ziconotide and alpha-beta-methyleneadenosine-5--triphosphate

1. We examined the effects of P2X purinoceptor agonists and P2 purinoceptor antagonists on mesenteric afferent nerves supplying the jejunum in the pentobarbitone sodium-anaesthetised rat. 2. ATP (0. 01-10 mg kg-1, i.a.) and alpha,beta-methylene-ATP (1-30 microg kg-1, i.a.) each induced dose-dependent increases in afferent nerve discharge and intrajejunal pressure. The effect on afferent nerves comprised an early (< 2 s after administration) intense burst of activity followed by a later increase (> 2 s after administration), less pronounced in comparison, which coincided with elevated intrajejunal pressure. 3. Pyridoxalphosphate-6-azophenyl-2', 4'-disulphonic acid (20 mg kg-1, i.v.) and suramin (80 mg kg-1, i.v. ) each antagonised both the early and later increases in afferent nerve discharge elicited by alpha,beta-methylene-ATP (30 microg kg-1, i.a.). 4. Co-administration of omega-conotoxin MVIIA and omega-conotoxin SVIB (each at 25 microg kg-1, i.v.), or treatment with the selective 5-HT3 receptor antagonist alosetron (30 microg kg-1, i.v.), did not affect the rapid burst of afferent nerve activity elicited by alpha,beta-methylene-ATP (30 microg kg-1, i.a.). However, toxin treatment did attenuate the elevations in intrajejunal pressure and the corresponding later phases of evoked afferent discharge, while alosetron inhibited basal afferent nerve activity. 5. In summary, ATP and alpha,beta-methylene-ATP each evoke excitation of mesenteric afferent nerves in the anaesthetised rat. We propose that the early increase in mesenteric afferent nerve activity represents a direct effect on the nerve ending, mediated by P2X receptors, whereas the later increase reflects activation of mechanosensitive fibres secondary to elevated intrajejunal pressure.

Topics: Action Potentials; Adenosine Triphosphate; Animals; Blood Pressure; Carbolines; Electrophysiology; Heart Rate; Jejunum; Male; Mesentery; Neurons, Afferent; omega-Conotoxins; Peptides; Purinergic P2 Receptor Agonists; Purinergic P2 Receptor Antagonists; Pyridoxal Phosphate; Rats; Rats, Wistar; Receptors, Purinergic P2; Serotonin; Suramin

Isolated guinea pig ileum displayed spontaneous contraction and fluctuation of membrane potential originating from the longitudinal muscle. Transmural stimulation of the enteric nerve plexus evoked contractions that were followed by lowered muscle tone and decreased spontaneous activity. The Na+ channel blocker tetrodotoxin, N-type Ca2+ channel blockers omega-conotoxins GVIA and MVIIA, the muscarinic receptor antagonist atropine, and inhibitory mediators alpha, beta-methylene-ATP and sodium nitroprusside all inhibited stimulation-evoked contraction while restoring spontaneous motility. L-type Ca2+ channel blockers nifedipine and calciseptine completely suppressed evoked and spontaneous contractions. Blockade of the large-conductance Ca(2+)-activated K+ (Kca) channel with charybdotoxin (but not of small-conductance Kca channel by apamin or of ATP-regulated K+ channel by glybenclamide) induced spikelike depolarization, inhibited nerve stimulation-evoked membrane hyperpolarization, and increased spontaneous activity. The results suggest that the large-conductance Kca channel is constitutively activated for modulations of spontaneous activity, and that muscle excitation, through elevation of Ca2+ levels, stimulates the large-conductance Kca channel to suppress spontaneous activity.

Topics: Adenosine Triphosphate; Animals; Atropine; Calcium; Calcium Channel Blockers; Electric Stimulation; Enzyme Inhibitors; Gastrointestinal Motility; Guinea Pigs; Ileum; Membrane Potentials; Muscle, Smooth; NG-Nitroarginine Methyl Ester; omega-Conotoxin GVIA; omega-Conotoxins; Parasympatholytics; Peptides; Potassium Channels; Tetrodotoxin