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Biomedical subjects

M S Barnette

Publications and source records attributed to M S Barnette.

28 records · Page 2Linked to original sources
PubMed1992

5-Hydroxytryptamine (5-HT) and SK&F 103829 contract canine lower esophageal sphincter smooth muscle by stimulating 5-HT2 receptors.

Addition of 5-HT or SK&F 103829 (2,3,4,5 tetrahydro-8[methyl-sulfonyl]-1 H-3-benzazepin-7-ol hydrobromide) contracts isolated strips of canine lower esophageal sphincter (LES) circular smooth muscle. 5-HT acts directly on the smooth muscle, since pretreatment with the neurotoxin TTX does not inhibit this contraction. Depletion of extracellular calcium or pretreatment with nifedipine inhibited the contraction to both 5-HT and SK&F 103829. Therefore, in this smooth muscle, the contraction produced by both 5-HT and SK&F 103829 requires extracellular calcium and is sensitive to inhibition by a voltage-dependent Ca2+ channel antagonist. In addition, with respect to 5-HT, SK&F 103829 appeared to act as a partial agonist. Receptor alkylation studies using phenoxybenzamine demonstrated no receptor reserve for the contractile response to 5-HT. Nonsurmountable antagonism of the contraction induced by 5-HT and SK&F 103289 was observed with several 5-HT2 antagonists, i.e., methysergide, ketanserin, cyproheptadine, and LY 53857. Using a method established for pseudoirreversible antagonism, the Ki values for these 5-HT2 receptor antagonists were estimated. Results suggested that both 5-HT and SK&F 103829 contract the canine LES by interacting at the same receptor site and that this receptor site has characteristics of the 5-HT2 receptor. Finally, neither bulbocapnine, domperidone, nor prazosin significantly alters the response to 5-HT or SK&F 103829. Thus, isolated strips of canine LES contain a contractile 5-HT2 receptor, and SK&F 103829 behaves as a partial agonist at this site.

Animals↗
PubMed1991

Human lower oesophageal sphincter relaxation is associated with raised cyclic nucleotide content.

Increases in cyclic adenosine monophosphate and cyclic guanosine monophosphate content accompany relaxation of isolated strips of opossum and canine lower oesophageal sphincter muscle. The aim of this investigation was to characterise these responses in isolated muscle from the human lower oesophageal sphincter. Electrical stimulation of enteric neurons produced a frequency dependent relaxation of the human lower oesophageal sphincter that was sensitive to tetrodotoxin. Furthermore, as previously shown in the opossum and canine lower oesophageal sphincter, cyclic guanosine monophosphate content was significantly raised in muscle strips frozen during maximum electrical field stimulation whereas cyclic adenosine monophosphate content was unchanged. In addition, sodium nitroprusside (EC50 = 0.1 microM) produced a concentration dependent relaxation of human lower oesophageal sphincter, significantly increased cyclic guanosine monophosphate content, but did not alter cyclic adenosine monophosphate content. Zaprinast (M&B 22948) and SK&F 94120, selective inhibitors of cyclic guanosine monophosphate and cyclic adenosine monophosphate phosphodiesterases, respectively, both relaxed human lower oesophageal sphincter with a potency similar to that seen in the dog or opossum lower oesophageal sphincter. Finally, the 8-bromo analogues of both cyclic adenosine monophosphate (EC50 = 420 microM) and cyclic guanosine monophosphate (EC50 = 100 microM) relaxed the human lower oesophageal sphincter. These studies suggest that in the human, as well as the canine and opossum lower oesophageal sphincter, increases in cyclic nucleotide content are associated with relaxation and increases in cyclic guanosine monophosphate are associated with the relaxation induced by stimulation of enteric neurons.

8-Bromo Cyclic Adenosine Monophosphate↗
PubMed1990

Inhibition of neuronally induced relaxation of canine lower esophageal sphincter by opioid peptides.

Opioid peptides have profound effects on gut motility. To assess their actions on enteric neurons regulating sphincteric smooth muscle, the ability of several opioid agonists to antagonize the neuronally induced relaxation of canine lower esophageal sphincter smooth muscle was examined. Opioid peptides selective for mu (FK 33-824) or delta [( D-Pen2,D-Pen5]enkephalin) receptors produced a concentration dependent inhibition of electrical field stimulation (EFS)-induced relaxation. In contrast, neither kappa (ketocycloclazine) or sigma (SK & F 10047) opioid agonists were potent inhibitors of EFS-induced relaxation. This inhibition was relatively selective for opioid agonists since BHT 933 (alpha 2 adrenoceptor agonist) and SK & F 89124 (D2 dopamine agonist) did not inhibit EFS-induced relaxation. Furthermore, naloxone antagonized the effects of both FK 33-824 and DPDPE. These functional data suggest that opioid receptors are present on sphincteric intrinsic inhibitory neurons and that stimulation of these neuronal receptors can regulate lower esophageal sphincter relaxation.

Animals↗
PubMed1990

Activation of cyclic AMP-dependent protein kinase during canine lower esophageal sphincter relaxation.

In isolated strips of opossum lower esophageal sphincter (LES) smooth muscle, elevation of cyclic AMP (cAMP) content is associated with relaxation. Because the activation state of cyclic nucleotide-dependent protein kinases may be a more sensitive measure of functionally important changes in cyclic nucleotide levels, we examined the ability of several pharmacological agents and electrical stimulation of the enteric neurons to activate cAMP dependent-protein kinase (cA-PK) and to relax isolated strips of LES smooth muscle. Addition of either isoproterenol or SK&F 94120, a selective inhibitor of the low Km cAMP phosphodiesterase to isolated strips of canine LES produced concentration-dependent increases in the activity ratio of cA-PK and concentration-dependent relaxations of canine LES. In contrast, although both zaprinast (M&B 22948), a selective inhibitor of the cyclic GMP selective phosphodiesterase, and sodium nitroprusside relaxed canine LES neither drug significantly increased the activity ratio of cA-PK. Furthermore, electrical stimulation of the enteric neurons produced a frequency-dependent relaxation but did not significantly activate cA-PK. To eliminate the possibility that the rapid metabolism of cAMP prevented us from observing a significant activation of cA-PK during electrical field stimulation, the ability of electrical field stimulation (1.0 Hz) to activate cA-PK was examined in the presence of 0.1 mM 3-isobutyl-1-methylxanthine. At at concentration of 0.1 mM, 3-isobutyl-1-methylxanthine, by itself, significantly increased the activity ratio of cA-PK; however, there was no additional activation produced by electrical field stimulation.(ABSTRACT TRUNCATED AT 250 WORDS)

3',5'-Cyclic-AMP Phosphodiesterases↗
PubMed1986

Lower esophageal sphincter relaxation is associated with increased cyclic nucleotide content.

Experiments were conducted to determine whether relaxation of the opossum isolated lower esophageal sphincter (LES), induced by electrical field stimulation (EFS) or various pharmacological agents, is associated with changes in cyclic nucleotide content. EFS relaxed the LES in a frequency-dependent manner with 0.7 Hz producing half-maximal relaxation. Control tissues and tissues stimulated at various frequencies were clamp-frozen and assayed for cyclic nucleotide content. EFS had no effect on adenosine 3',5'-cyclic monophosphate (cAMP) content but increased guanosine 3',5'-cyclic monophosphate (cGMP) content in a frequency-dependent manner. Tetrodotoxin eliminated both the relaxation and cGMP accumulation in response to EFS. Vasoactive intestinal polypeptide (VIP) relaxed the LES with an EC50 of 0.1 microM. In contrast to the results with EFS, VIP enhanced cAMP content but had no effect on cGMP content. Relaxation of the LES produced by sodium nitroprusside or atriopeptin II was accompanied by an increase in cGMP accumulation, whereas isoproterenol- and dopamine-induced relaxation was accompanied by an increase in cAMP content. The data indicate that, depending on the stimulus, increases in either cAMP or cGMP content can accompany LES relaxation. These results are consistent with the proposed role of cyclic nucleotides as second messengers mediating LES relaxation.

Animals↗
PubMed1986

Cyclic nucleotide-dependent protein kinases in the lower esophageal sphincter.

The characteristics and regulation of adenosine 3',5'-cyclic monophosphate (cAMP)-dependent and guanosine 3',5'-cyclic monophosphate (cGMP)-dependent protein kinases (PKs) in opossum, canine, and human lower esophageal sphincter (LES) were investigated. As measured by the incorporation of 32P from [gamma-32P]adenosine 5'-triphosphate (ATP) into histone, LES homogenates from all three species contained three distinct types of PK: cAMP-dependent PK, cGMP-dependent PK, and cyclic nucleotide-independent PK. In all three species, cAMP-dependent PK comprised approximately 80%, cGMP-dependent PK comprised approximately 10%, and cyclic nucleotide-independent PK comprised approximately 10% of the total PK activity in the LES. Diethylaminoethyl-sepharose column chromatography of the supernatant fraction of opossum LES homogenates revealed that of the total cAMP-dependent PK, 10% was Type I and 90% was Type II. In contrast, equal amounts of Type I and Type II cAMP-dependent PK were present in both the human and canine LES. Isoproterenol-induced relaxation of the isolated opossum LES was accompanied by an increase in cAMP content and an activation of cAMP-dependent PK. The results of this study support the proposal that cyclic nucleotides and cyclic nucleotide-dependent PKs regulate LES tone.

Adenosine Triphosphate↗
PubMed1985

Inhibition of calmodulin-stimulated phosphodiesterase activity by vasoactive intestinal peptide.

The effects of certain peptides of the glucagon family on calmodulin activity were determined from their capacity to inhibit a calmodulin-dependent form of phosphodiesterase. Vasoactive intestinal peptide and secretin were potent inhibitors of calmodulin activity, having IC50 values of 0.5 microM and 2 microM, respectively. By contrast, glucagon failed to inhibit calmodulin activity even at concentrations of 100 microM. None of these compounds significantly inhibited the basal activity of phosphodiesterase at concentrations up to 100 microM. These findings support the suggestion that important structural features of peptides for anticalmodulin activity include a net positive charge and a hydrophobic surface.

Animals↗
PubMed1983

Inhibition of calmodulin activity by insect venom peptides.

Several peptides found in insect venom, including melittin, apamin and mastoparan, inhibited the activity of calmodulin-stimulated phosphodiesterase at concentrations that had no appreciable effect on basal phosphodiesterase activity; the Ki value of melittin for inhibiting calmodulin activity was 30 nM. Acetylation of the peptides reduced their inhibitory effect on calmodulin, suggesting that a net positive charge was an important determinant of anti-calmodulin activity. An examination of other structural features of these peptides suggested that the most potent inhibitors of calmodulin had an alpha-helical conformation. Equilibrium dialysis experiments showed that melittin inhibited the calcium-dependent binding of 3H-chlorpromazine to calmodulin (IC50 0.9 microM); kinetic analyses of these data indicated that this inhibition was non-competitive, suggesting that melittin and chlorpromazine act at different sites on calmodulin. Since calmodulin regulates a number of processes that these peptides inhibit, our results raise the possibility that the inhibition of calmodulin activity by these insect venom peptides may explain some of their biochemical or toxicological effects.

Animals↗
PubMed1980

Pharmacological regulation of calmodulin.

A number of psychotropic drugs, particularly the phenothiazines and related antipsychotic compounds, inhibit a variety of calmodulin-dependent enzymes. The mechanism by which these compounds inhibit the activity of calmodulin is through a selective calcium-dependent binding to this protein. With the notable exception of certain stereoisomers, compounds that are clinically effective antipsychotic agents showed the greatest degree of binding to calmodulin. Other classes of pharmacological agents, including aminergic agonists and antagonists, and nonspecific central nervous system depressants and stimulants, showed little or no binding to calmodulin. In fact, the specificity with which antipsychotic drugs bind to calmodulin suggests the possibility of screening for new and clinically more effective antipsychotic agents based on their selective binding to calmodulin. Certain neuropeptides that produce behavioral effects in animals also were found to inhibit the activity of calmodulin, suggesting that there may be endogenous psychotogens or antipsychotic peptides that interact with calmodulin. Although under ordinary conditions the binding of antipsychotics to calmodulin is reversible, the binding of phenothiazine antipsychotics to calmodulin can be made irreversible either photochemically by ultraviolet irradiation, or enzymatically by a hydrogen peroxide-peroxidase system. Such a labeling technique should prove to be a useful tool to study the localization and turnover of calmodulin. These results indicate that several of the diverse biochemical actions of antipsychotic agents can be explained by a common mechanism, namely, by their binding to and inhibition of calmodulin, and raise the possibility that calmodulin may serve as one of the cellular receptors for certain antipsychotic compounds. However, further studies must be completed before we can state with any degree of certainty that these in vitro biochemical findings can explain the pharmacological and clinical actions of the antipsychotics.

3',5'-Cyclic-AMP Phosphodiesterases↗