Effects of N6,2'-O-dibutyryladenosine 3',5'-cyclic monophosphate, adenosine, and of oxotremorine and 3-isobutyl-1-methylxanthine on the electrically evoked [3H]acetylcholine secretion in the guinea-pig ileum myenteric plexus

Purinergic mechanisms in the control of gastrointestinal motility

For many years, ATP and adenosine have been implicated in movement regulation of the gastrointestinal tract. They act through three major receptor subtypes: adenosine or P1 receptors, P2X receptors and P2Y receptors. Each of these major receptor types can be subdivided into several different classes and is widely distributed amongst various neurons, muscle types, glia and interstitial cells that regulate intestinal functions. Several key roles for the different receptors and their endogenous ligands have been identified in physiological and pharmacological studies. For example, adenosine acting at A(1) receptors appears to inhibit intestinal motility in various pathological conditions. Similarly, ATP acting at P2Y receptors is an important component of inhibitory neuromuscular transmission, acting as a cotransmitter with nitric oxide. ATP acting at P2X and P2Y(1) receptors is important for synaptic transmission in simple descending excitatory and inhibitory reflex pathways. Some P2Y receptor subtypes prefer uridine nucleotides over purine nucleotides. Thus, roles for UTP and UDP as enteric transmitters in place of ATP cannot be excluded. ATP also appears to be important for sensory transduction, especially in chemosensitive pathways that initiate local inhibitory reflexes. Despite this evidence, data are lacking about the roles of either adenosine or ATP in more complex motility patterns such as segmentation or the interdigestive migrating motor complex. Clarification of roles for purinergic transmission in these common, but understudied, motility patterns will depend on the use of subtype-specific antagonists that in some cases have not yet been developed.

Muscarinic autoinhibition of acetylcholine release in mouse atria is not transduced through cyclic AMP or protein kinase C

1. The present study investigated the second messenger pathways that may mediate muscarinic receptor autoinhibition of acetylcholine release in mouse atria. The stimulation-induced (S-I) outflow of radioactivity from mouse isolated atria incubated with [3H]-choline was Ca(2+)-dependent and tetrodotoxin-sensitive and was used as an index of neuronal acetylcholine release. 2. The cell permeable analogue of cyclic AMP, 8-bromocyclic AMP (1 x 10(-3)M) enhanced the S-I outflow of radioactivity (33%), lower concentrations having no effect. Similarly, the adenylate cyclase activator forskolin (1 x 10(-5)M) had a small facilitatory effect on acetylcholine release. On the other hand the phosphodiesterase inhibitor 3-isobutylmethylxanthine (1 x 10(-4)M) had no effect on the S-I outflow of radioactivity. Together these results suggest that the adenylate cyclase/cyclic AMP system does not have an appreciable role in the modulation of acetylcholine release. 3. The protein kinase C activator phorbol dibutyrate (0.1-3 x 10(-6)M) enhanced the S-I acetylcholine release (maximally by 45%). The effects of phorbol dibutyrate were attenuated by the protein kinase inhibitor staurosporine (1 x 10(-7)M), which by itself had no effect on the S-I outflow of radioactivity. This latter result suggests that there is no tonic activation of protein kinase C during acetylcholine release. 4. Atropine (1 x 10(-7)M) markedly enhanced (232%) the S-I outflow of radioactivity, presumably by preventing feedback inhibition on acetylcholine release through prejunctional muscarinic receptors. This effect is unlikely to involve adenylate cyclase or protein kinase C since it was far greater than the effects of activation of either system with forskolin and phorbol dibutyrate, respectively. Furthermore, the facilitatory effect of atropine was not attenuated by staurosporine, which although a protein kinase C inhibitor, is also an effective inhibitor of cyclic AMP dependent protein kinase (protein kinase A).

Role of cyclic AMP in prostaglandin-induced modulation of acetylcholine release from the myenteric plexus of guinea pig ileum

Prostaglandins (PGs) have modulatory effects on spontaneous and nicotine-induced release of acetylcholine (ACh) from the myenteric plexus of guinea pig ileum. To determine whether cyclic AMP is involved in the mechanisms of these effects, we studied ACh release under conditions that inhibit PG synthesis. Indomethacin (IND), a cyclooxygenase inhibitor, inhibited ACh release concentration-dependently. The effect of the maximally inhibitory concentration of IND (2.8 microM) on nicotine-induced ACh release were reversed concentration-dependently by PGE2, forskolin, 3-isobutyl-1-methylxanthine (IBMX) and 8-bromo cyclic AMP. These compounds caused concentration-dependent reversal of the inhibition of spontaneous ACh release by IND, but their concentrations for restoration of spontaneous release were higher than those for restoration of nicotine-induced release. The effects of PGE2 and forskolin or IBMX were not additive in reversing the inhibition of nicotine-induced ACh release by IND. Neither forskolin nor 8-bromo cyclic AMP alone had any significant effect on either release. These results showed that increase in the level of cyclic AMP in myenteric cholinergic neurons restored ACh release from the tissue whose PG level had been lowered by IND and indicated that endogenous PGs may modulate the level of intraneuronal cyclic AMP.

Subtype classification of the presynaptic alpha-adrenoceptors which regulate [3H]-noradrenaline secretion in guinea-pig isolated urethra

1. The following experiments were carried out to investigate the presence and type of functional presynaptic receptors in adrenergic nerves of the guinea-pig urethra. 2. The urethra from male guinea-pigs was incubated with [3H]-noradrenaline and superfused with Tyrode solution in vitro. The fractional secretion of [3H]-noradrenaline evoked by 300 electrical pulses was measured. 3. The [3H]-noradrenaline secretion was positively frequency-dependent, yielding a half-maximal secretion at 8 +/- 5 Hz. Stimulation was usually applied at 5 Hz. 4. The [3H]-noradrenaline secretion was not altered by noradrenaline (1 or 100 microM), norephedrine (1 microM), isoprenaline (0.1 microM), 5-hydroxytryptamine (10 microM), oxotremorine (10 microM), adenosine (0.2 mM), propranolol (1 microM), atropine (1 microM) or 8-phenyltheophylline (10 microM). 5. The [3H]-noradrenaline secretion was enhanced by clonidine (3 microM), chlorpromazine (10 microM), metitepine (1 microM), 4-aminopyridine (0.5 mM), tetraethylammonium (2 mM), 3-isobutyl-1-methylxanthine (4 mM), 8-bromo cyclic AMP (1 mM) and forskolin (25 microM). 6. The alpha-adrenoceptor antagonists rauwolscine, yohimbine, phentolamine, prazosin and AR-C 239 maximally enhanced the [3H]-noradrenaline secretion to about 300% of control. The partial alpha-adrenoceptor agonist oxymetazoline maximally enhanced the secretion to about 200% of control. The order of apparent EC50 values was rauwolscine less than yohimbine less than phentolamine less than oxymetazoline less than prazosin less than AR-C 239.7. The enhancing effects of yohimbine (1 microM) with tetraethylammonium (2mM), 8-bromo cyclic AMP (1 mM), or forskolin (25,microM) were additive, but not those of yohimbine (1 microM) with prazosin (10 microM), 4-aminopyridine (0.5 mM), or 3-isobutyl-1-methylxanthine (4 mM).8. These results suggest that the [3H]-noradrenaline secretion in the guinea-pig urethra is regulated by presynaptic alpha2A-adrenoceptors which may, in a cyclic AMP-independent manner, be coupled to a 4-aminopyridine-sensitive potassium channel. The secretion is not influenced by compounds acting at beta-adrenoceptors, muscarinic cholinoceptors or adenosine receptors.