Contraction mediated by Ca++ release in circular and Ca++ influx in longitudinal intestinal muscle cells

The source of Ca++ responsible for contraction was examined in muscle cells isolated separately from the circular and longitudinal muscle layers of guinea pig and human intestine. Contraction was measured by scanning micrometry and cytosolic-free Ca++ ([Ca++]i) with the fluorescent indicator, quin2. In both species, contraction induced in circular muscle cells by cholecystokinin-8 (CCK-8) and acetylcholine was not affected by withdrawal of Ca++ from the medium or addition of the Ca++ channel blocker, methoxyverapamil, whereas contraction induced by both agonists in longitudinal muscle cells and by depolarizing concentrations of K+ in both cell types was abolished. Depletion of intracellular Ca++ stores with caffeine in Ca++-free medium abolished the response in circular muscle cells. Readdition of Ca++ to the medium for 30 sec restored the response in longitudinal but not circular muscle cells. [Ca++]i, measured in guinea pig muscle cells, increased 3- to 4-fold above resting levels (circular, 70.8 +/- 8.1 nM; longitudinal, 77.4 +/- 9.7 nM) in response to all three contractile agents. The increase in [Ca++]i induced by CCK-8 and acetylcholine in circular muscle cells was not affected by withdrawal of Ca++ from the medium or addition of methoxyverapamil, whereas the response to both agonists in longitudinal muscle cells and to 20 mM K+ in both cell types was abolished. It was concluded that cells from adjacent muscle layers of the intestine mobilize Ca++ differently during agonist-induced contraction, i.e., by Ca++ release in circular and Ca++ influx in longitudinal muscle cells.

Characterization of opioid receptors on isolated canine gallbladder smooth muscle cells

Smooth muscle cells were isolated from the fundus of the canine gallbladder and examined for the presence of opioid receptors. The cells contracted in a concentration-dependent manner in response to three opioid peptides (Met-enkephalin, dynorphin1-13 and Leu-enkephalin), which are known derivatives of opioid precursors present in myenteric neurons of the gut. The order of potency was Met-enkephalin greater than dynorphin1-13 greater than Leu-enkephalin. The contractile response to opioid agonists was selectively inhibited by opioid antagonists (naloxone and Mr2266) but not by muscarinic, CCK/gastrin or tachykinin antagonists. Equivalent responses to the three opioid peptides exhibited differential sensitivity to preferential antagonists of mu (naloxone) and kappa (Mr2266) opioid receptors consistent with the presence of the three main types of opioid receptors (mu, delta and kappa) on canine gallbladder muscle cells.

Receptors for substance P on isolated intestinal smooth muscle cells of the guinea pig

Two radioligands, 125I-labeled substance P (125I-SP) and 125I-labeled substance K (125I-SK), were used to characterize the kinetics and stoichiometry of binding of mammalian tachykinins [substance P (SP), substance K (SK), and neuromedin K (NK)] to smooth muscle cells isolated from the longitudinal muscle layer of guinea pig intestine. Specific binding of 125I-SP and 125I-SK was rapid, saturable, reversible, and temperature dependent. Binding attained 63-70% of steady-state binding within 1 min, coincidentally with the time of optimal contraction. The order of potency with which mammalian tachykinins and the SP antagonist, [D-Pro2, D-Trp7,9]SP, inhibited the binding of both radioligands was identical: SP greater than SK greater than NK greater than [D-Pro2, D-Trp7,9]SP, implying preferential interaction with a site that had highest affinity for SP. SK was 2-3 times, NK 3-4 times, and [D-Pro2, D-Trp7,9]SP 7-23 times less potent than SP (IC50 0.36 nM). Except for NK, the order of potency was similar to that for contraction of isolated muscle cells. The existence of binding sites with even higher affinity was suggested by the ability of muscle cells to contract in response to concentrations as low as 10(-13) M. These binding sites were not detectable at the concentration of radioligands used. It was concluded that a SP receptor is the only tachykinin receptor subtype present on intestinal muscle cells of the guinea pig.

Identification of muscarinic M2 receptors on single muscle cells of the human and guinea pig intestine

Muscle cells were isolated from the longitudinal muscle layer of guinea pig and human jejunum and used to identify the muscarinic receptor subtype (M1 or M2) that mediates contraction. Single muscle cells were anchored to the ceiling of a minichamber and their contraction was measured in response to acetylcholine, alone and in combination with three muscarinic antagonists: atropine, 4-diphenylacetoxy-N-methylpiperidine methiodide (4-DAMP), and pirenzepine. Estimates of the inhibitory dissociation constants (Ki) were closely similar in human and guinea pig muscle cells (atropine 2.5-5 X 10(-11) M, 4-DAMP 1.9-2.9 X 10(10) M, and pirenzepine 8.2-9.5 X 10(-8) M). Thus, pirenzepine, a preferential M1 antagonist, was 1900-3280 times less potent than atropine and 279-500 times less potent than 4-DAMP. Comparative measurements on longitudinal muscle strips from guinea pig jejunum confirmed the greater potency of atropine and 4-DAMP relative to pirenzepine. Inactivation of muscarinic receptors on single muscle cells with dibenamine showed that only a small fraction of receptors was responsible for the response to acetylcholine. It was concluded that intestinal muscle cells contain a large reservoir of muscarinic M2 receptors that exhibit considerable spareness and heterogeneity.

Suppression of inhibitory neural input to colonic circular muscle by opioid peptides

The aim of this study was to identify the neurotransmitter responsible for the dominant inhibitory neural input that normally masks myogenic phasic activity in the intestine. Opioid agonists, including opioid derivatives of proenkephalin and prodynorphin, caused direct tonic and indirect phasic contractions of strips from the circular muscle layer of rat colon. Both contractile responses appeared to be mediated by preferential interaction with delta opioid receptors. Elimination of most, if not all, neural input with tetrodotoxin (TTX) induced concentration-dependent phasic contractions. Neutralization of background vasoactive intestinal peptide (VIP) with VIP antiserum induced phasic contractions in previously quiescent muscle strips. The effect of VIP antiserum was concentration-dependent in the range of 1:960 to 1:60. A threshold concentration of VIP antiserum (1:960) increased the sensitivity of the phasic responses to [Met]enkephalin (100-fold) and TTX (20-fold) whereas exogenous VIP had the opposite effect. [Met]enkephalin and TTX inhibited basal VIP release by 43 +/- 8% (P less than .001) and 65 +/- 13% (P less than .01), respectively. It was concluded that VIP was the neurotransmitter responsible for the dominant inhibitory neural input to intestinal circular muscle and that opioid peptides induce phasic contractions by suppressing VIP release.

Role of somatostatin neurons in intestinal peristalsis: facilitatory interneurons in descending pathways

The role of somatostatin neurons in the regulation of peristalsis was examined in segments of rat colon that permit separate characterization of the ascending contraction and descending relaxation components of the peristaltic reflex. Release of somatostatin and vasoactive intestinal peptide (VIP) increased significantly only during descending relaxation. Preincubation of the segment with somatostatin antiserum (final concentration 1:40) decreased VIP release and descending relaxation. Addition of somatostatin (1 nM to 1 microM) augmented VIP release and descending relaxation in a concentration-dependent manner. Together the results implied that the increase in somatostatin release was coupled to, and responsible for, the increase in VIP release, which in turn was responsible for descending relaxation. The results are consistent with the topography of myenteric VIP neurons (which project into circular muscle) and somatostatin neurons (which project caudad within the plexus) and the pharmacological properties of the two peptides. Somatostatin antiserum had no effect on basal VIP release or ascending contraction, indicating that somatostatin neurons were not involved in the regulation of ascending contraction. The study suggests that somatostatin neurons of the myenteric plexus act as facilitatory interneurons in descending pathways.

Role of opioid neurons in the regulation of intestinal peristalsis

The participation of opioid neurons in the regulation of peristalsis was examined in a rat colonic segment that permits separate characterization of the components of the peristaltic reflex (ascending contraction and descending relaxation). Naloxone increased descending relaxation and decreased ascending contraction; opioid peptides [methionine-enkephalin (Met-Enk), dynorphin-13, and morphiceptin] had opposite effects. Naloxone increased, and Met-Enk decreased, vasoactive intestinal peptide (VIP) release during each component of the reflex. The changes in VIP release reinforced the direct effects of naloxone and opioid peptides on circular muscle tone, providing an explanation for the effects of these agents on the two components of the peristaltic reflex. Dynorphin release decreased during descending relaxation and increased during ascending contraction, reflecting corresponding changes in opioid neural activity. Based on these results a model is proposed, according to which a decrease in opioid neural activity during the initial phase (i.e., descending relaxation) results in direct and VIP-mediated decrease in circular muscle tone. Restoration of opioid neural activity during the subsequent phase (i.e., ascending contraction) increases circular muscle tone and reinforces the action of tachykinin and cholinergic motor neurons, which are the direct mediators of ascending contraction.

Prejunctional inhibition of vasoactive intestinal peptide release

The role of vasoactive intestinal peptide (VIP) and its homologue, peptide histidine isoleucine (PHI), as neurotransmitters of inhibitory motor nerves of the gut, were examined in strips of guinea pig taenia coli and gastric fundic muscle. The stoichiometry of VIP release and muscle relaxation was determined in the presence and absence of the bee venom peptide, apamin, and the existence of prejunctional VIP/PHI receptors capable of regulating VIP/PHI release was explored. In both types of muscle, relaxation induced by field stimulation was proportional to the amount of VIP released. Apamin inhibited relaxation and VIP release in a dose-dependent manner: maximal relaxation was inhibited by 85-96% at 10(-7)-10(-6) M apamin. Analysis of residual responses showed that apamin did not affect the stoichiometry of VIP release and muscle relaxation. Because apamin had no effect on basal tone or on relaxation induced by exogenous VIP, its effect on neurally induced relaxation was attributed to inhibition of VIP release. Both secretin and PHI inhibited neurally induced VIP release in the two types of muscle. At the optimal concentration of 10(-7) M, secretin inhibited VIP release by 52%, whereas the closer neural homologue, PHI, abolished VIP release. The dose-dependent inhibition of VIP release by PHI, which is cosynthesized and coreleased with VIP, indicates the existence of prejunctional inhibitory VIP/PHI autoreceptors capable of regulating VIP/PHI release.

Regional and cellular heterogeneity of cholecystokinin receptors mediating muscle contraction in the gut

The contractile action of cholecystokinin (CCK) on smooth muscle of the gut is either direct (gallbladder and gastric fundus) or both direct and neurally mediated (small intestine). These regional differences were used to characterize pharmacologically CCK receptors on smooth muscle cells and neurons of the gastric fundus, gallbladder, and ileum of the guinea pig. In circular and longitudinal ileal smooth muscle, tetrodotoxin was used to separate direct from neurally mediated contractile effects. Cholecystokinin receptors on smooth muscle cells were found in all locations. The muscle cells displayed a decreasing order of sensitivity to the C-terminal octapeptide of cholecystokinin as expressed in the median doses, and to the selective cholecystokinin antagonist, proglumide, as expressed in the inhibitory dissociation constants. The median doses of the octapeptide of cholecystokinin ranged from 5.5 nM in gallbladder muscle to 185 nM in circular ileal muscle; the corresponding inhibitory dissociation constants of proglumide ranged from 180 to 437 microM [corrected]. Cholecystokinin receptors on cholinergic neurons were confined to circular and longitudinal ileal muscle; the neurons were 80-300 times more sensitive to the octapeptide of cholecystokinin (D50's 0.5 and 2.3 nM) than the corresponding muscle cells, and 19-21 times more sensitive to proglumide (inhibitory dissociation constants, 20 microM [corrected]). The results provide clear evidence of cellular heterogeneity of cholecystokinin receptors (i.e., difference in sensitivity between muscle cells and neurons from the same location) as well as regional heterogeneity (i.e., difference in sensitivity between muscle cells from various locations).

Colonic peristaltic reflex: identification of vasoactive intestinal peptide as mediator of descending relaxation

Isolated segments of rat and guinea pig midcolon were used to examine the neurotransmitters responsible for ascending contraction and descending relaxation components of the peristaltic reflex. Graded radial stretch of the extreme orad end caused only descending relaxation accompanied by significant release of vasoactive intestinal peptide (VIP) in rat (82%, P less than 0.005) and guinea pig (47%, P less than 0.05). Radial stretch of the caudad end caused only ascending contraction without VIP release. VIP antiserum (1:480 to 1:60) inhibited descending relaxation in a concentration-dependent manner at all grades of stretch (40 +/- 12% to 74 +/- 15%) but augmented ascending contraction (25 +/- 7% to 108 +/- 21%). Axonal blockade with tetrodotoxin and ganglionic blockade with hexamethonium abolished both components, indicating the participation of cholinergic neurons. Atropine and the tachykinin antagonist [D-Pro2,D-Trp7,9]substance P inhibited ascending contraction but not descending relaxation; their combination abolished ascending contraction at all grades of stretch. We conclude that cholinergic neurons coupled to VIP motor neurons regulate descending relaxation and that cholinergic neurons coupled to tachykinin and cholinergic motor neurons regulate ascending contraction.

Receptors for mammalian tachykinins on isolated intestinal smooth muscle cells

The existence of receptors for three mammalian tachykinins, substance P (SP), substance K (SK), and neuromedin K (NK), was examined in smooth muscle cells, isolated separately from the longitudinal and circular muscle layers of guinea pig ileum. Tachykinin receptors capable of mediating contraction were present in muscle cells from both layers. The receptors were selectively blocked by the tachykinin antagonist [D-Pro2, D-Trp7,9]substance P but not by muscarinic, gastrin/cholecystokinin, or opiate antagonists (0.3 nM atropine, 1 mM proglumide, and 0.3 nM naloxone, respectively). The rank order of potency of tachykinins in causing contraction, NK greater than SP greater than SK, was similar in both muscle cell types. The results obtained in isolated muscle cells were closely paralleled by results obtained in intact muscle strips; the main difference was the greater sensitivity of isolated cells to tachykinin agonists (250-fold) and antagonist (210-fold). The inhibitory dissociation constant (Ki) of [D-Pro2, D-Trp7,9]substance P estimated from the displacement of dose-response curves (muscle cells) or from Schild plots (muscle strips) differed minimally or not at all, when either SP or SK was used as agonist, consistent with interaction of the two peptides with the same receptor subtype. The notion of a single receptor subtype in ileal muscle cells of the guinea pig was further supported by the occurrence of complete cross-desensitization between SP and SK in muscle strips.

Selective inhibition of pentagastrin- and cholecystokinin-stimulated exocrine secretion by proglumide

The effectiveness and selectivity of proglumide, a putative cholecystokinin/gastrin receptor antagonist in vitro, were examined on gastric acid and pancreatic secretion in vivo. Gastric secretion was measured in conscious dogs in the basal state and during infusion of pentagastrin, histamine, or bethanechol, alone or in combination with proglumide (300 mg/kg . h). Pancreatic secretion was measured in anesthetized rats in response to cholecystokinin-octapeptide or secretin, alone or in combination with proglumide (100 mg/kg). Proglumide inhibited pentagastrin-stimulated secretion but had no effect on basal, histamine-stimulated, or bethanechol-stimulated gastric acid secretion. Inhibition of pentagastrin-stimulated secretion was of the competitive type. An apparent inhibitory constant was calculated to be 300 mg/kg . h; this dose is capable of eliciting plasma concentrations of approximately 1 mM. This estimate corresponds closely to that derived from measurements in isolated canine parietal cells. Proglumide also inhibited cholecystokinin-stimulated but not secretin-stimulated pancreatic secretion. The lack of effect of proglumide on basal, histamine-stimulated, or bethanechol-stimulated gastric acid secretion implies that background gastrin has no direct or synergistic influence on stimulation by other secretagogues. The selective effect of gastrin receptor antagonists contrasts with the effectiveness of muscarinic and histamine H2-receptor antagonists against secretion induced by all types of stimulants. Accordingly, the antisecretory potential of gastrin receptor antagonists is confined to digestive secretion when the effect of gastrin is optimal. Their potential as antitrophic agents in duodenal ulcer disease, however, has not been explored yet.

Vasoactive intestinal peptide. Relaxant neurotransmitter in tenia coli of the guinea pig

Two main candidates have been proposed for the role of relaxant neurotransmitter in the intestine: (a) the purine nucleotide, 5'-adenosine triphosphate (ATP) and (b) the neuropeptide, vasoactive intestinal peptide (VIP). The candidacy of VIP is favored by its precise location in nerve fibers that innervate circular smooth muscle and tenia coli. We have used a photoaffinity analog of ATP, 3'-O-(4-Benzoyl)benzoyl ATP, that binds irreversibly to ATP receptors and inactivates them in the presence of light, and a specific VIP antiserum to examine the claims of VIP and ATP as relaxant neurotransmitters in tenia coli of the guinea pig. Both VIP and ATP caused dose-dependent, tetrodotoxin-insensitive relaxation of tenia coli. The effect of ATP was equipotent to that of its stable isostere alpha, beta-methylene ATP and resistant to degradation by adenosine deaminase, indicating interaction of ATP with purinergic-P2 receptors. Photoactivated 3'-O-(4-Benzoyl) benzoyl adenosine triphosphate selectively inhibited relaxation induced by ATP but had no effect on relaxation induced by VIP or by field (i.e., neural) stimulation. Vasoactive intestinal peptide antiserum (final dilution 1:60), on the other hand, inhibited relaxation caused by VIP and by field stimulation but had no effect on relaxation caused by ATP. Neither normal rabbit serum nor preneutralized VIP antiserum had any effect on relaxation induced by ATP, VIP, or field stimulation. Inhibition of neurally induced relaxation by VIP antiserum ranged from 52% +/- 7% (p less than 0.01) at the lowest frequency of stimulation to 15% +/- 4% (p less than 0.01) at the highest frequency, consistent with competitive interaction between antiserum and neurally released VIP. Near-maximal field stimulation at 1 Hz caused an eightfold (800% +/- 49%, p less than 0.01) increase in VIP release into the bathing medium. The results favor VIP (and probably peptide histidine isoleucine, a relaxant homologue known to be cosynthesized with VIP) as the main neural mediator of relaxation in tenia coli.

Vasoactive intestinal peptide as a neural mediator of gastric relaxation

Two main candidates, adenosine 5'-triphosphate (ATP) and vasoactive intestinal peptide (VIP), have been proposed as inhibitory transmitters at neuromuscular junctions in the gut. We have used a photoaffinity analogue of ATP, 3'-O-(4-benzoyl)benzoyl ATP or BzATP, that binds covalently to ATP receptors and inactivates them in the presence of light and a specific high-affinity VIP antiserum in order to examine the contributions of ATP and VIP to neurally induced relaxation in circular smooth muscle of the gastric fundus of the guinea pig. VIP and ATP caused dose-dependent relaxation; the effect of ATP was equal to that of its stable isostere, alpha, beta-methylene ATP, and was resistant to degradation by adenosine deaminase, indicating interaction of ATP with purinergic P2-receptors. Relaxation induced by VIP was selectively inhibited by VIP antiserum (final dilution 1:120), while that induced by ATP was selectively inhibited by photoactivated BzATP. Relaxation induced by electrical field (i.e., neural) stimulation was inhibited by VIP antiserum only; photoactivated BzATP had no effect. Inhibition of neurally induced relaxation ranged from 86% (P less than 0.01) at the lowest frequencies to 34% (P less than 0.01) at the highest frequencies. Maximal field stimulation caused an 11-fold increase in VIP release from intramural neurons. The results strongly favor VIP as the neural mediator of gastric relaxation.

Amylase isoenzyme clearances in normal subjects and in patients with acute pancreatitis

Renal clearances of amylase isoenzymes, expressed as percentages of creatinine clearances, were determined in 20 normal subjects and in 8 patients with acute pancreatitis. The isoenzyme assay employed thin layer isoelectric focusing, starch iodine staining, and densitometry. Normal clearance of pancreatic-like amylase (mean +/- SE: 3.00 +/- 0.40%) was greater than the clearance of salivary-like amylase (0.51 +/- 0.06%) in ea ch individual (P less than 0.001). However, the amount of pancreatic amylase in the serum was not the major determinant of amylase clearance. Normal clearance of pancreatic-like amylase was significantly (P less than 0.001) less than the clearance of total serum amylase in acute pancreatitis (6.49 +/- 1.07%). In pancreatitis the clearance of pancreatic-like amylase (7.29 +/- 1.19%) and the clearance of salivary-like amylase (4.55 +/- 1.02%) were both elevated compared to normal (P less than 0.001). These findings indicate that the increased clearance of amylase in pancreatitis results from a change in renal function rather than a change in serum amylase. Renal changes not reflected by increased serum creatinine or more than mild proteinuria may be manifestations of the systemic toxicity of acute pancreatitis.