A smooth muscle inhibitory material from the bovine retractor penis and rat anococcygeus muscles

Robert Furchgott (1916-2009): A scientist with a mission

Robert Furchgott was first noted for research on drug-receptor theory, autonomic neuroeffector mechanisms, and vascular pharmacology/physiology. His studies on drug-receptor interactions provided important knowledge about the properties of drug receptors long before methodologies were developed to study them directly. However, Furchgott achieved an enduring legacy for recognizing the importance of endothelial cells for the relaxation of vascular smooth muscle. On the basis of his own experiments and those of others, he proposed that acetylcholine interacted with muscarinic receptors at the surface of endothelial cells to release a substance called endothelium relaxing factor. Endothelium relaxing factor was later identified as nitric oxide, a colorless, odorless gas. Furchgott's discovery of an entirely new mechanism by which blood vessels dilate revolutionized studies on the physiology of the vascular system. His work also suggested new treatments for hypertension and heart disease, and was a key factor in the development of the anti-impotence drug sildenafil. In 1998, Robert Furchgott shared the Nobel Prize in Physiology or Medicine with Ferid Murad and Louis Ignarro.

Robert F. Furchgott, Nobel laureate (1916-2009)--a personal reflection

Robert F. Furchgott, pharmacologist and joint winner of the Nobel Prize for Medicine or Physiology (1998) died on the 12th of May 2009 aged 92. By unlocking the astonishingly diverse biological actions of nitric oxide, Furchgott leaves behind a rich legacy that has both revolutionized our understanding of human physiology and stimulated new and exciting opportunities for drug development in a wide range of pathological conditions. In this article, William Martin, who worked with Furchgott for 2 years (1983-1985), following the exciting discovery of endothelium-derived relaxing factor/nitric oxide, pays tribute to his close friend and colleague.

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.

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.

Nitric oxide and the non-adrenergic non-cholinergic neurotransmission

In the early 1960s, the first evidence was reported demonstrating neurally mediated responses in the presence of adrenergic and cholinergic antagonists, leading to the introduction of the concept of non-adrenergic non-cholinergic neurotransmission. The inhibitory component of this part of the autonomic nervous system has been illustrated in numerous organ systems mediating a wide range of physiological events. Since the discovery of these nerves, several substances have been proposed as putative neurotransmitter, with ATP and vasoactive intestinal polypeptide as main candidates. Finally, the ongoing research on the nature of the substance released by these nerves has generated the nitrergic theory proposing nitric oxide as putative neurotransmitter. By now, increasing evidence is reported to support the idea that inhibitory neurons release more neurotransmitters, interacting with each other at pre- and/or postsynaptic levels.

Non-adrenergic, non-cholinergic relaxation of the bovine retractor penis muscle: role of S-nitrosothiols

1. This study examined the possibility that an S-nitrosothiol, rather than nitric oxide, functions as the non-adrenergic, non-cholinergic (NANC) inhibitory neurotransmitter in the bovine retractor penis (BRP) muscle. 2. Treatment of BRP muscle with either of two sulphydryl inactivating agents, diamide (1 mM) and N-ethylmaleimide (0.3 mM), inhibited NANC relaxation and this was prevented by pretreating tissues with L-cysteine (3 mM), L-glutathione (3 mM) or dithiothreitol (3 mM). Inhibition was not specific, however, since the inactivating agents also inhibited the relaxant actions of authentic nitric oxide (0.3 microM), glyceryl trinitrate (0.001-1 microM) and isoprenaline (0.01-1 microM). 3. Reacting nitric oxide with L-cysteine in nominally oxygen-free solution at pH 3, followed by purging to remove free nitric oxide and neutralisation, produced greater and more prolonged relaxant activity when assayed on rabbit aortic rings than could be attributed to nitric oxide alone. H.p.l.c. analysis of the mixture identified a new peak distinct from either L-cysteine or nitric oxide which was responsible for the relaxant activity. The spectral absorption of this new compound had two bands with peaks at 218 and 335 nm. 4. Using a series of structural analogues of L-cysteine (all at 15 mM) it was found that removal of the carboxyl group (L-cysteamine), replacement of the carboxyl with an ester function (L-cysteine methyl ester) or substitution at the amino group (N-acetyl-L-cysteine) had no effect on the ability to generate relaxant activity upon reaction with nitric oxide (0.1 mM). In contrast, substitution at the sulphydryl group (S-methyl-L-cysteine, L-cysteinesulfinic acid and L-cysteic acid), or formation of disulphides(L-cystine and L-cystamine) led to a complete loss of ability to generate relaxant activity. L-Glutathione was also able to react with nitric oxide to produce relaxant activity, and this too was blocked upon substitution of the free sulphydryl group (S-methyl-L-glutathione). A free sulphydryl group was therefore required to generate relaxant activity following reaction with nitric oxide.5. Reacting L-cysteine (10 mM) with nitric oxide (~ 3 mM) under more stringent oxygen-free conditions followed by purging to remove free nitric oxide resulted in the generation of low relaxant activity and small absorption peaks at 218 and 335 nm and these were unaffected upon exposure to the air. In contrast, admitting air to the reaction chamber before purging enhanced both relaxant activity and the absorption peaks at 218 and 335 nm by some 40 fold and the solution turned pink due to the appearance of another absorption peak at 543 nm. This enhanced relaxant activity was not due to nitrogen dioxide being the reactive species, since at 0.1 mM this gas failed to react with L-cysteine to generate relaxant activity, and at 1 mM generated less activity than the equivalent concentration of nitricoxide.6. The relaxant activity generated by reacting nitric oxide with L-cysteine or L-glutathione was abolished following treatment with haemoglobin (3 MicroM), methylene blue (10 MicroM) or Nmethylhydroxylamine(100 MicroM), but was unaffected by N0-nitro-L-arginine (30 MicroM). Furthermore, two agents that generate superoxide anion, pyrogallol (0.1 mM) and hydroquinone (0.1 mM), also inhibited this relaxant activity as well as that induced by authentic nitric oxide (0.3 MicroM) but as previously reported, had no effect on relaxation induced by NANC nerve stimulation. Superoxide dismutase(100 u ml1) reversed the actions of pyrogallol and hydroquinone but had no effect on NANC relaxation.7. In conclusion, the reaction of nitric oxide with L-cysteine or L-glutathione generates relaxant activity which exceeds that of nitric oxide alone and probably results from formation of S-nitrosocysteine and S-nitrosoglutathione, respectively. The effects of pyrogallol and hydroquinone suggest that the NANCneurotransmitter is a superoxide anion-resistant, nitric oxide-releasing molecule and that neither S-nitrocysteine nor S-nitrosoglutathione is a suitable candidate for this.

Contribution of neural intrapancreatic non-cholinergic non-adrenergic mechanisms to glucose-induced insulin release in the isolated rat pancreas

In the isolated rat pancreas the effect of intrapancreatic non-adrenergic non-cholinergic nerves was examined upon insulin, glucagon and somatostatin release during perturbations of perfusate glucose. Elevation of glucose from 1.6 to 8.3 mmol/l increased insulin and somatostatin secretion and inhibited glucagon release. The first phase of insulin secretion was significantly reduced by the neurotoxin tetrodotoxin to 55% of the controls (p < 0.05). The somatostatin response was attenuated by tetrodotoxin while the change of glucagon remained unaffected. In contrast the combined adrenergic and cholinergic blockade with atropine, phentolamine and propranolol (10(-5) mol/l) did not modify the insulin, glucagon and somatostatin response. When glucose was changed from 8.3 to 1.6 mmol/l, the reduction of insulin and somatostatin release was not modified by tetrodotoxin, but stimulation of glucagon was significantly attenuated by 60-70% (p < 0.03), which was similar to the effect of combined adrenergic and cholinergic blockade. Subsequently, the effect of neural blockade was examined during more physiological perturbations of perfusate glucose levels. When glucose was changed from 3.9 to 7.2 mmol/l, tetrodotoxin also attenuated first phase insulin response by 40% while cholinergic and adrenergic blockade had no effect. The nitric oxide synthase inhibitor NG-Nitro-L-arginine-methyl-ester (L-NAME) did not alter the glucose-induced insulin response indicating that nitric oxide is not involved in this mechanism. It is concluded that neural non-adrenergic non-cholinergic mechanisms contribute to the first, but not second phase of glucose-induced insulin release. Non-adrenergic non-cholinergic effects do not participate in regulation of glucagon and somatostatin secretion under the conditions employed.(ABSTRACT TRUNCATED AT 250 WORDS)

L-NG-nitro-arginine and its methyl ester are potent inhibitors of non-adrenergic, non-cholinergic transmission in the rat anococcygeus

1. The effects of L-NG-nitro-arginine (L-NOARG) and some other arginine analogues on non-adrenergic, non-cholinergic (NANC) relaxations of the rat anococcygeus muscle were investigated. 2. L-NOARG (5-200 microM) produced concentration-related inhibition of the NANC response; 100 microM L-NOARG produced 90% inhibition. 3. L-Arginine (5-200 microM) produced a concentration-related reversal of the inhibitory effect of 20 microM L-NOARG; a five fold excess of L-arginine (100 microM) was required to obtain the maximum reversal of 90%. D-Arginine (100 microM) produced no such reversal, but significant reversal was produced by L-citrulline, L-arginine-L-aspartate, L-homoarginine and L-arginine-methyl-ester (all at 100 microM). 4. L-NG-nitro-arginine-methyl-ester (L-NAME; 5-200 microM) also reduced NANC relaxations, with a potency similar to that of L-NOARG; both L-NOARG and L-NAME were some ten times more potent than L-NG-monomethyl-arginine (L-NMMA). Like L-NOARG, the effects of L-NAME (20 microM) were reversed by 100 microM L- but not D-arginine. 5. Neither L-NOARG nor L-NAME (both 20 microM) affected submaximal relaxations induced by 10 microM sodium nitroprusside or 20 microM hydroxylamine. 6. D-NOARG, L-NG-tosyl-arginine and L-N alpha-(t-butyl-oxycarbonyl)-NG-nitro-arginine (all at 100 microM) had no effect on NANC relaxations. 7. Thus, in the rat anococcygeus, L-NOARG and L-NAME are more potent than L-NMMA as prejunctional inhibitors of NANC transmission. The reversibility of the effect of L-NOARG by arginine analogues suggests that the NANC system of the anococcygeus shows similarities to the endogenous nitrate system recently described in the brain.

L-NG-monomethyl arginine and L-NG-nitro arginine inhibit non-adrenergic, non-cholinergic relaxation of the mouse anococcygeus muscle

1. The effects of L-NG-monomethyl arginine (L-NMMA) and L-NG-nitro arginine (L-NOARG) on non-adrenergic, non-cholinergic (NANC) relaxations of the mouse anococcygeus were investigated. 2. L-NMMA (10-200 microM) produced a concentration-related inhibition of the NANC response; the inhibitory effect of 50 microM L-NMMA was completely reversed by L-arginine but not D-arginine (both 100 microM). 3. L-NOARG (1-50 microM) also produced a concentration-related inhibition of the NANC response and was some 30-50 times more potent than L-NMMA; again, the effects of 10 microM L-NOARG were reversed by 100 microM L-, but not D-, arginine. By itself 100 microM L-arginine did not relax the tissue, but did cause a slight potentiation of the NANC response. 4. Sodium nitroprusside (0.01-10 microM), hydroxylamine (0.1-100 microM), sodium azide (1-100 microM) and nitric oxide (3-120 microM) all relaxed carbachol-induced tone; relaxations to submaximal concentrations of these nitrovasodilators were unaffected by either 50 microM L-NMMA or 10 microM L-NOARG. 5. L-NOARG 10 microM did not inhibit, but rather potentiated, contractions of the mouse anococcygeus due to stimulation of its sympathetic nerves. 6. The inhibitory effects of 10 microM L-NOARG on NANC relaxations were reversed by L-arginine (by 131%), L-citrulline (by 75%), L-arginine methyl ester (by 46%) and L-homoarginine (by 22%), but were unaffected by a variety of other amino acids and their derivatives (all at 100 microM). 7. The results provide strong evidence that NANC relaxations of the mouse anococcygeus are mediated by an endogenous nitrate material, probably derived from L-arginine, and confirm that L-NOARG provides a very useful and potent drug for the investigation of endogenous nitrate function.

Mechanism for the generation of active smooth muscle inhibitory factor (IF) from bovine retractor penis muscle (BRP)

The active smooth muscle inhibitory factor (IF) is generated by acidifying the extracts from the bovine retractor penis muscle (BRP). The activated IF is very similar to endothelium-derived relaxing factor (EDRF). However, the mechanism for the generation of active IF is yet unknown. From reverse-phase high performance liquid chromatographic data, the activities of nitrite loss were closely related to the contents of both sulfhydryl (SH) groups and L-cysteine. Thus, under acidic conditions, RSH and nitrite in the extracts are considered to react as follows: RSH + HNO2----RSNO + H2O, RSNO----(1/2) RSSR + NO, NO + (1/2)O2----NO2, NO2 + (1/2)H2O----(1/2)HNO2 + (1/2)HNO3. In addition to NO, RSNO is also a labile but potent vasodilator. Thus, activated IF seems to consist of RSNO and NO, and thiol-containing small molecules (molecular weight of less than 1,000) from BRP are closely involved in the generation of active IF.

N-methylhydroxylamine inhibits and M&B 22948 potentiates relaxations of the mouse anococcygeus to non-adrenergic, non-cholinergic field stimulation and to nitrovasodilator drugs

1. The effects of N-methylhydroxylamine (NMH) and of M&B 22948 on relaxations of the mouse anococcygeus to non-adrenergic, non-cholinergic (NANC) field stimulation and to a number of smooth muscle relaxant drugs were investigated. 2. Relaxations to NANC field stimulation (10 Hz; 60 s train) were reversibly blocked by NMH (1-5 mM), which also caused weak, transient reductions of carbachol (50 microM)-induced tone. N,N-dimethylhydroxylamine (2 mM) and hydroxylamine (5 microM) reduced tone to the same extent as NMH, but neither produced any inhibition of NANC relaxations. 3. M&B 22948 10 microM, which by itself reduced tone by 12%, potentiated submaximal but not maximal relaxations to NANC field stimulation; overall the log frequency-response curve was displaced to the left by a factor of 2. 4. Sodium nitroprusside (0.01-1 microM), hydroxylamine (0.5-100 microM), and nitric oxide (2-200 microM) all relaxed carbachol-induced tone; relaxations to submaximal concentrations of these nitrovasodilators were reduced in the presence of 2 mM NMH, and potentiated in the presence of 10 microM M&B 22948. 5. Neither NMH (2 mM) nor M&B 22948 (10 microM) affected relaxations induced by submaximal concentrations of vasoactive intestinal peptide (VIP; 1 microM), papaverine (10 microM), 3-isobutyl-1-methyl-xanthine (10 microM), or 8-bromo-cyclic guanosine monophosphate (100 microM); relaxations to adenosine 5'-triphosphate (ATP, 2 mM) were unaffected by M&B 22948, but were potentiated by NMH. 6. The selective inhibition by NMH, and potentiation by M&B 22948, of NANC and nitrovasodilator-induced relaxations of the mouse anococcygeus suggests that the NANC transmitter is neither VIP nor ATP, but resembles the nitrovasodilator drugs in its mode of action. The NANC transmission system is therefore similar to that recently described in the bovine retractor penis.

What is the relationship between the endothelium derived relaxant factor and nitric oxide?

Nitric oxide gas in solution (NO) relaxes blood vessels with similar actions and pharmacodynamics as the endothelium derived relaxant factor (EDRF) and has been proposed to be a component of the materials released from stimulated endothelial cells. Certain data however suggest that EDRF and NO may not be identical. In some non-vascular smooth muscles, NO and EDRF exhibit markedly different pharmacologic profiles. Furthermore the interaction of EDRF and NO with anion exchange resins differ. The hypothesis that EDRF is identical to nitric oxide gas in solution or a nitrogen oxide containing compound is discussed.

Influence of haemoglobin and erythrocytes on the effects of EDRF, a smooth muscle inhibitory factor, and nitric oxide on vascular and non-vascular smooth muscle

1. The relaxant action of endothelium-derived relaxing factor (EDRF), the smooth muscle inhibitory factor (IF) isolated from the bovine retractor penis (BRP), nitric oxide (NO) and sodium nitroprusside (NaNP) on four vascular and non-vascular smooth muscle preparations has been examined. Their sensitivity to EDRF, the IF and NO was the same, suggesting all might be NO. Sodium nitroprusside produced complete relaxation of the rat anococcygeus at low doses, suggesting an action additional to the intracellular release of NO. 2. Haemoglobin added to solutions of EDRF, activated IF or NO completely removed their relaxant properties, consistent with all three acting by virtue of NO. 3. Suspensions of red blood cells with a haemoglobin concentration equivalent to to that used in the previous experiments were as effective as haemoglobin in abolishing the relaxant effect of EDRF or NO but were ineffective against the activated IF. 4. The similarity in sensitivity of a series of smooth muscles and the binding by haemoglobin are consistent with NO being the active principle of both EDRF and the acid activated IF. The abolition of the effect of EDRF by red blood cells (RBCs) is further confirmation for this hypothesis, but the ineffectiveness of RBCs against acid-activated IF suggests that either the latter is not NO or that it is bound in a way which makes it unable to diffuse through cell membranes.

Evidence that inhibitory factor extracted from bovine retractor penis is nitrite, whose acid-activated derivative is stabilized nitric oxide

1. Unactivated extracts of bovine retractor penis (BRP) contains 3-7 microM nitrite. Acid-activation of these extracts at pH 2 for 10 min followed by neutralization generates the active form of inhibitory factor (IF; assayed by its vasodilator action on rabbit aorta), and is associated with partial loss of nitrite. 2. Increasing the time of acid-activation at pH 2 from 10 to 60 min with intermittent vortex mixing generates greater vasodilator activity and increases nitrite loss. 3. When acid-activated and neutralized extracts are incubated at 37 degrees C or 30 min or boiled for 5 min, vasodilator activity is lost and nitrite content increased. Reactivation of these samples at pH 2 for 10 min followed by neutralization leads to partial recoveries of vasodilator activity with loss in nitrite content. 4. Addition of sodium nitrite to BRP extracts increases acid-activatable vasodilator activity pro rata. 5. Acid-activation of aqueous sodium nitrite solutions results in less loss of nitrite and generation of less vasodilator activity than BRP extracts. Vasodilatation is only transient and is rapidly abolished on neutralization, whereas responses to acid-activated BRP extracts are more prolonged and activity is stable on ice. 6. Bovine aortic endothelial cells yield vasodilator activity that is indistinguishable from that isolated from BRP. It is activated by acid, stable on ice, abolished by boiling or by haemoglobin, and appears to be due to the generation of nitric oxide (NO) from nitrite. 7. The data provide confirmatory evidence that nitrite in BRP extracts is IF, that acid-activation of BRP extracts yields NO which is responsible for its vasodilator action, and that inactivation occurs by decay of NO to nitrite and nitrate. They further suggest that BRP extracts contain a NO-stabilizing agent which favours conversion of nitrite to NO. 8. The finding that bovine aortic endothelial cells yield an agent indistinguishable from IF suggests that nitrite in endothelial cells may likewise be the precursor of endothelium-derived relaxing factor (EDRF), itself identified as NO.

Endothelium-derived relaxing factor inhibits in vitro platelet aggregation

We studied the effects of endothelium-derived relaxing factor (EDRF), bovine retractor penis muscle inhibitory factor and sodium nitroprusside, three stimulants of guanylate cyclase, on the in vitro aggregation of washed human platelets. Platelet aggregation induced either by collagen or by the thromboxane A2 analogue U46619 was inhibited by all three agents. The anti-aggregatory effect of each agent was inhibited by haemoglobin. The anti-aggregatory effect of EDRF was potentiated by superoxide dismutase. These findings are discussed in relation to a potential role for EDRF in haemostasis.

The mechanisms by which haemoglobin inhibits the relaxation of rabbit aorta induced by nitrovasodilators, nitric oxide, or bovine retractor penis inhibitory factor

The mechanisms by which haemoglobin and methaemoglobin inhibit the vasodilator actions of glyceryl trinitrate, sodium azide, nitric oxide, and the bovine retractor penis inhibitory factor (IF) were studied on rabbit endothelium-denuded aortic rings. Methaemoglobin was less effective than haemoglobin against each vasodilator, it was more effective at inhibiting the relaxation to azide than that to glyceryl trinitrate. Glyceryl trinitrate was neither bound nor inactivated when passed through columns of haemoglobin-agarose or methaemoglobin-agarose. Azide was reversibly bound but less by haemoglobin-agarose than by methaemoglobin-agarose. Inhibition of the vasodilator actions of glyceryl trinitrate is not attributable therefore to a direct interaction with the haemoproteins, although a small part of the inhibition of azide-induced relaxation by methaemoglobin is likely to be due to a direct interaction. Columns of haemoglobin-agarose were more effective than columns of methaemoglobin-agarose in removing nitric oxide from solution. The greater ability of haemoglobin, compared to methaemoglobin, to inhibit vasodilatation induced by nitrovasodilators may therefore reflect the greater ability of haemoglobin to bind nitric oxide which is the active principle of the nitrovasodilators. Neither the acid-activated nor the inactive forms of IF were bound or inactivated when passed through columns of methaemoglobin-agarose. Neither form of IF was retained on passage through columns of haemoglobin-agarose, but the resulting activity in the eluates was less than control, was unstable and, unlike the original activity, decayed rapidly on ice. The greater ability of haemoglobin, compared to methaemoglobin, to inhibit vasodilatation induced by IF might therefore reflect the greater ability of haemoglobin to interact with this vasodilator and inactivate it.

Atriopeptin II-induced relaxation of rabbit aorta is potentiated by M&B 22,948 but not blocked by haemoglobin

We examined the effects of haemoglobin (which inhibits the vascular responses to stimulation of soluble guanylate cyclase) and of M&B 22,948 (which selectively inhibits cyclic GMP phosphodiesterase) on the relaxation induced in rabbit aorta by the atrial natriuretic peptide, atriopeptin II (which stimulates particulate guanylate cyclase). Pretreatment with M&B 22,948 (100 microM) produced a 2.3 fold potentiation of atriopeptin II-induced relaxation of endothelium-denuded rings of rabbit aorta. Pretreatment with haemoglobin (10 microM) had no effect on the relaxation or the 10.9 fold increase in cyclic GMP content induced by atriopeptin II in endothelium-denuded rings of rabbit aorta. The potentiation by M&B 22,948 suggests a causal role for cyclic GMP in mediating atriopeptin II-induced vasodilatation of rabbit aorta. The inability of haemoglobin to block the atriopeptin II-induced rise in cyclic GMP suggests that it does not block stimulation of particulate guanylate cyclase. Thus, it is unlikely that a ferrous haem-containing receptor site is involved in the activation of the particulate form of guanylate cyclase as it is with soluble guanylate cyclase.

Evidence against VIP-involvement in neurogenic relaxations of the mouse anococcygeus muscle

Vasoactive intestinal polypeptide (VIP) antiserum, alpha-chymotrypsin, and repeated exposure to VIP markedly reduced relaxations of the mouse anococcygeus muscle to VIP but not to field stimulation. This evidence suggests that VIP does not mediate non-adrenergic, non-cholinergic relaxations in the mouse anococcygeus.

Is vasoactive intestinal polypeptide the principal transmitter involved in human penile erection?

Previous work from this laboratory reported on the effects of several autacoids and other agents on strips of human corpus cavernosum (cc) muscle. These investigations indicated the presence in the cc muscle of a) atropine-sensitive cholinoceptors, b) alpha- and beta-adrenoceptors and c) a non-adrenergic non-cholinergic mechanism. Several recent publications have presented evidence in support of the possibility that vasoactive intestinal polypeptide (VIP) is an important, or the chief, transmitter in human penile erection. This paper describes the actions of VIP and other compounds on the cc muscle and the effect of intracavernous injection of VIP in volunteers. Among the agents tested, VIP was the most potent relaxant of the cc muscle. This effect, which was seen at a dose as low as 0.03 nM, was suppressed by VIP antiserum. The response of the isolated penile vasculature to VIP was similar. VIP antiserum had no effect on the relaxation of the cc muscle produced by field stimulation. In five of the seven subjects given intracavernous VIP (1.0 micrograms.) some degree of penile enlargement was evident, but none had an erection. It is suggested that local release of VIP, withdrawal of the alpha-adrenoceptor mediated tonic supply to the penis and the activation of the latter's beta-adrenoceptors are all probably involved in penile erection in man.

A comparison of the action of the endothelium-derived relaxant factor and the inhibitory factor from the bovine retractor penis on rabbit aortic smooth muscle

The dependence of relaxation of rabbit aortic strips by carbachol and by the inhibitory factor from the bovine retractor penis (BRP) on the presence of endothelium has been compared. Carbachol-induced relaxation is abolished by removing the endothelium, inhibitory factor-induced relaxation is unimpaired. The inhibitory factor, therefore, does not act by releasing an endothelium-derived relaxing factor (EDRF). The effect of inhibitors of eicosanoid metabolism on relaxation was examined. Quinacrine and nordihydroguaiaretic acid abolished the relaxant effect of carbachol and flurbiprofen had no effect. The relaxation produced by the inhibitory factor was unaffected by quinacrine and flurbiprofen while nordihydroguaiaretic acid potentiated the response. No eicosanoid appears, therefore, to be involved in the relaxant effect of the inhibitory factor from the BRP. Methylene blue, a drug reported to inhibit guanylate cyclase, in a concentration of 10 microM selectively abolished the relaxation produced by carbachol. However, at the higher concentration of 30 microM it abolished almost completely the response to inhibitory factor from the BRP and reduced inhibition by sodium nitroprusside. It is not possible from these results to exclude the possibility that the EDRF and the inhibitory factor from the BRP are chemically related.

The effect of hypoxia on neuroeffector transmission in the bovine retractor penis and rat anococcygeus muscles

The effects of reducing the PO2 of the bathing fluid were studied on non-adrenergic non-cholinergic (NANC) transmission in isolated preparations of the bovine retractor penis muscle, the rat anococcygeus muscle, the guinea-pig taenia caeci and the guinea-pig urinary bladder. Hypoxia rapidly and reversibly impaired NANC transmission in the bovine retractor penis and rat anococcygeus muscles but did not affect transmission in the guinea-pig taenia caeci or bladder, suggesting that different NANC mechanisms are involved. Although neurally-evoked relaxation of the bovine retractor penis was impaired by hypoxia, relaxations produced by vasoactive intestinal peptide, prostaglandin E1, sodium nitroprusside or an inhibitory factor isolated from the bovine retractor penis were unaffected. Since the inhibitory factor is similar to, and may actually be the NANC transmitter, the results suggest that the site of action of hypoxia in impairing transmission is prejunctional at the inhibitory nerve endings.

Evidence for vasoactive intestinal polypeptide as a neurotransmitter in smooth muscle of the urogenital tract

The localization of vasoactive intestinal polypeptide (VIP) in the anococcygeus muscle of the rat, rabbit and cat was investigated by radioimmunoassay and immunohistochemistry. High concentrations of VIP-like immunoreactivity were found in the anococcygeus muscle of all 3 species. VIP-like immunoreactivity was observed in a network of nerve fibres branching throughout the muscle which were unaffected by treatment with 6-hydroxydopamine. VIP, when applied exogenously to isolated preparations of the anococcygeus, produced dose-related relaxations of tone in all 3 species with a time course and maximum response similar to non-adrenergic, non-cholinergic inhibitory nerve stimulation. These observations provide further evidence in support of VIP as a suitable candidate for the non-adrenergic, non-cholinergic inhibitory neurotransmitter in the anococcygeus muscle.

Cyclic GMP mediates neurogenic relaxation in the bovine retractor penis muscle

Field stimulation of the non-adrenergic, non-cholinergic inhibitory nerves to the bovine isolated retractor penis muscle evoked a relaxation that was preceded by a rise in the tissue content of cyclic GMP. There was no change in the content of cyclic AMP. The selective cyclic GMP phosphodiesterase inhibitor, 2-o- propoxyphenyl -8- azapurin -6-one (M&B 22948), elevated the tissue's cyclic GMP content, and potentiated both the relaxation and the rise in cyclic GMP produced by inhibitory nerve stimulation. Sodium nitroprusside and an inhibitory factor extracted from the bovine retractor penis muscle mimicked the effects of inhibitory nerve stimulation in that they each produced relaxation associated with a selective rise in cyclic GMP concentration. Haemoglobin (in the form of erythrocyte haemolysate) and N- methylhydroxylamine , which are known to block guanylate cyclase, blocked the relaxation and the rise in cyclic GMP content produced by inhibitory nerve stimulation, inhibitory factor and sodium nitroprusside. Haemoglobin itself caused a rise in muscle tone and at the same time reduced the cyclic GMP content of the tissue. 8-Bromocyclic GMP, a permeant derivative of cyclic GMP, produced a relaxation of the muscle that, as expected, was not blocked by haemoglobin. Vasoactive intestinal polypeptide, prostaglandin E1 and forskolin each produced relaxation associated with a selective rise in cyclic AMP content. Their effects were not blocked by haemoglobin or N- methylhydroxylamine . It is concluded that inhibitory nerve stimulation in the bovine retractor penis muscle produces a relaxation that is mediated by cyclic GMP, although some substances relax the muscle without affecting cyclic GMP levels. The results are also compatible with the view that the extracts of muscle contain the inhibitory neurotransmitter.

Neuropeptide-induced contraction and relaxation of the mouse anococcygeus muscle

Isometric tension responses to neuropeptides were recorded from anococcygeus muscles isolated from male mice. This smooth muscle tissue is innervated by inhibitory nonadrenergic, noncholinergic nerves that resemble, ultrastructurally, the peptidergic neurons of the gastrointestinal tract; the physiological function of the anococcygeus is not known. Slow sustained contractions were produced by oxytocin (0.2-20 nM), [Arg8]vasopressin (0.4-200 nM), and [Arg]-vasotocin (0.4-100 nM); the mouse anococcygeus is, therefore, one of the few examples of nonvascular smooth muscle from male mammals to respond to low concentrations of oxytocin and related peptides. Substance P (0.5-8 microM) caused distinctive, biphasic increases in muscle tone of some, but not all, preparations. Other neuropeptides producing contractions were neurotensin (2-100 microM) and thyrotropin-releasing hormone (2-100 microM); the responses were of similar time course and displayed selective cross-desensitization, suggesting that these two peptides act through a common distinct mechanism. Tetradecapeptide somatostatin (10-80 microM) and its analog urotensin II (0.1-5 microM), a dodecapeptide from the urophysis of the teleost fish Gillichthys mirabilis, produced similar slowly developing relaxations of carbachol-induced tone. Piscine urotensin II, of which there are no reported effects on nonvascular mammalian systems, was 20-50 times more potent than somatostatin, a well-established mammalian hormone. Of the peptides studied, only vasoactive intestinal polypeptide (0.05-1 microM) caused rapid powerful relaxations in low concentrations; this is consistent with its proposed involvement in nonadrenergic, noncholinergic neurotransmission in the mouse anococcygeus.

Neurogenic vasodilatation in isolated bovine and canine penile arteries

Field stimulation of isolated, perfused bovine or canine penile arteries produced dilatation, after the adrenergic motor component of the response had been blocked with guanethidine and the vessels had developed a background tone. The vasodilatation was blocked by tetrodotoxin but not by atropine. The vasodilator responses to field stimulation were compared with those produced by ATP, by vasoactive intestinal peptide (VIP), and by the inhibitory factor extracted from the bovine retractor penis muscle. Of the three putative transmitters, the inhibitory factor produced responses that most closely resembled those to field stimulation. Haemoglobin, which blocks non-adrenergic, non-cholinergic inhibitory transmission in the bovine and canine retractor penis muscles, did not impair the vasodilatations produced by ATP or VIP, but slowly reduced or abolished those produced by field stimulation or by the inhibitory factor. Haemoglobin itself produced a powerful constriction of the isolated penile arteries. The results are compatable with the possibility that the inhibitory factor from the bovine retractor penis muscle (which may be the inhibitory transmitter in that muscle) is, or closely resembles, the transmitter of non-adrenergic, non-cholinergic vasodilator fibres in the penile arteries of dog and ox.

The effect of an inhibitory factor from the bovine retractor penis on the gastro-intestinal tract and gall bladder of the guinea-pig

1 The effect of a smooth muscle inhibitory factor extracted from the bovine retractor penis has been examined on a variety of in vitro smooth muscle preparations from the guinea-pig alimentary canal and on the guinea-pig gall bladder. 2 The inhibitory factor caused relaxation of spontaneous and carbachol-induced tone in the taenia coli, the stomach fundal strip and the duodenum and colon. There was little effect on the ileum. Sensitivity was highest in the taenia coli where the response to the inhibitory factor mimicked the response to stimulation of the non-adrenergic, non-cholinergic (NANC) inhibitory nerves. 3 In the taenia coli the inhibitory response to stimulation of the NANC nerves and to ATP was abolished by apamin 5 x 10(-8) M, whereas this or higher concentrations had no effect on the response to the inhibitory factor. This makes it unlikely that the latter is the neurotransmitter in these NANC nerves. 4 The inhibitory factor had no effect on the gall bladder. Inhibitory responses to field stimulation were obtained in this tissue but these were insensitive to tetrodotoxin in concentrations greater than those needed to block the motor cholinergic nerves.

Evidence against ATP being the nonadrenergic, noncholinergic inhibitory transmitter in guinea pig stomach

THe adenosine-sensitive P1 purinoceptor antagonists 8-phenyl theophylline and 8-(p-bromophenyl)theophylline (4 microM) antagonized ATP-induced relaxation of spontaneous tone in guinea pig stomach, but caused no significant modification of relaxation to electrical field stimulation (0.2 - 10 Hz, 1 msec for 30 sec, atropine 1.5 microM, and guanethidine 4 microM treated). These results suggest that in fundic muscle ATP acts via hydrolysis to adenosine, with subsequent activation of P1 purinoceptors, and that ATP is not the nonadrenergic, noncholinergic inhibitory transmitter in stomach fundus.

Block of some non-adrenergic inhibitory responses of smooth muscle by a substance from haemolysed erythrocytes

1. A preparation of haemolysed rat erythrocytes (the haemolysate) blocked the relaxations of both the bovine retractor penis and the rat anococcygeus muscles in response to field stimulation of their non-adrenergic inhibitory nerves. The effective concentration range was 5-20 mul./ml. of haemolysate, equivalent to 0.25-1.0 mul./ml. of blood. The active principle in the haemolysate was a non-dialysable, heat-labile material of molecular weight between 50,000 and 100,000 daltons. If, as appeared probable, the active component of the haemolysate was oxyhaemoglobin, its effective blocking concentration was 0.5-2 muM.2. Haemolysate (5-20 mul./ml.) also blocked the relaxation of both the bovine retractor penis and the rat anococcygeus to the inhibitory factor extracted from the bovine retractor penis, an observation supporting the possibility that this inhibitory factor may be the transmitter released by the inhibitory nerves in these tissues. In the bovine retractor penis, haemolysate was also effective in blocking relaxations in response to sodium nitroprusside, but relaxations produced by prostaglandin E(1) or isobutylmethylxanthine were unchanged or only slightly reduced.3. In contrast, in the taenia of the guinea-pig caecum, haemolysate did not block the non-adrenergic inhibitory response to field stimulation, nor the relaxation produced by ATP, although it did block the relaxation produced by the inhibitory factor.4. In spiral strips of isolated rabbit aorta, haemolysate (10 mul./ml.) increased the contraction produced by noradrenaline and blocked the relaxation produced by the inhibitory factor. These were shown to be independent effects.5. Apamin, which blocked the relaxation of the taenia of the guinea-pig caecum elicited by either ATP or field stimulation of its non-adrenergic nerves, was without effect on relaxations of the bovine retractor penis or rat anococcygeus muscles in response to field stimulation of inhibitory nerves or to inhibitory factor.6. These differences in the blocking effects of apamin and haemolysate suggest either that the transmitter in the bovine retractor penis and rat anococcygeus differs from that in the guinea-pig taenia, or, if the transmitter is the same, then its mechanism of action differs.

The effect of ethanol on inhibitory and motor responses in the rat and rabbit anococcygeus and the bovine retractor penis muscles

1 Ethanol (200 mM) reduced the response to inhibitory nerve stimulation in the rat and rabbit anococcygeus and the bovine retractor penis (BRP) muscles. Ethanol also reduced the response to the inhibitory extract from the BRP consistent with the inhibitory factor in these extracts playing some part in the response to inhibitory nerve stimulation. 2 Ethanol's effect on the response to other inhibitory stimuli was examined in the rabbit anococcygeus and the BRP. In the anococcygeus the response to carbachol was reduced, to bradykinin and isobutylmethylxanthine (IBMX) unaltered, and to isoprenaline and adenosine 5'-triphosphate (ATP) potentiated. In the BRP responses to IBMX and sodium nitroprusside were unaltered but in this tissue the response to isoprenaline was reduced. Ethanol's ability to reduce inhibitory responses is, therefore, selective and confined to inhibitory nerve stimulation, inhibitory extract, carbachol, and, in the BRP, isoprenaline. 3 Ethanol reduced the rate of development of inhibition even where the magnitude of the inhibitory response was unaltered. 4 In the rat anococcygeus, ethanol (200 mM) potentiated the response to motor nerve stimulation and to noradrenaline (NA) at low frequencies and low concentrations respectively. Higher ethanol concentrations (400 mM) reduced the response to both motor nerve stimulation and NA. The motor response to carbachol was also reduced. 5 Ethanol (200 mM) itself caused an easily reversible contraction in all three tissues. This was not due to the release of NA but was highly sensitive to the removal of external calcium from the medium. 6 A unified explanation of these varied effects of ethanol based on a reduction in membrane binding of calcium and a reduced efficiency of receptor coupling is suggested.

Some physical and chemical properties of the smooth muscle inhibitory factor in extracts of the bovine retractor penis muscle

1. A method of extracting and partially purifying a smooth muscle inhibitory factor from the bovine retractor penis is described. This consists of extraction in methanol followed by adsorption on an anion exchange resin, elution from the resin with 500 mM-sodium chloride solution and, if necessary, removal of adenine nucleotides by adsorption on alumina. 2. The inhibitory factor exists in a stable pharmacologically inactive form and an unstable pharmacologically active form. Conversion to the active form is by a brief exposure to acid at pH 2.0. 3. The inhibitory factor is insoluble in ether or acetone but soluble in methanol. Anhydrous methanol, however, irreversibly destroys pharmacological activity especially if the inhibitory factor is in the active form. This effect of methanol is prevented by the presence of 20-30-% water. 4. The inhibitory factor binds to an anion exchange resin but not to a cation exchange resin. It can be eluted from the resin by 500 mM-sodium chloride solution. 5. The molecular weight of the inhibitory factor, as judged by the ability to pass ultrafiltration membranes, is about 500. 6. Inhibitory activity is unaffected by the proteases trypsin, subtilisin or pepsin or by leucine aminopeptidase, pyroglutamate aminopeptidase or carboxypeptidase. The inhibitory effect of the extract and the inhibitory response to stimulation of the non-adrenergic, non-cholinergic nerves are also unaffected by the protease inhibitor, aprotinin. The active material, therefore, is unlikely to be a peptide. 7. Inhibitory activity is abolished by exposure of the extracts to periodic acid or sodium periodate. Acetic anhydride in pyridine also abolishes activity but the vehicle pyridine is also effective. 8. Sodium borohydride but not borate abolishes inhibitory activity when added to the acid-activated material at pH 2.0 but has no effect or may even potentiate activity if added to the stable inactive form at pH 9.0. When added to the acid-activated but neutralized material at pH 6.8 it usually abolishes inhibitory activity but occasionally has no effect. 9. These results suggest the smooth muscle inhibitory factor in these extracts is potent and probably novel. It does not appear to be a peptide or a lipid but may contain a carbohydrate as part of the molecule. Its possible physiological role is discussed.

Actions on the cardiovascular system of an inhibitory material extracted from the bovine retractor penis

1 A partially purified material has been isolated from methanol extracts of the bovine retractor penis muscle. This material exerts biological activity only after treatment with acid and subsequent neutralisation. The active principle in this extract, which appears to be no known autacoid, mimics the response to stimulation of the non-adrenergic, non-cholinergic nerves in the bovine isolated retractor penis muscle. 2 This inhibitory extract did not alter the heart rate or blood pressure of the anaesthetized rat when administered either by intravenous or intra-arterial injection, nor did it have any obvious effect on isolated cardiac muscle. 4 The extract produced relaxation of spiral strips of various arteries isolated from ox, cat, rabbit or rat, in which tone was induced by noradrenaline, K+ of Ba2+. 5 The extract also produced dilatation of the resistance vessels of the rat isolated mesenteric circulation and the rat hindquarters perfused with Krebs solution; tone was induced in these vessels by adrenaline or noradrenaline. 6 Lack of vasodilator activity of the extract in the whole animal appeared to be due to rapid inactivation in the blood, probably by binding to the erythrocytes.