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Biomedical subjects

K N Bitar

Publications and source records attributed to K N Bitar.

48 records · Page 3Linked to original sources
PubMed1985

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.

Adenosine Triphosphate↗
PubMed1984

Isolation and culture of human intestinal smooth muscle cells.

Intestinal smooth muscle cells were isolated from human bowel and maintained in culture through several passages. These cells were obtained by enzyme digestion of slices taken from the circular layer of the muscularis propria of human jejunum. When subcultured, they initially flattened out and then began proliferating after 3 days. After 3 weeks in culture, they began aggregating into ridges. Fluorohistochemical staining revealed numerous prominent actin stress fibers. When these cells were exposed to the C-terminal octapeptide of cholecystokinin they contracted in a dose-dependent fashion. The availability of human intestinal smooth muscle cells in culture will considerably enhance our ability to study the contractile, proliferative and connective tissue responses of the smooth muscle of the human gastrointestinal tract.

Cell Division↗
PubMed1982

Relaxation of isolated gastric smooth muscle cells by vasoactive intestinal peptide.

Vasoactive intestinal peptide caused a prompt, dose-dependent relaxation of isolated gastric smooth muscle cells of the guinea pig and a significant increase in intracellular adenosine 3',5'-monophosphate coincidentally with optimum relaxation. Relaxation was augmented by a threshold concentration of isobutyl methylxanthine. The direct relaxant effect of vasoactive intestinal peptide and the distribution of nerves containing this peptide to circular smooth muscle support the view that vasoactive intestinal peptide is the neuromuscular transmitter of enteric inhibitory nerves.

1-Methyl-3-isobutylxanthine↗
PubMed1982

Receptors on smooth muscle cells: characterization by contraction and specific antagonists.

Smooth muscle cells were isolated from the stomach of the guinea pig, and the kinetics, stoichiometry, and specificity of contraction in response to the C-terminal octapeptides of cholecystokinin (CCK-OP), gastrin-17, and acetylcholine were examined. All three agonists elicited dose-dependent peak contraction that did not depend on the presence of extra-cellular calcium. The potencies of CCK-OP and gastrin-17 were equal (D50, 10(-11) M) and 10 times greater than the potency of acetylcholine (D50, 10(-10) M). A combination of low doses of acetylcholine and CCK-OP was synergistic; however, its effect did not exceed the maximal responses to either agonists alone or to high extracellular concentrations of calcium. The specificity of the receptors was established by the use of atropine and the two CCK-receptor antagonists dibutyryl cGMP and proglumide. The span of the dose-response curves was wide, suggesting the existence of receptor heterogeneity. It is concluded that gastric smooth muscle cells of the guinea pig possess distinct, high-affinity receptors for CCK-gastrin and acetylcholine; the receptors mediate contraction that is not immediately dependent on the presence of extracellular calcium.

Animals↗
PubMed1982

Regulation of gastrin and somatostatin secretion by cholinergic and noncholinergic intramural neurons.

The regulation of gastrin and somatostatin secretion by intramural neurons was examined in the isolated vascularly perfused rat stomach. The optimal modalities of transmural electrical stimulation of the antrum were established to be 40 V and 10 Hz. Stimulation at increasing cycle durations (0.1-4 ms) caused increasing gastrin secretion that was progressively more resistant to atropine. The maximal gastrin response to 4-ms cycles was equal to the maximal response to methacholine. However, the response to methacholine was inhibited 70-90% by atropine, whereas the response to 4-ms cycles was inhibited by 15% only. Stimulation at all cycle durations caused a decrease in somatostatin secretion. Atropine converted the decrease to an increase, from which it was concluded that before atropine somatostatin secretion was the net result of cholinergic inhibition and noncholinergic stimulation of somatostatin. The results indicate that cholinergic and noncholinergic intramural neurons are predominantly but not exclusively activated by 0.1- and 4-ms cycles, respectively. The existence of distinct neurons was supported by results of stimulation of preganglionic vagal fibers with 0.2- and 4-ms cycles. The findings are consistent with a model according to which gastrin secretion is regulated by two interdependent intramural neurons: a cholinergic neuron that stimulates gastrin secretion indirectly by inhibition of somatostatin secretion and a noncholinergic neuron that stimulates gastrin secretion directly by release of a peptide stimulant, probably bombesin.

Animals↗
PubMed1982

Cholinergic and peptidergic receptors on isolated human antral smooth muscle cells.

Smooth muscle cells were isolated from segment so of human antrum obtained at operation. The presence, activity, and specificity of receptors to acetylcholine, the C-terminal octapeptide of cholecystokinin, methionine-enkephalin, and vasoactive intestinal peptide were examined. Muscarinic cholinergic receptors sensitive to low concentrations of atropine, cholecystokinin receptors sensitive to proglumide and dibutyryl cyclic guanosine monophosphate, enkephalin receptors sensitive to naloxone were demonstrated and found to mediate contraction. Vasoactive intestinal peptide receptors were demonstrated also and found to mediate relaxation. The stoichiometry and specificity of responses were closely similar to those previously found in isolated gastric smooth muscle cells of the guinea pig. The existence of high-affinity functional receptors on smooth muscle cells, together with the presence of the peptides in nerve terminals adjacent to these cells, makes it possible for these peptides to act as neurotransmitters.

Cholecystokinin↗
PubMed1980

Gastrin and somatostatin secretion by perfused rat stomach: functional linkage of antral peptides.

The isolated vascularly perfused rat stomach was used to study the polarity, kinetics, and stoichiometry of gastrin and somatostatin secretion and the interaction of the two antral peptides. The secretion of gastrin (79%) and somatostatin (95%) was predominantly in the circulation. Methacholine (5 x 10(-8) to 5 x 10(-4) M) produced a biphasic dose-dependent increase in gastrin secretion. The maximal gastrin response (434 +/- 89% above basal levels; P less than 0.001) was partially inhibited by 10(-8) M atropine and completely inhibited by 10(-7) M atropine. Methacholine produced a dose-dependent inhibition of somatostatin secretion; the inhibition was blocked by atropine. An inverse relationship between the secretion of gastrin and somatostatin was noted in the basal state and during infusion of methacholine or prostaglandin E2; the latter had effects on gastrin and somatostatin secretion opposite to those of methacholine. The data, together with data reported elsewhere that somatostatin antiserum stimulates gastrin secretion in the perfused stomach, are consistent with the hypothesis that gastric somatostatin secretion exerts a continuous restraint on basal gastrin secretion and that stimulation of gastrin secretion may be mediated in part by inhibition of somatostatin secretion.

Animals↗
PubMed1980

Neural release of vasoactive intestinal peptide from the gut.

The release of vasoactive intestinal peptide (VIP) from the canine gut and its possible neural origin were studied using two agents, oxytocin and neostigmine, known to increase peripheral levels of VIP. Oxytocin and neostigmine increased the portal concentrations of VIP by threefold and sevenfold, respectively. A considerable portal/femoral vein gradient ranging from twofold in the basal state to sevenfold during stimulation with neostigmine indicated that the gut was the main source of circulating VIP. The contribution of the brain was minor, and that of the uterus was undetectable. Release of VIP occurred from the entire gut: After enterectomy, the residual gut (stomach, pancreas, and proximal duodenum) released spontaneously a large amount of VIP which masked the effect of oxytocin. Tetrodotoxin and hexamethonium, but not atropine, inhibited oxytocin-stimulted release of VIP by 80% and 60% respectively. This prompted the conclusion that the release of VIP was predominantly neurally mediated and that the chain of transmission involved a preganglionic cholinergic pathway. Hexamethonium strongly inhibited neostigmine-stimulated release of VIP. Atropine was even more potent in that it abolished the effect of neostigmine. The effect of atropine was attributed to a blockade of ganglionic muscarinic receptors, which are preferentially activated by cholinesterase inhibitors like neostigmine. The results of this study and those derived from electrical stimulation of the vagus nerve are consistent with the hypothesis that circulating VIP is released from intrinsic neurons of the gut under preganglionic cholinergic control.

Animals↗
PubMed1979

Interaction of acetylcholine and cholecystokinin with dispersed smooth muscle cells.

Isolated gastric smooth muscle cells were prepared from the stomach of Bufo marinus by successive incubation in collagenase without added trypsin. Contraction was determined by image-splitting micrometry and expressed as the mean percentage decrease in cell length from control. Peak contractile response was attained within 30 s. Dose-response curves constructed from peak responses showed that the maximal responses to CCK-OP (37.2 +/- 3.8%), acetylcholine (35.3 +/- 2.5%), and Ca2+ (42.3 +/- 0.9%) were similar. The D50s for octapeptide of cholecystokinin (CCK-OP) and acetylcholine were around 10(-12) M and 10(-11) M, respectively. The response to a combination of submaximal concentrations of acetylcholine and CCK-OP exceeded the individual responses but did not exceed the maximal response to either agent alone. A low concentration of atropine (5 X 10(-10) M) inhibited specifically the maximal response to acetylcholine. A high concentration of atropine (5 X 10(-8) M) inhibited partially the maximal response to CCK-OP but had no effect on the maximal response to Ca2+. It was concluded that 1) dispersed gastric smooth muscle cells are highly sensitive to stimulation; 2) CCK-OP has a direct (myogenic) contractile effect on gastric smooth muscle; and 3) the effect of CCK-OP and acetylcholine are mediated by separate receptors.

Acetylcholine↗
PubMed1978

Inhibition of VIP-stimulated intestinal secretion and cyclic AMP production by somatostatin in the rat.

The effect of somatostatin on colonic secretion induced by 10(-8) M vasoactive intestinal peptide (VIP), 10(-2) M theophylline, and 2 X 10(-3) M dibutyryl cyclic AMP was studied in muscle-stripped everted open rat colon sacs. The secretory response to VIP, measured as the decrease in net absorptive flow rate (microliters 30 min-1 mg-1 of dry weight), was maximal and equalled the responses to theophylline or dibutyryl cyclic AMP. Somatostatin (10(-5) M) blocked completely the secretory response to VIP but only partially the secretory response to theophylline or dibutyryl cyclic AMP. This difference in the extent of inhibition suggested that somatostatin exerted an inhibitory effect both before and after the point of generation of intracellular cyclic AMP. In order to test the hypothesis that one component of the action of somatostatin involved inhibition of the production of cyclic AMP, measurements of this nucleotide were made in isolated rat colon cells. Control levels of cyclic AMP measured by radioimmunoassay (12.6 +/- 1.6 pmoles per 10(6) cells) were not affected by 10(-5) M somatostatin. VIP (5 X 10(-8) M) increased cyclic AMP levels 2-fold (P less than 0.01) and this increase was blocked by somatostatin. The results indicated that somatostatin inhibits colonic secretion by exerting effects at two sites: one site lies at, and another beyond, the point of generation of intracellular cyclic AMP.

Adenylyl Cyclase Inhibitors↗
PubMed1978

Binding of secretin to plastic surfaces.

Five carboxyl-terminal fragments of secretin ranging in size from 6 to 21 amino acid residues were tested for pancreatic secretory activity in the rat. None of the fragments displayed activity when given alone but each displayed significant activity when given after secretin. This apparent activity was shown to be the result of displacement of secretin bound to the walls of the injection catheter. The activity was abolished by dissolving secretin in 2% bovine serum albumin. The finding emphasizes the ease with which secretin can bind to plastic surfaces and consequently the need to reevaluate previous dose-response studies and the caution required in the design of future studies.

Animals↗