Possible interaction of cholinergic nerves with two different (pre and post) sites of the neuromuscular junction in guinea-pig vas deferens

Cholinergic innervation of the guinea-pig isolated vas deferens

Recently, a cholinergic neurogenic component of contraction has been characterised in the aganglionic mouse vas deferens. In this paper, a cholinergic component of contraction in the guinea-pig vas deferens is characterised pharmacologically. A residual, tetrodotoxin-sensitive (TTX, 0.3 microM), neurogenic contraction was revealed after prolonged exposure (5 h) to the adrenergic neurone blocker bretylium (20 microM) or in the presence of prazosin (100 nM) and alpha,beta-methylene ATP (1 microM), a purinergic agonist which desensitizes P2X receptors. The bretylium-resistant component was potentiated by the acetylcholinesterase (AChE) inhibitor neostigmine (10 microM) and inhibited by the muscarinic-receptor (mAChR) antagonist cyclopentolate (1 microM). Nicotine (30 microM) enhanced the bretylium-resistant component. Neostigmine increased the second component of contraction in the presence of prazosin and alpha,beta-methylene ATP, whilst yohimbine (1 microM), an alpha(2) adrenergic receptor antagonist, enhanced both the first and second components of the electrically evoked contraction. These enhanced contractions were blocked by cyclopentolate in both cases. Nicotine enhanced the cholinergic component of contraction revealed by neostigmine but failed to have any detectable effects in the presence of cyclopentolate. Neostigmine alone increased the slow component of contraction which was reversed by cyclopentolate to control levels. The M(3) receptor-antagonist 4-DAMP (10 nM) markedly inhibited the cholinergic component of contraction to a level comparable with cyclopentolate. Laser microscopy has shown that neostigmine also increased the frequency of spontaneous Ca(2+) transients remaining in smooth muscle cells after perfusion with prazosin and alpha,beta-methylene ATP, an effect blocked by 4-DAMP. These experimental data show that there is a functional cholinergic innervation in the guinea-pig vas deferens whose action is limited by acetylcholinesterase, blocked by cyclopentolate and mediated through M3 receptors. Moreover, by blocking the cholinesterase, the increased amount of ACh generates spontaneous Ca(2+) transients in smooth muscle cells.

Physiological features of visceral smooth muscle cells, with special reference to receptors and ion channels

Visceral smooth muscle cells (VSMC) play an essential role, through changes in their contraction-relaxation cycle, in the maintenance of homeostasis in biological systems. The features of these cells differ markedly by tissue and by species; moreover, there are often regional differences within a given tissue. The biophysical features used to investigate ion channels in VSMC have progressed from the original extracellular recording methods (large electrode, single or double sucrose gap methods), to the intracellular (microelectrode) recording method, and then to methods for recording from membrane fractions (patch-clamp, including cell-attached patch-clamp, methods). Remarkable advances are now being made thanks to the application of these more modern biophysical procedures and to the development of techniques in molecular biology. Even so, we still have much to learn about the physiological features of these channels and about their contribution to the activity of both cell and tissue. In this review, we take a detailed look at ion channels in VSMC and at receptor-operated ion channels in particular; we look at their interaction with the contraction-relaxation cycle in individual VSMC and especially at the way in which their activity is related to Ca2+ movements and Ca2+ homeostasis in the cell. In sections II and III, we discuss research findings mainly derived from the use of the microelectrode, although we also introduce work done using the patch-clamp procedure. These sections cover work on the electrical activity of VSMC membranes (sect. II) and on neuromuscular transmission (sect. III). In sections IV and V, we discuss work done, using the patch-clamp procedure, on individual ion channels (Na+, Ca2+, K+, and Cl-; sect. IV) and on various types of receptor-operated ion channels (with or without coupled GTP-binding proteins and voltage dependent and independent; sect. V). In sect. VI, we look at work done on the role of Ca2+ in VSMC using the patch-clamp procedure, biochemical procedures, measurements of Ca2+ transients, and Ca2+ sensitivity of contractile proteins of VSMC. We discuss the way in which Ca2+ mobilization occurs after membrane activation (Ca2+ influx and efflux through the surface membrane, Ca2+ release from and uptake into the sarcoplasmic reticulum, and dynamic changes in Ca2+ within the cytosol). In this article, we make only limited reference to vascular smooth muscle research, since we reviewed the features of ion channels in vascular tissues only recently.

Noradrenergic, purinergic, and cholinergic transmission in the mouse vas deferens: influence of field-stimulation parameters, reserpinization, 6-hydroxydopamine and 4-aminopyridine

In the field-stimulated mouse vas deferens the twitch inhibiting potency of prazosin (1 microM) and alpha,beta-methylene ATP (MeATP, 10 microM) was studied, using two types of stimulation-response curves, (a) variation of frequency from 3 to 100 Hz at a constant pulse width of 0.1 ms and (b) variation of pulse width from 0.04 to 0.8 ms at a constant frequency of 15 Hz. Prazosin and MeATP reduced the twitch response by eliminating the noradrenergic and purinergic component, respectively. After the combined application of both compounds a small third twitch component remained that was most prominent at high frequencies. Reserpinization reduced the effect of prazosin but enhanced that of MeATP and increased the cholinergic component. 6-Hydroxydopamine enhanced the effects of prazosin and MeATP to the same extent, but left the cholinergic component intact. In vasa pre-loaded with [3H]-noradrenaline, field stimulation induced a larger release of tritium at high frequency and short pulse duration (100 Hz, 0.1 ms) than at lower frequency and long pulse duration (15 Hz, 0.3 ms). Prazosin (1 microM) augmented both the spontaneous and the stimulation-induced overflow of tritium, whereas MeATP (10 microM) had only a negligible negative effect on the outflow of label. In conclusion, the twitch contraction of the mouse vas deferens has a small cholinergic component in addition to the noradrenergic and purinergic components. Adrenergic and purinergic transmission seem not to run strictly in parallel: the purinergic component dominates during stimulation at low frequency and long pulse duration, and after reserpinization; 4-aminopyridine enhances the adrenergic mechanism more than the purinergic one.

Two alpha-adrenergic contractions with different Ca2+ activation mechanisms in response to field nerve stimulation in the circular smooth muscle of guinea-pig vas deferens

1. Two noradrenergic components of contraction were induced by field nerve stimulation of the circular smooth muscle of guinea-pig vas deferens. One component occurred during the stimulation (former-response) and the other component occurred after stimulation had ceased (after-response), with a distinct tension fall between these two responses. 2. The effects of stimulation frequency and duration upon these components were examined. 3. Prazosin (0.05-0.5 microM) suppressed both responses, whereas nifedipine (0.02 microM) suppressed only the after-response. 4. It is suggested that in this tissue there are two neurogenic alpha-adrenergic contractions which possess different Ca2+ activation mechanisms. The former-response is due to an increase in cytoplasmic Ca2+ concentration through a pathway independent of action potentials, while the after-response is attributed to action potentials arising as a secondary response to the released noradrenaline.

Involvement of cholinergic nerves in excitatory junction potentials through prejunctional nicotinic receptors in the guinea-pig vas deferens

1. The effects of various drugs on excitatory junction potentials (EJPs) elicited by field stimulation of the longitudinal smooth muscle of the guinea-pig vas deferens were examined using the sucrose-gap method. 2. Field stimulation with a single pulse (1.0 ms, 20 Vcm-1) produced a depolarization of 3-7 mV. When a single pulse was repetitively applied, the depolarization showed summation, resulting in action potential firing. 3. Tetrodotoxin (1 microM) and guanethidine (1 microM) completely suppressed EJPs whilst propranolol (1 microM) or atropine (0.1 microM) did not affect them. Phentolamine (1 microM) slightly potentiated EJPs. 4. d-Tubocurarine (dTC) at lower concentrations (0.5-2 microM) suppressed EJPs, but at a higher concentration (20 microM) the drug little affected them. 5. Hexamethonium (C6) suppressed EJPs at low concentrations (0.5-5 microM), but potentiated them at a higher concentration (50 microM). 6. Either dTC or C6, each at a concentration of 0.5 microM, did not affect the depolarization induced by exogenously applied ATP or noradrenaline. 7. These results suggest that in this tissue cholinergic nerve activity can contribute to the magnitude of EJPs, and that this effect is mediated through nicotinic receptors.