Effects of some peptides on isolated human penile erectile tissue and cavernous artery

Contractant and relaxant effects of four peptides known to occur in nerves innervating human penile vessels and erectile tissue, namely substance P (SP), vasoactive intestinal polypeptide (VIP), neuropeptide Y (NPY) and somatostatin, were studied in isolated preparations from the corpus cavernosum (CC), corpus spongiosum (CS) and cavernous artery (Acc). In addition, the actions of another peptide, arginine vasopressin (AVP), were investigated. In erectile tissue proper, SP induced concentration-dependent contractions. No effect of this peptide was observed in Acc segments. CC and CS preparations contracted by noradrenaline (NA) were relaxed by 30-40%; the effect in NA-contracted Acc preparations was inconsistent. AVP had a potent contractant effect in preparations from all the tissues studied, the effect being most conspicuous in CS strips. VIP was without contractant actions in any of the preparations. NA-contracted preparations were relaxed by VIP, and electrically induced contractions inhibited. The inhibitory effect was particularly marked in electrically stimulated CC and CS preparations. NPY had no effects; somatostatin contracted Acc segments, and in high concentrations CC and CS strips. It is concluded that among the peptides studied only VIP has effects compatible with a role as a neurotransmitter in penile erection.     

Characterization of the contractile effect of neuropeptide Y in feline cerebral arteries

The action of neuropeptide Y (NPY), which coexists with noradrenaline (NA) in perivascular sympathetic nerves, has been examined on feline cerebrovascular smooth muscle using a sensitive in vitro system. The direct cerebrovascular responses of peptides with structural similarities with NPY, peptide YY (PYY), avian (APP), and bovine (BPP) and human (HPP) pancreatic polypeptides, have been compared with that of NPY on isolated feline cerebral arteries. The relative potency for contractions induced by the peptides is: NPY, PYY greater than APP greater than BPP, HPP. The alpha-adrenoceptor antagonist rauwolscine, which blocked the response to noradrenaline (NA), had no effect on NPY-induced contractions. Neuropeptide Y significantly potentiated contractions induced by 10(-6) M NA, but not by 10(-5) M. Withdrawal of Ca2+ from the extracellular medium for 30 min reduced the contractile response to NPY in cerebral vessels by about 80%. Subsequent readdition of Ca2+ caused reproducible contractions which were inhibited by the calcium entry blocker nimodipine. Nimodipine also relaxed isolated middle cerebral artery segments contracted by NPY and NA in a concentration-dependent manner. The data suggest that NPY mediates contraction of cerebrovascular smooth muscle via a mechanism that is dependent on the concentration of extracellular calcium.     

Effects of neuropeptide Y (NPY) on isolated guinea-pig heart

Neuropeptide Y (NPY) is present in nerve fibres throughout the mammalian heart. We have elucidated the effects of NPY on the isolated papillary muscle and heart (Langendorff) from the guinea-pig. The paced papillary muscle was studied with regard to duration of the action potential, peak force, maximum rate of force development, time to peak force, and time from peak force to half relaxation; all these parameters were identical whether or not NPY (5 X 10(-7) M) was present in the medium. When a stimulation with trains of pulses was superimposed, the paced papillary muscle exhibited enhanced contractions. This increase in contractility was not observed in the presence of the beta-adrenoceptor antagonist propranolol (10(-6) M) and was thus considered to be adrenergically mediated. The latter (adrenergic) response was markedly attenuated by NPY. Since NPY did not interfere with the response to exogenous noradrenaline (NA) it is suggested that the peptide exerts a pre-junctional inhibitory affect on adrenergic nerve-mediated positive inotropy. Neuropeptide Y did not influence the electrocardiogram from the spontaneously beating heart (Lagendorff), nor did the peptide modify the positive chronotropic effect of exogenously applied NA. In conclusion, the results indicate that NPY is without effect on the heart muscle proper but that the sympathetic terminals of the heart possess pre-junctional receptors for NPY (and/or related peptides) related to suppression of stimulated NA release.     

Contractile effects of endothelin-1 and localization of endothelin binding sites in rabbit lower urinary tract smooth muscle

In isolated rabbit bladder and urethral smooth muscle, endothelin-1 caused concentration-related, slowly developing contractions that were difficult to wash out. Relative to contractions induced by K+ (124 mM), contractions in bladder preparations reached a higher amplitude than in urethral preparations. There was a marked tachyphylaxis to the effects of the peptide. The endothelin-1-induced contractions were not significantly affected by phentolamine or indomethacin in the urethra, or by scopolamine or indomethacin in the bladder. Incubation for 30 min in a Ca2(+)-free solution abolished the endothelin-1-induced contractions. Nifedipine did not affect the actions of endothelin-1 in the urethra but had a marked inhibitory action on its effects in the bladder. In the presence of endothelin-1, Ca2(+)-induced contractions were significantly blocked by nifedipine in the bladder but not in the urethra. Urethral preparations at resting tension responded to electrical stimulation by tetrodotoxin-sensitive, frequency-dependent contractions sensitive to alpha-adrenoceptor blockade. Pretreatment with endothelin-1 (10(-9) M) produced a significant increase in the nerve-induced contractions but had no significant effect on contractions induced by exogenous noradrenaline. Endothelin-1 did not affect spontaneous or stimulation-induced efflux of 3H-labelled noradrenaline in urethral smooth muscle. Preparations contracted by endothelin-1 were frequency-dependently relaxed by electrical stimulation. The peptide had no significant effect on the responses induced by electrical stimulation in the bladder preparations. In both bladder and urethra, [125]endothelin-1 binding sites were found mainly in the outer longitudinal muscle layer, in vessels and in the submucosa. The highest density of binding sites appeared to be in vessels and the outer muscle layer in both types of muscle. The results suggest that in the rabbit both bladder and urethral smooth muscle contain binding sites for endothelin. The peptide has contractant effects dependent on extracellular calcium in both types of tissue, but voltage-operated calcium channels seem to involved in activation only of bladder smooth muscle. The functional importance of endothelin-1 in the rabbit lower urinary tract remains to be elucidated.     

Functional responses of different muscle types of the female rat urethra in vitro

The influence of muscle type on functional responses of the female rat urethra was investigated using morphological and functional in-vitro techniques. The urethral submucosa was found to contain longitudinally or obliquely oriented smooth muscle cells. The muscularis layer is composed of circularly oriented muscle cells. Near the bladder orifice smooth muscle fibres dominate, but in the mid-urethra the vast majority is circularly oriented striated muscle cells. Circular preparations responded to electrical field stimulation in vitro with a rapid contraction. Stimulation with single impulses resulted in a twitch response; frequencies exceeding 5-10 Hz induced a summation and tetanus. The response was unaffected by phenoxybenzamine, propranolol and scopolamine and had a low sensitivity to calcium-free solution but was sensitive to suxamethonium and tetrodotoxin. Using longer impulse trains stimulation evoked also a slow contraction, sensitive to calcium-free solution. In longitudinal preparations stimulation induced a relaxation followed by a contraction, responses much smaller than those seen in the circular preparations. Both preparations relaxed on addition of calcitonin gene-related peptide or capsaicin. The relaxation to calcitonin gene-related peptide was larger than that to capsaicin in longitudinal preparations but equally large in the circular ones. Substance P and 5-hydroxytryptamine contracted both preparations. The longitudinal urethra showed a larger contraction to 5-hydroxytryptamine than to substance P, whereas both substances induced similar responses in the circular preparations. The study shows a similar muscle arrangement in the female rat urethra as described in humans and further points to a functional differentiation between the different types of muscle.     

Contractile effects of endothelin-I and localization of endothelin binding sites in rabbit lower urinary tract smooth muscle

In isolated rabbit bladder and urethral smooth muscle, endothelin-1 caused concentration-related, slowly developing contractions that were difficult to wash out. Relative to contractions induced by K⁺ (124 mM), contractions in bladder preparations reached a higher amplitude than in urethral preparations. There was a marked tachyphylaxis to the effects of the peptide. The endothelin-l-induced contractions were not significantly affected by phentolamine or indomethacin in the urethra, or by scopolamine or indomethacin in the bladder. Incubation for 30 min in a Ca²⁺-free solution abolished the endothelin-l-induced contractions. Nifedipine did not affect the actions of endothelin-1 in the urethra but had a marked inhibitory action on its effects in the bladder. In the presence of endothelin-1, Ca²⁺-induced contractions were significantly blocked by nifedipine in the bladder but not in the urethra. Urethral preparations at resting tension responded to electrical stimulation by tetrodotoxin-sensitive, frequency-dependent contractions sensitive to α-adrenoceptor blockade. Pretreatment with endothelin-1 (10^-9′ M) produced a significant increase in the nerve-induced contractions but had no significant effect on contractions induced by exogenous noradrenaline. Endothelin-1 did not affect spontaneous or stimulation-induced efflux of ³H-labelled noradrenaline in urethral smooth muscle. Preparations contracted by endothelin-1 were frequency-dependently relaxed by electrical stimulation. The peptide had no significant effect on the responses induced by electrical stimulation in the bladder preparations. In both bladder and urethra, [¹²⁵]endothelin-l binding sites were found mainly in the outer longitudinal muscle layer, in vessels and in the submucosa. The highest density of binding sites appeared to be in vessels and the outer muscle layer in both types of muscle. The results suggest that in the rabbit both bladder and urethral smooth muscle contain binding sites for endothelin. The peptide has contractant effects dependent on extracellular calcium in both types of tissue, but voltage-operated calcium channels seem to involved in activation only of bladder smooth muscle. The functional importance of endothelin-1 in the rabbit lower urinary tract remains to be elucidated.     

Localization of Adrenergic Receptors in Guinea Pig Ileum and Rabbit Jejunum to Cholinergic Neurons and to Smooth Muscle Cells

The localization of adrenergic receptors mediating a relaxing action was investigated in innervated and denervated longituclinal muscle strips from guinea pig ileum and rabbit jejunum. Denervated preparations were contracted by drugs that had a direct effect on smooth muscle cells, such as acetylcholine and histamine, but not by stimuli acting on cholinergic neurons, such as electrical field stimulation or nicotine. After blockade of β-adrenoceptors, norepinephrine relaxed the innervated guinea pig ileum contracted by electrical field stimulation, by stimulating α-adrenoceptors. Norepinephrine in low concentrations did not relax denervated preparations contracted by agents acting directly on smooth muscle. In high concentrations, it relaxed denervated preparations by a nonadrenergic mechanism, resistant to α- and/or β-receptor blockade, but which was also activated by l-(3,4-dihydroxyphenyl) ethanol. Phenylephrine only had a weak agonistic effect on the electrically stimulated innervated preparation and did not relax the denervated one. The denervated rabbit intestine contracted by acetylcholine was relaxed by norepinephrine and phenylephrine by stimulation of α-adrenoceptors. In the innervated preparations both drugs were more effective in inhibiting contractions induced by electrical field stimulation or eserine than those induced by exogenous acetylcholine. Both the denervated guinea pig and rabbit intestine were relaxed by stimulation of β-adreno-ceptors. It is suggested that in the guinea pig ileum α-adrenoceptors mediating relaxation are located only in cholinergic neurons, whereas in rabbit jejunum they are located both in these neurons and in the smooth muscle cells. Beta-adrenoceptors are located in the smooth muscle cells of both organs.     

Relaxation of sheep urethral muscle induced by electrical stimulation of nerves: involvement of nitric oxide

Isolated smooth muscle preparations from the sheep urethra responded to electrical field stimulation with contraction when basal tension was low (5-6 mN), but with relaxation when the preparations were contracted with noradrenaline (NA), clonidine, or prostaglandin F2a. No relaxant response could be elicited in high K+ (124 mM) contracted preparations. Electrically induced relaxations had a threshold of less than 1 Hz and a maximum at 8 Hz. Both contractant and relaxant responses were abolished by tetrodotoxin, indicating that they were caused by transmitters released from nerves. The amplitude of the relaxant responses showed a highly significant correlation to the tension induced by noradrenaline. A coefficient (R/T) was calculated relating relaxation to noradrenaline-induced tension. In this way it is possible to separate the effect of drugs on muscle tension (non-specific effect) from their action on the electrically induced relaxation (specific effect). Chemical sympathectomy with 6-OHDA did not significantly modify the relaxant response to 6 Hz in noradrenaline contracted strips, as evaluated by the R/T coefficient. The electrically induced relaxation was not affected by hexamethonium, propranolol, phentolamine, muscarinic receptor blockade, cocaine, indomethacin, or methysergide. Both nifedipine and Bay K 8644 inhibited significantly the response induced by electrical stimulation, decreasing its maximum. Nifedipine, but not Bay K 8644, significantly reduced the level of tension induced by noradrenaline, and its effect, evaluated by the R/T coefficient, was an increase in the electrically induced relaxation, whereas Bay K 8644 had a significant inhibitory effect.(ABSTRACT TRUNCATED AT 250 WORDS)     

Direct and indirect contractile responses of the human vas deferens and actions of noradrenaline and of calcium antagonists

Contractile responses of the isolated human vas deferens, obtained from vasectomy operations, were measured. Large single electrical shocks gave a twitch response with short latency (0.36 s) which was insensitive to prazosin (5 microM) or TTX (0.2 microM) and was thus identified as due to direct muscle stimulation. A train of 100 low intensity shocks gave a response with a longer latency (1.9 s) which was substantially sensitive to both prazosin and TTX; we assume this response is dominated by an indirect nerve-induced contraction. Relaxations, presumably caused by activation of circular muscle, were recorded from regions of some preparations both by direct and indirect stimulation. Noradrenaline (10-20 microM) induced a tonic contracture, spontaneous contractions and a large potentiation of the response to direct stimulation--but not to indirect stimulation implying a strong presynaptic inhibition. Noradrenaline also speeded the relaxation from contractions. Verapamil (1-100 microM) and nifedipine had no effect on the direct responses but verapamil (10 microM) inhibited the indirect response. Calcium removal prevented most, and 5 mM-EDTA all, of the direct response. However, even with EDTA, noradrenaline was able to support spontaneous and stimulus-induced contractions. Thus contraction of the vas, though sustained by external calcium, does not appear to directly depend on it.     

Effect of neurotensin on contractile activity and [3H]acetylcholine release in cat terminal ileum during different postnatal periods.

The effect of neurotensin (NT) on the contractile activity of circular and longitudinal strips from the terminal ileum of 15-, 30-, 60-day-old and adult cats as well as on the resting and electrically-evoked release of [3H]acetylcholine (ACh) was studied. Radioactivity was measured by liquid scintillation spectrometry and the effect of NT was evaluated by the S2/S1 ratio. In the circular muscle strips NT (1-100 nM) inhibited spontaneous contractions in all age groups. In the longitudinal strips the effect of NT was concentration- and age-dependent. NT at a concentration of 1 nM had no effect on the spontaneous activity in 15-day-old cats, but in the other age groups in 70-80% of the cats it inhibited spontaneous contractions. The response to 10 and 100 nM NT was either biphasic (relaxation followed by contraction) or inhibitory: in 15-day-old cats the response was biphasic only and with increasing age the percentage of strips responding with inhibition of the contractions increased. Neither substances affecting adrenergic and cholinergic transmission nor TTX changed the inhibitory response to NT. The contractile component of the biphasic response was TTX-resistant in all age groups and was significantly decreased by scopolamine in 60-day-old and adult cats. NT increased both resting and electrically-evoked release of [3H]ACh which was not changed by TTX. In the presence of the peptide the S2/S1 ratio increased as NT-induced [3H]ACh release in the strips of adult cats was higher than that in young cats.(ABSTRACT TRUNCATED AT 250 WORDS)     

The adrenal medulla as a wet sponge: a role for the intramedullary venous vasculature?

Synchronous contractions in the intramedullary venous vasculature have been postulated to assist in the discharge of hormones from the stimulated adrenal medulla in a manner analogous to the squeezing of a wet sponge. This study reports on two experimental approaches to support the hypothesis that contractions in the venous vasculature may contribute to the hormonal efflux. Firstly, the bovine adrenal medulla was perfused retrogradely via the bovine central adrenomedullary vein and changes in the vascular volume were assessed as changes in wet weight of the perfused tissue. Stimulation with acetylcholine and carbachol resulted in repetitive, transient weight losses, suggesting cholinergically mediated reductions in the vascular volume. Secondly, the contractile properties of the longitudinal layers of smooth muscle cells in the intramedullary venous system were characterized, using the bovine central adrenomedullary vein as a model. The results showed that the longitudinal layers of this vein were, similarly to the circular layers, selectively contracted by endothelin-1 via an ET_A-like receptor, by neuropeptide Y and by membrane depolarization (high K⁺). However, the vein was insensitive to electrical stimulation, acetylcholine and carbachol, as well as to catecholamines. These results suggest neuropeptide Y, released from the cholinergically stimulated chromaffin cells, as the most likely mediator of stimulus-evoked synchronous contractions of the venous vasculature in the bovine adrenal medulla. Together, these experiments provide support for the `wet sponge' hypothesis for hormonal discharge from the adrenal gland.     

Inhibition of the endothelium-dependent relaxation by 18beta-glycyrrhetinic acid in the guinea-pig aorta.

The effect of 18beta-glycyrrhetinic acid (GA), an agent which interferes with gap junction conductivity, on endothelium-dependent relaxation produced by substance P was investigated in isolated aortic rings of the guinea-pig. In nor-adrenaline (NA)-contracted aortic rings, substance P (10(-7) M) induced an endothelium-dependent, transient relaxation. The relaxation was only slightly reduced by the co-application of nitroarginine and diclofenac. When GA (2x10(-5) M) was applied first, it slightly reduced substance P-induced relaxation, and a subsequent co-application of nitroarginine and diclofenac strongly reduced the relaxation. In aortic rings contracted with high-K solution ([K(+)](o) = 29.4 mM), substance P-induced relaxation was reduced by the simultaneous application of GA, nitroarginine and diclofenac, but not by GA alone. In endothelium-denuded aortic rings, GA reduced the threshold concentration of NA required to produce contractions and increased the amplitude of NA-induced contractions. GA increased the amplitude of contraction produced by small increases of [K(+)](o) (<30 mM) but reduced those produced by higher concentrations of [K(+)](o) (>54 mM). In NA-contracted aortic rings, Y-26763, a K(+)-channel opener, could relax muscles with reduced amplitude in the presence of GA. It is concluded that in guinea-pig aortic rings, GA inhibits mainly the EDHF-induced components of endothelium-dependent relaxation. GA also modulated contractions produced by NA or high-K solutions. The possible effects of inhibition of gap junctions by GA on endothelium-dependent relaxation were discussed.     

Cholinergic and adrenergic neural control of smooth muscle function in the non-pregnant rat uterine cervix

The isolated circular smooth muscle of the uterine cervix from spayed rats was investigated in vitro. One group of animals was treated with oestrogen. Preparations from these rats showed no spontaneous contractile activity, but responded with contractions to electrical field stimulation. The contractions were blocked by approximately 85% with atropine, scopolamine and tetrodotoxin, but were unaffected by the adrenergic antagonists propranolol, phenoxybenzamine, and yohimbine. This neurogenic effect was potentiated by neostigmine. Noradrenaline inhibited the nerve-induced contractions and also lowered resting tension in a concentration-dependent way. The latter effect of noradrenaline was inhibited by propranolol. The indirect effects of noradrenaline on the electrically induced contractions were also inhibited by propranolol, but potentiated with phenoxybenzamine and, to a less extent, yohimbine. Nerve-induced relaxation was never observed. Acetylcholine contracted the smooth muscle by an effect that was counteracted by atropine. A second group of spayed rats was not given any steroid treatment. The cervix preparations from these animals showed spontaneous activity, which was uninfluenced by atropine, propranolol, phenoxybenzamine, yohimbine, and tetrodotoxin. Noradrenaline inhibited the spontaneous activity. This noradrenergic effect was counteracted by propranolol and potentiated by phenobenzamine. Acetylcholine increased the smooth-muscle tone by an atropine-sensitive action. The results suggest the presence of a cholinergic motor innervation on the non-pregnant rat uterine cervix. The cervix also receives an adrenergic innervation, which may act both post-junctionally (lowering resting tension and inhibiting spontaneous activity) and prejunctionally (modulating the cholinergic nerves) via beta-adrenergic as well as alpha-adrenergic receptors.     

Neuropeptide Y (NPY) co-exists with tyrosine hydroxylase and potentiates the adrenergic contractile response of vascular smooth muscle in the human uterine artery.

The human uterine artery was studied by immunocytochemistry and in vitro pharmacology. Nerve fibres containing immunoreactivity for neuropeptide Y (NPY) were encountered in the media, adventitia, smooth muscle layers and surrounding the vasa vasorum. Approximately 50% of the nerve fibres containing NPY also stored immunoreactivity for tyrosine hydroxylase (TH) and TH activity was found in no other fibres. Noradrenaline (NA) contracted the isolated uterine artery in a concentration dependent manner. The presence of increasing doses of NPY shifted the concentration-response curve for NA to the left. Consequently the pD2-values were increased indicating a potentiation of the adrenergic effects induced by NPY. The results demonstrate the existence of NPY in adrenergic nerve fibres surrounding the human uterine artery. A close co-operation between NPY and NA in the neuronal control of smooth muscle is suggested.     

Tetrodotoxin-resistant contractions induced by electrical stimulation of bladder muscle from man, rabbit and rat

Isolated detrusor preparations from man, rabbit and rat were suspended in an organ bath and isometric tension was recorded. The preparations were stimulated electrically in the presence of Bay K8644 and nifedipine before and after neuronal blockade with tetrodotoxin. Transmural electrical stimulation produced frequency-dependent contractions in all preparations. Bay K8644 significantly increased and nifedipine decreased these contractions. TTX effectively suppressed the response to electrical field stimulation in all species. When Bay K8644 was added to TTX blocked preparations, the responses to electrical stimulation were partly restored in bladder strips from man and rat. No increase in response was seen in the rabbit preparations. However, if the extracellular K+-concentration was increased to 10 mM (which per se did not affect the response) Bay K8644 significantly increased the contractions. All responses elicited by electrical stimulation in the presence of TTX were abolished by nifedipine. It is concluded that if the bladder smooth muscle is exposed to factors that can increase its sensitivity to contractile agents, this may result in uncontrolled (unstable) bladder contractions. Such contractions may use the 'normal' transmitter substances, but may be triggered at a lower stimulus intensity than normal. As a non-specific increase in membrane excitability seems to be associated with an influx of calcium through voltage-sensitive calcium channels, calcium antagonists, together with agents specifically blocking relevant transmitter substances, would offer an effective therapy against the unstable bladder.     

Concentrations and contractile effects of substance P in the human ampullary-isthmic junction

From 20 women undergoing hysterectomy, strip preparations were isolated from the outer, longitudinal and the inner, circular smooth muscle layer of the ampullary-isthmic junction (AIJ), together with small arterial segments dissected as ring preparations from the root of the mesosalpinx. The specimens were mounted in organ baths and isometric tension was recorded. In addition, tissue concentrations of substance P (SP) in the ampulla, AIJ and utero-tubal junction were determined by radioimmunoassay. Tissue concentrations of SP expressed as pmol X g tissue-1 (wet weight, +/- SE) amounted to 3.09 +/- 1.40 in the utero-tubal junction, 1.08 +/- 0.299 in the AIJ and 0.742 +/- 0.299 in the ampulla. In strips of circular muscle, SP at concentrations of 10(-7) -3 X 10(-6) mol X l-1 elicited a combined phasic and tonic response and in longitudinal muscle a mainly tonic contraction was produced. In both tissues, contractions elicited by SP were rapidly abolished in calcium-free medium. Nifedipine abolished the phasic contraction elicited in circular muscle by SP while the tonic response was resistant. The contraction in longitudinal muscle was reduced by 20-30%. Vasoactive intestinal polypeptide (VIP) decreased tension in preparations contracted by SP, prostaglandin F2 alpha and K+-depolarization (124 mmol X l(-1). In unstimulated oviductal arterial preparations, SP had no effect, while the peptide induced a transient relaxation of noradrenaline contracted preparations, and slightly decreased tension of K+-depolarized vessels. The results suggest that SP may be involved in the control of motility of the human AIJ.     

Stimulation of intestinal smooth muscle by atropine, procaine, and tetrodotoxin.

In order to determine whether or not atropine, procaine, and tetrodotoxin (TTX) can stimulate intestinal smooth muscle directly, we examined the effects of these drugs on the mechanical and electrical activities of several types of cat intestinal smooth muscle preparations. The preparations consisted of isolated rings of 1) intact intestinal wall, 2) intact longitudinal and circular muscle, 3) ganglion-free circular muscle, and 4) ganglion-free circular muscle devoid of its dense layer and plexus muscularis profundus. Atropine and procaine (greater than 10(-4) M) stimulated all four types of preparation. On the other hand, TTX (up to 5 X 10(-6) M) stimulated only preparations 1 and 2. It is concluded that whereas atropine and procaine can directly stimulate intestinal smooth muscle, the excitatory effect of TTX is neurally mediated.     

Actions of endothelin on isolated corpus cavernosum from rabbit and man

The effects of endothelin, a vasoconstrictor peptide produced by vascular endothelial cells, were investigated in isolated rabbit and human corpus cavernosum (CC). Preparations from both rabbit and man were potently contracted by endothelin in a concentration-dependent manner. The contractions developed slowly, could not be reversed despite frequent washings, and were only partly inhibited by the Ca2+ channel blocker nimodipine. Even in Ca2(+)-free medium containing the chelator EGTA a small contractile component persisted. In rabbit CC, the contractions in Ca2(+)-free medium were not affected by nimodipine, the Ca2(-)-channel agonist BAY K 86(44), or by depletion of intracellular Ca2(+) stores sensitive to noradrenaline (NA) and caffeine, but were almost abolished by the protein kinase C inhibitor H7. In both rabbit and man, carbachol and vasoactive intestinal polypeptide concentration-dependently relaxed preparations contracted by endothelin. The relaxations induced by carbachol were antagonized by atropine. Endothelin enhanced concentration-dependently the contractions induced by exogenously applied NA in rabbit CC. The enhancement was more pronounced at low concentrations of NA. This study shows that endothelin potently contracts isolated penile erectile tissue. The contraction seems to be mediated mainly by influx of Ca2+ through the cell membrane, which partly occurs through a pathway other than voltage-operated calcium channels. However, involvement of other mechanisms cannot be excluded. The results suggest that endothelin can play a role in penile erectile mechanisms.     

Mechanisms underlying pre- and postjunctional effects of neuropeptide Y in sympathetic vascular control

The effects of porcine neuropeptide Y (NPY) regarding sympathetic vascular control were studied in vitro on isolated rat blood vessels. The 10(-9)M NPY enhanced (about two-fold) the contractile responses to transmural nerve stimulation (TNS), noradrenaline (NA) and adrenaline (about two-fold) in the femoral artery. Higher concentrations of NPY (greater than 10(-8)M) caused an adrenoceptor-resistant contraction per se. The TNS-evoked [3H]NA efflux was significantly reduced by NPY in a concentration-dependent manner (threshold 10(-9)M). The calcium antagonist, nifedipine, abolished the contractile effects of NPY and the NPY-induced enhancement of NA contractions but did not influence the prejunctional inhibition of [3H]NA release. Receptor-binding studies showed that the ratio of alpha 1-to alpha 2-adrenoceptors in the femoral artery was 30:1. The NPY did not cause any detectable change in the number of alpha 1-or alpha 2-adrenoceptor binding sites or in the affinity of alpha 2-binding sites, as revealed by prazosin- and clonidine-binding, respectively. The NPY also inhibited the TNS-evoked [3H]NA release (by 42-86%) in the superior mesenteric and basilar arteries and in femoral and portal veins. The NPY still depressed TNS-evoked [3H]NA secretion from the portal vein in the presence of phentolamine. The NPY caused a clear-cut contraction in the basilar artery, increased the contractile force of spontaneous contractions in the portal vein, while only weak responses were observed in the superior mesenteric artery and femoral vein. The NA-induced contraction was markedly enhanced by NPY in the superior mesenteric artery, only slightly enhanced in the portal vein and uninfluenced in the femoral vein. In conclusion, in all blood vessels tested, NPY depresses the TNS-evoked [3H]NA secretion via a nifedipine-resistant action. Furthermore, NPY exerts a variable, Ca2+-dependent vasoconstrictor effect and enhancement of NA and TNS contractions.     

Electrical components contributing to the nerve-mediated contractions in the smooth muscles of the rabbit ear artery

Adrenergic transmissions were investigated by recording electrical and mechanical responses of the smooth muscle cells in the rabbit ear artery. Perivascular nerve stimulation generated an excitatory junction potential (e.j.p.) and a slow depolarization. The latter but not the former was suppressed by prazosin or phentolamine. Both the e.j.p. and slow depolarization were suppressed by tetrodotoxin (TTX) or guanethidine. Facilitation processes of e.j.p.s produced by repetitive stimulation of the nerves were not modified by prazosin, phentolamine, or yohimbine. Increasing the stimulus frequency increased the amplitude of e.j.p.s and slow depolarizations and, at high frequencies (greater than 5 Hz) generated a spike potential. Nicardipine (10(-7) M) blocked the spike potential and reduced the e.j.p. amplitude, but did not affect the slow depolarization. Amplitude of muscle contractions produced by transmural nerve stimulation increased with increase in the stimulus frequency. The nerve-mediated contractions produced by high-frequency stimulation (10 Hz) were reduced to 49% of the control value by prazosin (10(-6) M), to 79% by nicardipine (10(-7) M), to 34% by prazosin (10(-6) M) plus nicardipine (10(-7) M), and to 1.2% by TTX (3 X 10(-7) M). Exogenously applied noradrenaline depolarized the smooth muscle membrane and produced the muscle contraction. These effects of noradrenaline were antagonized by prazosin or phentolamine. Thus, in the rabbit ear artery, perivascular nerve stimulation produced three types of electrical responses, i.e., e.j.p., spike potential, and slow depolarization. The latter but not the former two was produced through activation of alpha 1-adrenoceptors. Nerve-mediated muscle contractions were the results of stimulation of alpha 1-adrenoceptors, generation of spike potentials, and of e.j.p.s.