longitudinal strips from the duodenum the effect was resistant to tetrodotoxin, while in the circular strips a tetrodotoxin-sensitive component of the effect of leu-enkephalin was observed. Since leu-enkephalin evoked opposite effects in the longitudinal and circular layers of one and the same region, it is concluded that leu-enkephalin-induced modulation of the motility of the small intestine in the cat is a physiological phenomenon. Electrical stimulation, at a frequency of 5 Hz, evoked contractile responses in the longitudinal strips and relaxant, as well as low-amplitude, contractile responses in the circular strips. Rebound contractions developed after the end of stimulation in all preparations tested, with the exception of the longitudinal strips from the duodenum. Leu-enkephalin decreased the contractile components and tended to potentiate the relaxant components of the responses in a naloxone-dependent manner. Atropine inhibited the contractile components of the responses and significantly depressed the rebound contractions. Leu-enkephalin, applied after atropine, was ineffective suggesting that leu-enkephalin-induced modulation was mediated mainly through interaction with cholinergic transmission.