Pharmacological control of the synthesis and metabolism of cyclic nucleotides

Effects of modulatory agents on neurally-mediated responses of trout intestinal smooth musclein vitro

Mediators and mechanisms responsible for the inhibitory modulation of trout intestinal smooth muscle were examined using a series of putative mediators and substances known to modulate neurotransmission in mammalian systems. Frequency response relationships to transmural stimulation and concentration response relationships to 5-hydroxytryptamine, carbachol, and substance P were established on paired segments of rainbow trout intestinein vitro in the presence and absence of putative modulatory agents. Modulation of neurally-mediated contractions of trout intestine was achieved with dibutyryl cyclic AMP and forskolin, agents that increase intracellular levels of cyclic AMP. The effect appears to be at the level of the smooth muscle, since the adenylate cyclase activator, forskolin, inhibited muscarinic and serotoninergic contractions as well as transmurally stimulated contractions. Substance P-induced contractions were unaffected by forskolin. The endogenous agonists/neurotransmitters which would increase cyclic AMP levels in rainbow trout intestinal smooth muscle are as yet unknown. The effects do not appear to be modulated by vasoactive intestinal peptide (VIP), calcitonin, calcitonin gene-related peptide (CGRP), or agents that activate β-adrenoceptors. Prostaglandin E2 (PGE2) and α2-adrenergenic agonists are possible agents which will decrease contractility of the smooth muscle. They were only active in the proximal intestine and on transmurally stimulated contractions. The effects of both PGE2 and α2-agonists appear to be prejunctional, decreasing release of contractile neurotransmitters in the enteric nervous system.

Mechanism for shifting the phase of a circadian rhythm by serotonin: involvement of cAMP

Serotonin (5-hydroxytryptamine, 5-HT) shifts the phase of the circadian rhythm in the eye of Aplysia. We have examined the role of cAMP in mediating the effects of 5-HT on the rhythm. The phase shifts produced by 5-HT are mimicked by treatments that should increase intracellular levels of cAMP. An analogue of cAMP-8-benzylthio-cAMP, advanced and delayed the rhythm at phases in which 5-HT had similar effects on the rhythm. In addition, two phosphodiesterase inhibitors, Ro-20-1724 and papaverine, caused advance phase shifts where 5-HT advances the rhythm. The phosphodiesterase inhibitors Ro-20-1724 and 3-isobutyl-1-methylxanthine each potentiated the effect of subthreshold doses of 5-HT on the rhythm. The effects of 5-HT and 8-benzylthio-cAMP on the rhythm were nonadditive, indicating that 5-HT and 8-benzylthio-cAMP affect the rhythm through a common pathway. Finally, 5-HT produced large changes (13-fold) in the levels of cAMP in the eye. These results indicate that cAMP mediates the effect of 5-HT on the rhythm. There are two possible roles for cAMP in the circadian system. Either the cAMP system is an intracellular step in an entrainment pathway or it is part of the biological clock. Because 5-HT, 8-benzylthio-cAMP, and three phosphodiesterase inhibitors inhibit impulses from the eye, cAMP may also mediate the inhibition produced by 5-HT, or it might be involved in regulating the frequency of spontaneous impulses throughout the day.

The ligand specificity of the (adenosine 3',5'-monophosphate)-binding site of yeast glyceraldehyde-3-phosphate dehydrogenase. Interaction with adenosine derivatives and pharmacologically-active compounds

The high-affinity cAMP-binding site of form-II yeast glyceraldehyde-3-phosphate dehydrogenase has a marked specificity for adenosine derivatives, such ligands including N6-substituted adenosine derivatives active as cytokinins n plant systems and adenine nucleotides. Of a wide range of nucleotides and nucleosides examined only adenosine derivatives bind to the cAMP binding site. A variety of antimitotic compounds (including colchicine, colcemid and phenylcarbamate derivatives), adrenergic receptor antagonists (alprenolol and propranolol) and non-steroidal anti-inflammatory agents (notably indomethacin and flufenamic acid) displace cAMP from glyceraldehyde-3-phosphate dehydrogenase. Colchicine, colcemid, N6-furfuryladenosine, indomethacin, flufenamic acid and propranolol inhibit cAMP binding to the enzyme in an apparently competitive fashion. Given the evolutionary conservatism and abundance of glyceraldehyde-3-phosphate dehydrogenase, the affinity of the cAMP-binding site of this enzyme for a variety of structurally-disparate pharmacologically-active compounds compromises simple one-site interpretations of physiological responses to these agents.

Hemicyst formation stimulated by cyclic AMP in dog kidney cell line MDCK

Certain epithelial cell lines have morphologic, physiologic, biochemical and pharmacologic characteristics of transporting epithelia from intact organs. In this paper we show that dibutyryl cyclic AMP, 5' AMP, adenosine and cyclic AMP phosphodiesterase inhibitors stimulate hemicyst formation by the dog kidney cell line MDCK. It is suggested that this effect is explained by elevation of intracellular cyclic AMP levels by means of an exogenous non-metabolizable source of cyclic AMP, phosphodiesterase inhibition or adenyl cyclase stimulation. Since hemicyst formation is in part due to transepithelial fluid transport, these findings raise the possibility that this fraction might be modulated by cAMP in an established cell line. We believe that cultured epithelial cells may provide an exploitable model system to investigate at the cellular and subcellular levels, the mechanism by which cyclic AMP modifies water and solute movements across epithelia.

Phosphodiesterase inhibition by succinic and related acid biphenylalkyl monoesters

Biphenylalkyl monoesters of succinic and related acids represent a potent new class of cyclic AMP phosphodiesterase inhibitors. The biphenyl group is necessary for potent inhibition. The spatial relationship of the carboxyl and ester groups and alkyl chain length are important to inhibitory potency.

FMRFamide increases the adenylate cyclase activity and cylic AMP level of molluscan heart

FMRFamide (phenylalanyl-methionyl-arginyl-phenylalanine amide) is a cardioexcitatory peptide recently isolated and identified in molluscan ganglia. Both FMRFamide and 5-hydroxytryptamine (5HT), the cardioexcitatory neurotransmitter in molluscs, were tested on the ventricle of the bivalve Mercenaria mercenaria. Both agents increased myocardial contractility, the intracellular cyclic AMP concentration of intact hearts and the adenylate cyclase activity of a myocardial membrane fraction. FMRFamide was 5--10 times more potent than 5HT. All of the effects of 5HT, and none of those of FMRFamide, were blocked by methysergide, a specific 5HT antagonist.