Breakdown of membrane choline-phospholipids induced by endogenous and exogenous muscarinic agonist is potentiated by VIP in rat submandibular gland

Effects of muscarinic agonists in the guinea-pig prostate

1. The contractile response to transmural stimulation of the guinea-pig prostate is largely due to activation of noradrenergic neurons but there is a small contribution from cholinergic neurons. Carbachol and acetylcholine have been reported to act via muscarinic M(1) cholinoceptors to facilitate contractions produced by neuronal stimulation of the tissue. This action of cholinomimetics was further investigated in isolated ventral lobes of the prostate. 2. Oxotremorine-M, bethanecol, pilocarpine, xanomeline and McN-A-343 produced facilitation of the response to transmural stimulation of the prostate. When carbachol was applied as the first agonist, the facilitatory response to the latter four agonists above was absent or reduced, compared with the effect observed when the other agonist was applied first, indicating that the effect of these agonists is readily desensitized. Only oxotremorine-M was unaffected by prior exposure of the tissue to carbachol. When applied first, pilocarpine and xanomeline produced a smaller degree of facilitation than carbachol indicating they were partial agonists for the effect. The facilitation produced by McN-A-343, when applied as the first agonist, was variable. In some preparations the facilitation was less than that of carbachol but in others it exceeded that produced by a subsequent application of carbachol. 3. The release of endogenous choline from the prostate was measured at rest and during transmural stimulation using a chemiluminescent technique. A statistically significant negative correlation existed between pmol mg(-1) of endogenous choline released during transmural stimulation and the mass of the ventral lobe of the prostate. As the guinea-pig prostate is known to undergo postpubertal stromal hypertrophy the finding suggests that endogenous choline release from the prostate is largely from epithelial, rather than stromal tissue. 4. The possible involvement of facilitatory M(1) autoreceptors on cholinergic neurons in the effect of cholinomimetics was investigated. Tissue was incubated with (3)H-choline to label neuronal stores of acetylcholine and the subsequent release of (3)H-choline from the tissue was measured. Carbachol per se increased the release of (3)H-choline. 5. Transmural stimulation usually increased the release of (3)H-choline but in c. 30% of preparations there was a decrease. In the presence of carbachol there was a significant increase in the release of (3)H-choline during transmural stimulation of prostate lobes. However, there was no correlation between the two effects of carbachol; facilitating contractions produced by transmural stimulation and enhancing (3)H-choline release during transmural stimulation. The finding provides no evidence that facilitatory M(1) autoreceptors on cholinergic neurons play a major role in the facilitation of contractions.

Rapid eye movement sleep induction by vasoactive intestinal peptide infused into the oral pontine tegmentum of the rat may involve muscarinic receptors

In rats, rapid eye movement sleep can be induced by microinjection of either the cholinergic agonist carbachol or the neuropeptide vasoactive intestinal peptide into the oral pontine reticular nucleus. Possible involvement of cholinergic mechanisms in the effect of vasoactive intestinal peptide was investigated using muscarinic receptor ligands. Sleep-waking cycles were analysed after infusion into the oral pontine reticular nucleus of vasoactive intestinal peptide (10 ng in 0.1 microl), carbachol (20 ng), atropine (200 ng) and pirenzepine (50, 100 ng), performed separately or in combination at 15-min intervals. The increase in rapid eye movement sleep due to the combined infusion of vasoactive intestinal peptide and carbachol (+58.7+/-4.6% for 8 h, P<0.05) was not significantly different from that induced by each compound separately. The enhancement of rapid eye movement sleep by vasoactive intestinal peptide was totally prevented by infusion of atropine, but not pirenzepine, a relatively selective M1 antagonist. On their own, none of the latter two compounds affected the sleep-waking cycle. Quantitative autoradiographic studies using [3H]quinuclidinyl benzylate (1 nM) and pirenzepine (0.5 microM) indicated that muscarinic receptors correspond to pirenzepine-insensitive binding sites in the oral pontine reticular nucleus. In vitro, vasoactive intestinal peptide (1-100 nM) significantly increased (+30-40%) the specific binding of [3H]quinuclidinyl benzylate to the oral pontine reticular nucleus in rat brain sections. This effect appeared to be due to an increased density, with no change in affinity, of pirenzepine-insensitive binding sites in this area. These data suggest that pirenzepine-insensitive muscarinic binding sites are involved in the induction of rapid eye movement sleep by vasoactive intestinal peptide at the pontine level in the rat.

Muscarinic receptor subtypes in the submandibular gland and the urinary bladder of the rabbit: in vivo and in vitro functional comparisons of receptor antagonists

1. In pentobarbitone-anaesthetized rabbits, the inhibitory effects of muscarinic receptor antagonists with different selectivity profiles were examined on carbachol-evoked submandibular secretion and urinary bladder contractions, and on parasympathetically nerve-evoked secretion. On isolated submandibular gland fragments, the inhibitory effects of the antagonists were studied on carbachol-evoked release of potassium and on the overflow of tritium in response to electrical field stimulation. 2. In vivo, 4-DAMP equipotently inhibited simultaneously carbachol-evoked submandibular secretory and contractile responses of the urinary bladder, while pirenzepine was found to be four times as potent in inhibiting the secretory response compared with the contractile response. 3. The inhibition of carbachol-evoked salivation caused by atropine, 4-DAMP and pirenzepine was as great as their inhibition of parasympathetic nerve-evoked salivation. Methoctramine exerted less inhibitory effect on nerve-evoked salivation than on carbachol-evoked, thus seemingly causing greater presynaptic inhibition. 4. In vitro, pirenzepine was only 30 times less potent in inhibiting carbachol-evoked potassium release than 4-DAMP (pA2, 9.58 vs 8.10). Whereas atropine, 4-DAMP and pirenzepine abolished the overflow of tritium from isolated glands in response to electrical field stimulation, methoctramine increased it. 5. It is concluded that the muscarinic secretory response in the rabbit submandibular gland is exerted via both muscarinic M1 and M3 receptors, while the contractile response of the urinary bladder to muscarinic agonists is exerted via muscarinic M3 receptors. The release of acetylcholine from nerve terminals in the gland can be inhibited via M2 autoreceptors in rabbits.

M1 muscarinic receptor-mediated facilitation of acetylcholine release in the rat urinary bladder

1. Release of [3H]ACh in response to electrical field stimulation (10 Hz) was measured in strips of rat urinary bladder and cardiac atrial tissues previously incubated with [3H]choline. 2. The volley output of [3H]ACh release was positively correlated with frequency of stimulation in the urinary bladder but negatively correlated in the atrium. 3. The quantity of [3H]ACh release was influenced by the pattern and duration of stimulation. Continuous stimulation (CS) with trains of 100 shocks released 10 times larger amounts of ACh than the same number of shocks presented as short trains of intermittent stimulation (IS): ten shocks per train with 5 s inter-train intervals. 4. The facilitation of transmitter release was antagonized completely by the administration of atropine (1 microM) or pirenzepine (0.05 microM), a selective M1 antagonist. Eserine, an anticholinesterase agent, markedly facilitated ACh release induced by CS and IS. This effect was blocked by atropine. 5. Release of ACh from atrial strips did not exhibit CS-induced facilitation. Eserine decreased IS- and CS-evoked ACh release in the atrium. 6. It is concluded that continuous stimulation of postganglionic cholinergic nerves in the rat urinary bladder leads to the activation of M1 muscarinic, facilitatory presynaptic receptors which enhance the release of ACh. Presynaptic facilitation may be an important mechanism for modulating neural input to the bladder during micturition.

Carbachol-induced phosphatidylcholine hydrolysis and choline efflux in rat submandibular gland involves phospholipase D activation and is modulated by protein kinase C and calcium

The role of calcium and protein kinase C activation in carbachol-induced choline efflux from submandibular glands was investigated. The participation of phospholipase D in this signal transduction pathway was demonstrated by the formation of [14C]phosphatidylethanol in [14C]lysophosphatidylcholine-labelled submandibular gland cells treated with carbachol or noradrenaline in the presence of ethanol. Chelation of the intracellular calcium with BAPTA/AM reduced the carbachol stimulated outflow of [3H]choline. The calcium ionophore A23187 in a high concentration (10 microM) increased the basal [3H]choline outflow, but decreased the carbachol-induced outflow. Removal of the extracellular calcium enhanced the carbachol-stimulated outflow, which returned to control when calcium was re-added to the medium. Activation of protein kinase C by phorbol-12,13-dibutyrate (100 nM) or 1-oleyl-2-acetyl-sn-glycerol (20 microM) was without effect per se, but enhanced the carbachol-mediated outflow of [3H]choline. Phorbol-12,13-dibutyrate in combination with 1 microM A23187 induced a small efflux of [3H]choline. A 2 h treatment with phorbol-12,13-dibutyrate (1 microM), causing down-regulation of protein kinase C, significantly decreased the carbachol-stimulated [3H]choline outflow. In conclusion, elevation of intracellular calcium levels and protein kinase C activation are of importance for the carbachol-stimulated outflow of [3H]choline. Inflow of calcium, if anything, reduces the carbachol-stimulated outflow of [3H]choline. Since phosphatidylcholine breakdown generates diacylglycerol and this could lead to activation of protein kinase C, activation of this signal transduction pathway may be important for the protein content of the saliva and for the known trophic effects of parasympathetic innervation.