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.     

Different effects of neuropeptide Y on electrically induced contractions in the longitudinal and circular smooth muscle layers of the female rabbit urethra

The effects of neuropeptide Y (NPY) on preparations of isolated longitudinal and circular smooth muscle from rabbit urethra were studied. In both types of muscle, electrically induced contractions and relaxations could be abolished by tetrodotoxin, (TTX). In the longitudinal muscle preparations the contraction was slightly reduced by prazosin, but markedly reduced by scopolamine and NPY. The NPY effect was not influenced by pretreatment with rauwolscine. Pretreatment with NPY had no effect on contractions induced by noradrenaline (NA) or carbachol and the peptide did not relax preparations contracted by these agents. In circular muscle an initial, fast response, not sensitive to prazosin or scopolamine was occasionally observed following electrical stimulation. A slow contraction component was regularly seen; this response was abolished by prazosin. Neuropeptide Y did not influence any of these responses. The preparations were concentration-dependently contracted by NA, whereas carbachol had no effect. Pretreatment with NPY did not affect contractions induced by NA, nor did the peptide relax NA-contracted preparations. In neither longitudinal nor circular muscle strips did NPY affect the electrically induced TTX sensitive relaxation of NA-contracted preparations. The results suggest that in the rabbit urethra NPY reduces contractions in the longitudinal muscle layer by selectively inhibiting the release of acetylcholine from cholinergic nerves. Neuropeptide Y did not appear to have any significant postjunctional effects nor to interfere with the release, or effects of NA or other transmitter agents. The physiological importance of the urethral effects of NPY remains to be established.     

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)     

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.     

Regulation of voltage-dependent excitatory responses to alpha,beta-methylene ATP, ATP and non-adrenergic nerve stimulation by dihydropyridines in the guinea-pig vas deferens

Simultaneous recordings of mechanical and intracellular electrical activity were obtained from the guinea-pig vas deferens, where nerve stimulation, ATP and the stable nucleotide analogue alpha,beta-methylene ATP elicited excitatory responses. Excitatory junction potentials and action potentials were elicited by low-frequency (trains of pulses, generally less than or equal to 2 Hz) field stimulation. alpha,beta-Methylene ATP and ATP elicited only concentration-dependent depolarizations at low concentrations, while higher concentrations elicited a superimposed action potential discharge which was accompanied by mechanical contraction. The voltage threshold at which action potential discharge was initiated by these three stimuli was about -45 mV (resting membrane potential averaged -66 mV). Action potential discharges and contractile responses were antagonized by nifedipine and augmented by Bay K 8644 at concentrations (1 and 0.5 microM, respectively) which exhibited only small effects on either excitatory junction potential amplitudes or nucleotide-induced depolarizations. Bay K 8644 enhanced and nifedipine antagonized the repolarization (rectification) phase of action potential discharge elicited by nerve stimulation and drugs; after-hyperpolarizations were prominent in the presence of Bay K 8644 (0.1-5 microM). Excitatory junction potentials were antagonized after exposure to alpha,beta-methylene ATP. This antagonistic effect of alpha,beta-methylene ATP was also observed following depolarizations elicited in the absence and presence of nifedipine (1 microM). Noradrenaline was approximately 50-100 times less potent than alpha,beta-methylene ATP in eliciting action potential discharge and contraction. It was only when a high concentration of noradrenaline was used (about 60-100 microM) that the noradrenaline-induced depolarization attained the voltage threshold for action potential initiation. These results illustrate the similarity of the electrical components which underlie excitation by nerve stimulation and adenine nucleotides in the vas deferens, and demonstrate the ability of dihydropyridines to regulate voltage-dependent events associated with both the generation and inactivation of muscle action potentials. These are probably voltage-dependent calcium currents and calcium-activated potassium currents, respectively. Neither excitatory junction potentials nor the mechanism of desensitization of the ATP purinoceptor by alpha,beta-methylene ATP involve voltage-dependent calcium channels.     

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.     

Effects of Bay K 8644 on spontaneous and evoked transmitter release at the mouse neuromuscular junction

The dihydropyridine, Bay K 8644, was applied in vitro to mouse phrenic nerve-diaphragm muscle preparations. The drug increased both spontaneous and evoked release of acetylcholine from the motor nerve terminal in a concentration- and time-dependent manner. The rise in miniature endplate potential frequency, however, was the result of an increased intraterminal mobilization of free calcium, rather than well-established activation of voltage-dependent calcium channels. This view is supported by the following observations: (1) an increase in frequency was apparent in Ca2+-free medium; (2) Bay K 8644 is known to require a moderate depolarization to affect Ca2+ channels, but no membrane depolarization was detected; and (3) exposure to low Ca2+ and high Mg2+ medium did not diminish the effect on miniature endplate potential frequency. In a medium containing low Ca2+ and high Mg2+, Bay K 8644 increased quantal content of the evoked endplate potentials to a greater degree and with a faster time course than the frequency of miniature endplate potentials. This enhancement in evoked release did not appear to be caused solely by an increase in cytoplasmic Ca2+, but rather reflected at least in part the Bay K 8644-induced activation of voltage-gated Ca2+ channels, perhaps L-type, at the presynaptic nerve terminal. Thus, we propose that Bay K 8644 exerts dual effects on the motor nerve endings, characterized by a primary action on the presynaptic Ca2+ channels and a secondary action associated with the elevation of intracellular Ca2+ concentration.     

Mechanical and electrical responses modulated by excitation of inhibitory nerves during stimulation with high-potassium solutions in circular smooth muscle of the rabbit rectum.

The properties of the mechanical responses produced by solutions containing high concentrations of potassium ion (high-K solution, [K(+)](o) = 9-27 mM) were investigated in circular smooth muscle preparations isolated from the rabbit rectum. Isometric recording of mechanical responses of the muscle revealed spontaneous contractions, which successively decreased and finally disappeared in most preparations. Stimulation of the smooth muscle with high-K solutions elicited an increase in both amplitude and frequency of twitch contractions (sustained component), with about a 2 min delay in the beginning (initial inhibition), and a transient large contraction shortly after the cessation of stimulation (after contraction). Transmural nerve stimulation (TNS) with electrical pulses for 1 min at 1 Hz frequency produced a sustained inhibition, but a transient contraction followed after termination of TNS. In the presence of tetrodotoxin (TTX), the TNS-induced responses were abolished, while a high-K solution elicited increased twitch contractions with a short delay and abolished the after contraction. Suramin produced effects similar to TTX on the responses produced by high-K solutions or TNS, but this was not the case for atropine, guanethidine or N(omega)-nitro-L-arginine (L-NA). Recording membrane potentials with microelectrodes revealed that TNS evoked an inhibitory junction potential (i.j.p.) which was non-adrenergic, non-cholinergic and non-nitrergic in nature. High-K solutions elicited a tri-phasic change in the membrane potential; an initial hyperpolarization, followed by a sustained depolarization and finally a transient depolarization on cessation of high-K stimulation. TTX or suramin inhibited the i.j.p.s and altered the tri-phasic change in the membrane potential produced by a high-K solution to a mono-phasic depolarization. No significant modulation of electrical responses of the membrane induced by TNS or high-K solution was elicited by atropine, guanethidine or L-NA. The results indicated that the circular smooth muscle of the rabbit rectum is innervated by inhibitory nerves, and that stimulation with high-K solutions caused inhibitory neuronal modulation of both electrical and mechanical responses of the smooth muscle, in a suramin-sensitive way.     

Effects of prostanoids on isolated feline cerebral arteries. II. Roles of extra- and intracellular calcium for the prostaglandin F2 alpha-induced contraction

The roles of extra- and intracellular calcium for the contractile effects of PGF2 alpha in the feline basilar artery (BA) were investigated. Comparisons were made with contractions induced by K+ and noradrenaline (NA). Addition of nifedipine to PGF2 alpha- or K+ (124 mM)-contracted arteries resulted in an incomplete relaxation, whereas NA-contracted vessels were completely relaxed. Incubation of the preparations in a calcium-free medium containing 10(-5) M EGTA for 5-10 min almost abolished contractions induced by K+ and NA. In contrast, 63% of the response to PGF2 alpha remained after pretreatment of the arteries in a calcium-free solution for 40 min; PGF2 alpha produced a biphasic contraction in 17 out of 20 preparations consisting of a rapidly developing initial phase followed by a second increase in tension after 1-6 min. The second phase was absent if the EGTA-concentration was increased to 10(-4) M, or if the arteries were pre-treated with nifedipine. After incubation of the arteries in a calcium-free medium for 40-120 min and K+-depolarization, re-addition of calcium elicited contractions at lower concentrations in the presence of PGF2 alpha than in controls. The results suggest that PGF2 alpha-induced contractions in the feline BA are considerably less dependent on extracellular calcium than contractions evoked by K+ or NA. PGF2 alpha appears to be able to release calcium from two cellular stores, and may also promote calcium influx through the cell membrane.     

Effects of nimodipine, Bay K 8644 and pinacidil on alpha 1- and alpha 2-adrenoceptor-mediated vasoconstriction in human hand veins

The effects of nimodipine, Bay K 8644 and pinacidil, three drugs interfering with transmembrane Ca2+ fluxes in different ways, were investigated in isolated human hand veins. Their ability to influence the concentration-response relationship for noradrenaline (NA) was assessed in the absence and presence of prazosin or rauwolscine. The contractile response to NA was almost abolished in Ca2+ -free medium. Nimodipine and pinacidil depressed the NA concentration-response curve both in the absence and presence of alpha-adrenoceptor blockers. The NA response was only partially inhibited by nimodipine, indicating that NA may activate nimodipine-insensitive influx pathways, presumably receptor-operated calcium channels. Pinacidil inhibited the contractile response to 124 mM K+ and reduced the NA-induced contraction in the presence of nimodipine, suggesting that pinacidil has actions other than the opening of potassium channels and subsequent membrane hyperpolarization. Bay K 8644 increased the NA potency fourfold in the presence of rauwolscine, whereas it had no effect on the NA response in the presence of prazosin and in the absence of alpha-adrenoceptor blockade. Such an action of Bay K 8644 can be reconciled with alpha 1-adrenoceptor activation causing membrane depolarization and opening of potential-operated calcium channels. It may be concluded that both alpha 1- and alpha 2-adrenoceptor-mediated contractions in human hand veins are highly dependent on Ca2+ influx, although the mechanisms utilized to bring about this influx partly differ between the two receptor subtypes.     

Electrical field stimulation of rat mesenteric small arteries: force and free cytosolic calcium during neurogenic contractions and mechanisms of non-neurogenic relaxations

The effect of transmural electrical field stimulation (TEFS) of rat mesenteric small arteries was studied. Stimulation parameters were selected to cause tetrodotoxin (TTX) sensitive contractions. In arteries precontracted with PGF_2α in the presence of phentolamine, TTX insensitive relaxation could be induced by TEFS. The relaxing effect of TEFS required higher stimulation amplitude and duration than the contractions. Thus, by appropriately choosing stimulation parameters, contractile responses could be elicited which were little affected by any relaxing effect, while contractions were abolished by TTX at any stimulation conditions in the present study. The contractions were abolished by cold storage and almost completely inhibited by phentolamine. Thus, contractions were neurogenic and primarily caused by noradrenaline. At low frequencies, TEFS caused phentolamine sensitive increases in free cytosolic calcium with no contractions. At higher frequencies, there was a further increase in free cytosolic calcium, associated with contraction. Only at high frequencies, noradrenaline from nerves caused sensitization of the contractile filaments to free cytosolic calcium as during stimulation with exogenous noradrenaline. The relaxations were associated with decreases in free cytosolic calcium and were probably non-neurogenic since they were resistant to TTX, cold storage, capsaicin, and repeated stimulation. Furthermore, relaxations were almost completely abolished by increasing extracellular potassium to 40 mM or by adding tetraethylammonium chloride or 4-aminopyridine. Relaxations were also reduced by ouabain and potassium free conditions.     

Dihydropyridine calcium channel antagonists block and agonists potentiate high potassium contractures but not twitches in frog skeletal muscle.

The effects of two dihydropyridine calcium channel antagonists (nitrendipine and nifedipine) and two agonists (Bay K8644 and CGP-28392) were tested on high K(+)-induced contractures of frog's toe muscles. All four drugs depressed or blocked maximum contractures induced by 123 mM K+. Agonist effects, i.e., an increase in contracture amplitude, were found with smaller contractures produced by lower high K+ concentrations (i.e., 10, 20, and 25 mM). Bay K8644 produced its maximum agonist effect at 10(-7) M and only depressed contractures with 10(-6) M. CGP-28392 had its greatest agonist effect with 10(-9) M and had only antagonist effects with 10(-7) M or more. Nitrendipine had no agonist effects but nifedipine produced agonist effects with all concentrations tested (10(-9) to 10(-4) M). These results support previous results indicating that these contractures are initiated by extracellular Ca2+ ions entering via the voltage-sensitive, slow calcium channels in the t-tubules. The results obtained in the present study also are consistent with the known pharmacological effects of these drugs on calcium channels.     

Calcium influx through voltage-sensitive calcium channels regulates in vivo serotonin N-acetyltransferase (NAT) activity in hen retina and pineal gland.

The involvement of transmembrane transport of Ca2+ in regulation of retinal and pineal serotonin N-acetyltransferase (NAT) activity was studied in vivo in hens. Intramuscular administration to hens of organic antagonists of voltage-sensitive calcium channels (VSCC), nimodipine, nifedipine and verapamil, resulted in a marked inhibition of the nocturnal increase of NAT activity in retina and pineal gland. In both tissues the inhibitory effect of nimodipine on the night-time rise of NAT activity was abolished by Bay K 8644, a VSCC agonist. Bay K 8644 administered to hens at the end of the light phase of the light-dark cycle significantly enhanced the dark-dependent increase in retinal and pineal NAT activity. It is suggested that the nocturnal increase of NAT activity in hen retina and pineal gland is similarly regulated in vivo by Ca2+ influx through the VSCC.     

On the usefulness of Fura-2 measurements of intrasynaptosomal calcium levels in rat cortical synaptosomes to study mechanisms of presynaptic function

Levels of [Ca²⁺]_i in rat cortex synaptosomes were measured using the Ca²⁺indicator Fura-2. Ca²⁺influx was induced by veratridine in a concentration-dependent manner (1–10 μm ). The resulting increase in [Ca²⁺]_i was inhibited by tetrodotoxin (TTX). K⁺(18 mm ) increased the [Ca²⁺]_i which was not influenced by TTX. K⁺-channel blockers such as 4-aminopyridine, α-and δ-dendrotoxin per se were ineffective. The veratridineinduced Ca²⁺influx in synaptosomes was reduced by L-type Ca²⁺-channel blockers, such as felodipine, nifedipine and PN-200-110, verapamil and diltiazem. ω-Conotoxin, an N-type Ca²⁺-channel blocker, did not inhibit the veratridine-stimulated [Ca²⁺]_i increase. Bay K 8644, an L-channel agonist, stimulated an increase of [Ca²⁺]_i in synaptosomes which was not sensitive to TTX. R-N⁶-Phenyl-isopropyl-adenosine (R-PIA) and clonidine, agonists at adenosine A₁-receptors and α₂-adrenoceptors, respectively, did not influence the veratridinestimulated [Ca²⁺]_i increase. R-PIA did not interact with Bay K 8644-stimulated [Ca²⁺]_i increase in synaptosomes. The results for all the substances used show major differences between the effects on Ca²⁺influx in synaptosomes and on the electrically evoked neurotransmitter release in slice preparations. Thus, the synaptosome preparation is not a generally applicable experimental model for the study of Ca²⁺mechanisms of presynaptic neuromodulation.     

Calcium dependence of BAY K 8644 effects on the rabbit gall-bladder

In the present study, we characterized the effects of the calcium (Ca²⁺) channel activator BAY K 8644 on sodium (Na⁺) absorption and transepithelial potential difference (Pd) in the rabbit gall-bladder. In gall-bladders mounted in an Ussing chamber it was observed that serosal BAY K 8644 (10^±5 M) inhibited Na⁺ absorption in the presence, but not in the absence of serosal Ca²⁺. Serosal nifedipine (a Ca²⁺ channel antagonist) at 10^±5 M did not reverse the Na⁵⁺ transport inhibition caused by BAY K 8644. Another effect of serosal BAY K 8644 (10^±5 M) was to induce oscillations in Pd. These Pd-oscillations had a frequency of about one per minute and an amplitude of 20–40 V. The appearance of Pd-oscillations was dependent on the presence of Ca²⁺ in the serosal medium. The oscillations were abolished by 1–3×10^±5 M serosal nifedipine and by bilateral application of 3 mM barium (Ba²⁺) (a K⁺ channel blocker). In a sac preparation of the rabbit gall-bladder, spontaneous cyclic contractions of smooth muscle cells in the gall-bladder wall were observed as oscillations in the transmural pressure. These spontaneous contractions were not accompanied by oscillations in Pd. Serosal BAY K 8644 (10^±5 M) evoked oscillations in Pd in half of the sac preparations, but in each gall-bladder the frequencies of Pdoscillations and pressure oscillations were different. Serosal nifedipine (2×10^±5 M) abolished both types of oscillation. The results show that the BAY K 8644-induced Na⁺ transport inhibition and initiation of Pd-oscillations can be ascribed to specific epithelial Ca²⁺-dependent effects of BAY K 8644. We postulate that the Pd-oscillations involve Ca²⁺-activated, Ba²⁺-sensitive K⁺ channels in both the luminal and the basolateral cell membrane.Part of this study was presented at the 9th Conference of the European Society for Comparative Physiology and Biochemistry, Copenhagen, 1987 [13]     

Effects of nimodipine,Bay K 8644 and pinacidil on α1- and α2-adrenoceptor-mediated vasoconstriction in human hand veins

The effects of nimodipine, Bay K 8644 and pinacidil, three drugs interfering with transmembrane Ca²⁺ fluxes in different ways, were investigated in isolated human hand veins. Their ability to influence the concentration-response relationship for noradrenaline (NA) was assessed in the absence and presence of prazosin or rauwolscine. The contractile response to NA was almost abolished in Ca²⁺-free medium. Nimodipine and pinacidil depressed the NA concentration-response curve both in the absence and presence of α-adrenoceptor blockers. The NA response was only partially inhibited by nimodipine, indicating that NA may activate nimodipine-insensitive influx pathways, presumably receptor-operated calcium channels. Pinacidil inhibited the contractile response to 124 mM K⁺ and reduced the NA-induced contraction in the presence of nimodipine, suggesting that pinacidil has actions other than the opening of potassium channels and subsequent membrane hyperpolarization. Bay K 8644 increased the NA potency fourfold in the presence of rauwolscine, whereas it had no effect on the NA response in the presence of prazosin and in the absence of α-adrenoceptor blockade. Such an action of Bay K 8644 can be reconciled with α-adrenoceptor activation causing membrane depolarization and opening of potential-operated calcium channels. It may be concluded that both α₁- and α₂-adrenoceptor-mediated contractions in human hand veins are highly dependent on Ca²⁺ influx, although the mechanisms utilized to bring about this influx partly differ between the two receptor subtypes.     

Effects of nifedipine on potassium-induced contraction and noradrenaline release in cerebral and extracranial arteries from rabbit

The present study was designed to evaluate the effects of the calcium antagonist nifedipine on potassium-evoked contractions and release of noradrenaline from sympathetic nerves in rabbit basilar and facial arteries. Contractions were measured isometrically in a small volume organ bath. While noradrenaline (NA) produced strong contractions in facial arteries, the majority of the basilar arteries responded only to the highest concentrations of NA employed (greater than 10 microM) with weak contraction. Prazosin (1 microM) and phentolamine (1-10 microM) effectively antagonized the responses to NA in both types of vessel. In contrast, contractions evoked by potassium (K+, 124 mM) were only slightly reduced by the alpha-adrenoceptor blocking agents, indicating that the participation of endogenous NA in maintaining the contractile response to K+ is either small or negligible in the vessel types studied. Nifedipine concentration-dependently inhibited K+-induced contractions in basilar and facial arteries, the former being significantly more affected as evidenced by the maximum inhibitions (approximately 80% compared to approximately 60%) and IC50 values (approximately 10 nM vs. approximately 30 nM). A combination of nifedipine (0.3 microM) and prazosin (1 microM) or phentolamine (1 microM) further suppressed the K+-evoked contractile response in facial arteries, but failed to do so in basilar arteries, when compared with the effect of nifedipine alone. The depressant effect of the alpha-adrenoceptor blockers was, however, still obtainable after reserpine treatment of the facial artery in vitro. Fluorescence histochemical demonstration of noradrenaline revealed a dense network of adrenergic nerve fibres in the walls of the basilar and facial artery. The vessels were also shown to accumulate 3H-NA and release it upon depolarization with K+. The uptake and subsequent release of 3H-NA were significantly reduced by desipramine (10 microM). Nifedipine (0.3-3.0 microM) failed to alter the K+-evoked 3H-NA efflux from sympathetic nerves in neither of the two vessel types. It may be concluded that nifedipine effectively inhibits K+-evoked contractions in isolated basilar and facial arteries from rabbit without interfering with nerve-mediated NA release. Possible explanations for this selective effect of nifedipine on muscle contraction are discussed.     

The ionic mechanism of phenylephrine-induced rhythmic contractions in rabbit mesenteric arteries treated with ryanodine

Phenylephrine induces endothelium-independent rhythmic contractions in ryanodinetreated rabbit mesenteric arteries. To elucidate the ionic mechanism of this rhythmic behaviour, rabbit mesenteric arterial rings were suspended in an organ chamber for isometric tension studies. Yohimbine, propranolol, and atropine had no effect on these contractions, minimizing the possibility that transmitter release from nerve terminals was involved. Additionally, the oscillatory contractions were not altered by diphenhydramine, cimetidine, and indomethacin, thus ruling out the involvement of histamine and prostaglandins. This oscillatory response was completely abolished after the removal of extracellular Ca 2+, as well as after Ca 2+ channel blockade by diltiazem or nifedipine. Sparteine and quinidine, Ca 2+-activated K+ channel antagonists, also abolished the oscillation. In contrast, tetraethylammonium and 3,4-diaminopyridine, voltage-dependent K+ channel antagonists, augmented the response. Glibenclamide, an antagonist of the ATP-sensitive K+ channel, had no effect on the rhythmic contractions. These results suggest that the rhythmic contractions observed in rabbit mesenteric arteries after ryanodine treatment were caused by the movement of Ca 2+ and K+ across the plasmalemma via the voltage-dependent Ca 2+ channel and the Ca 2+-activated K+ channel, respectively.     

电刺激脊髓损伤模型兔肛周皮肤调控神经源性膀胱内压

背景：以往研究表明通过不同频率电刺激阴部神经及其分支会抑制膀胱逼尿肌的过度活动,从而治疗神经源性膀胱,基于此,能否通过电刺激阴部神经分支来抑制逼尿肌过度活动呢？ 目的：探讨电刺激脊髓损伤兔肛周皮肤调控神经源性膀胱内压可行性。 方法：选择成年雌性兔10只,在T9-T10水平横断脊髓,饲养4周后开始进行实验,电刺激采用一对钩状电极刺激肛门周围的皮肤,在膀胱内插入双腔球囊导管注入生理盐水,测量膀胱内压力。 结果与结论：当膀胱灌注量高于排尿容积阈值时,以低于10 Hz的频率电刺激肛周,会显著抑制膀胱逼尿肌过度活动;当膀胱灌注量低于排尿容积阈值时,刺激频率在20-50 Hz时,会引起膀胱收缩。最佳抑制刺激    (6 Hz)能显著增加膀胱容积(30±10)%,最佳的兴奋刺激(25 Hz)能引起大幅度(超过25 cm H2O即24.5 kPa)、长时间(20 s)的膀胱收缩。结果表明电刺激脊髓损伤兔肛周皮肤可以有效调控神经源性膀胱压力。  中国组织工程研究杂志出版内容重点：肾移植;肝移植;移植;心脏移植;组织移植;皮肤移植;皮瓣移植;血管移植;器官移植;组织工程