Evidence for nitric oxide as an inhibitory neurotransmitter in rabbit isolated anococcygeus

The nonadrenergic noncholinergic relaxation of anococcygeus muscles of bile duct-ligated rats

Previous studies have shown the naloxone-induced withdrawal syndrome and the development of tolerance in the tissues of cholestatic animals. Increased neuronal nitric oxide synthase (nNOS) expression is reported to exist in morphine-tolerant animals. This, together with evidence for nitric oxide (NO) overproduction in cholestasis, suggested the possibility of an alteration of nonadrenergic noncholinergic (NANC) relaxation of anococcygeus muscles of cholestatic rats. To study this, we used three main groups of animals: unoperated, sham-operated and bile duct-ligated. Electrical field stimulation, in the presence of atropine and guanethidine, caused NANC relaxation in the anococcygeus muscle which was enhanced in bile duct-ligated animals. N(G)-nitro-L-arginine methyl ester (L-NAME), a NOS blocker, caused a dose-dependent inhibition of the NANC relaxation. The IC(50)'s of L-NAME in 7-day (7.30+/-0.87 microM), 14-day (6.98+/-0.70 microM) and 21-day (8.25+/-1.40 microM) bile duct-ligated groups were significantly different from those of unoperated (1.69+/-0.30 microM) and sham-operated groups (1.90+/-0.27 microM). L-NAME (100 microM) completely inhibited the NANC relaxation response, suggesting that NANC relaxation in the rat anococcygeus muscle is mediated mainly via NO. The contraction response of the intact muscle to phenylephrine, an alpha(1)-adrenoceptor agonist, and the relaxation response of the phenylephrine-contracted muscle to sodium nitroprusside, an NO donor, were not different in unoperated, sham-operated and 7-day bile duct-ligated groups. These results showed that the smooth muscle component of NANC relaxation is not altered in anococcygeus muscles of bile duct-ligated rats. It can thus be concluded that the NANC relaxation in the anococcygeus of cholestatic rats is more resistant to a NOS blocker, providing evidence for increased nitrergic neurotransmission in the anococcygeus muscles of cholestatic rats.

Biology of the anococcygeus muscle

The anococcygeus is a smooth muscle tissue of the urogenital tract which, in the male, runs on to form the retractor penis. The motor innervation is classically sympathetic with noradrenaline as transmitter, but the relaxant parasympathetic transmitter has only recently been identified as nitric oxide. Indeed, the anococcygeus has provided an extremely useful model with which to probe the mechanisms underlying this novel nitrergic system, including the importance of physiological antioxidants in maintaining the potency of nitric oxide as a neurotransmitter. The cellular mechanisms of contraction and relaxation are slowly being clarified, with particular interest in the contribution of capacitative calcium entry and the guanylyl cyclase/cyclic GMP system. Many questions remain unanswered, however, including the precise physiological role of the muscle, the identity of substances released from subcellular vesicles of nitrergic nerves, the unusual sensitivity of the tissue to certain peptides (oxytocin and urotensin II), and the nature of store-operated channels through which calcium enters the cell to maintain contraction.

Distribution of NADPH diaphorase-positive neurons in the enteric nervous system of the rabbit intestine

Nitric oxide (NO) has been proposed as an inhibitory transmitter in gastrointestinal muscle relaxation. We analyzed the distribution of nitric-oxide producing neurons in the rabbit intestine through nicotinamide-adenine-dinucleotide-phosphate-diaphorase histochemistry. By this reliable and convenient method, we visualized neuronal nitric-oxide-synthase, the enzyme responsible for nitric oxide generation, in the rabbit intestine. In the ileum and rectum, nitric-oxide-synthase-related diaphorase activity was present in the myenteric plexus ganglion cells, and in the nerve fibers in the internodal strand, secondary, and tertiary plexuses. These fibers were particularly abundant in the deep circular rather than in the outer longitudinal muscle layer. In the inner submucosal plexus, we found scarce labeled neurons. Labeled neural somata showed a range of sizes and shapes suggesting different functional roles. The present basic information is required to use the rabbit as an experimental animal in neurochemical NO enteric research.

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.

Inhibition of nitrergic relaxations by a selective inhibitor of the soluble guanylate cyclase

1. The actions of 1H-[1,2,4]oxadiazolo[4,3,-a]quinoxalin-1-one (ODQ), a specific inhibitor of the soluble guanylate cyclase (SGC), were investigated in the rabbit anococcygeus muscle. 2. ODQ (1 nM-1 microM) inhibited in a concentration-dependent manner the relaxations induced by electrical field stimulation (EFS; 50 V, 0.3 ms duration, 1 Hz, for 5 s, every 120 s). 3. ODQ (1 microM) also inhibited the relaxations elicited by EFS (50 V, 0.3 ms duration, 1, 2.5, 5, 10 Hz, for 5 s) and sodium nitroprusside (SNP; 1 microM) without affecting those induced by isoprenaline (1 microM), atrial natriuretic peptide (ANP; 100 nM) or an analogue of cyclic GMP (8-pCPT-cyclic GMP; 500 microM). 4. ODQ (1 microM) inhibited the elevations in the concentration of cyclic GMP induced by SNP or EFS, but not by ANP. ODQ did not affect the concentrations of cyclic AMP. 5. Nitrergic relaxation in this tissue appears, therefore, to be mediated via activation of SGC.

Characterization of nitrergic neurotransmission during short- and long-term electrical stimulation of the rabbit anococcygeus muscle

1. Isolated preparations of rabbit anococcygeus muscle were exposed to electrical field stimulation (EFS; 50V, 0.3 ms duration, 0.08-40 Hz) for periods of 1-60 s (short-term EFS) or 10 min-2 h (long-term EFS). 2. Both short- and long-term EFS caused a contractile response which was enhanced by the nitric oxide (NO) synthase inhibitor, NG-nitro-L-arginine (L-NOARG), showing that it is modulated by endogenous NO. 3. In preparations treated with scopolamine and guanethidine and in which a constrictor tone was induced by histamine, both short- and long-term EFS resulted in relaxation of the tissue. 4. Such relaxations were reversed by tetrodotoxin (TTX), omega-conotoxin, inhibitors of NO synthase and the NO scavenger, oxyhaemoglobin, indicating that they are neuronal in origin and nitrergic in nature. 5. The relaxations to long-term EFS persisted for the duration of the stimulation and were associated with sustained release of oxidation products of NO (NOx). The EFS-induced release of NOx was decreased by N-iminoethyl-L-ornithine (L-NIO), an inhibitor of NO synthase, and by TTX. 6. Inhibitors of NO synthase, in addition, increased the basal tone of the tissue and reduced the basal output of NOx. The basal output of NOx was also reduced by TTX. 7. Long-term EFS which induces approximately 50% of the maximum relaxation could be enhanced by addition of L-, but not D-, arginine to the perfusion medium. 8. These data show that there is a continuous basal release of NO from nitrergic nerve terminals which maintains a relaxant tone in the rabbit anococcygeus muscle. 9. In addition, NO is released during short- and long-term EFS which further relaxes the preparation and modulates sympathetic transmission. Activation of the L-argimne: NO pathway for periods up to2 h does not exhaust nitrergic transmission in any appreciable way.