Comparison of the effects of BRL 34915 and verapamil on electrical and mechanical activity in rat portal vein.

The effects of the novel anti-hypertensive agent BRL 34915, (+/-) 6-cyano-3,4-dihydro-2,2-dimethyl-trans-4-(2-oxo-1-pyrrolidyl)-2H-b enzo[b]pyran-3-ol, have been compared with those of verapamil on rat isolated portal vein. BRL 34915 produced a concentration-dependent reduction in mechanical responses to noradrenaline but had relatively little inhibitory effect on K+-induced contractions. Verapamil reduced the magnitude of both noradrenaline and K+-induced mechanical responses. BRL 34915 delayed the appearance of the reduced noradrenaline contractions, a property not shared by verapamil. BRL 34915 abolished spontaneous electrical and mechanical discharges and hyperpolarized the portal vein cells close to their calculated potassium equilibrium potential. Verapamil inhibited spontaneous electrical and mechanical discharges, effects associated with a small depolarization. BRL 34915 produced a significant increase in the 86Rb efflux rate coefficient whilst verapamil was without effect on this parameter. The inhibitory effects of BRL 34915 were rapid in onset and readily reversible by washing, whilst those of verapamil were slower in onset and only slowly reversible. It is concluded that the inhibitory effects of BRL 34915 in rat portal vein are produced by the opening of potassium channels in the smooth muscle cells. This inhibits spike activity and in sufficient concentration holds the membrane potential at or close to the potassium equilibrium potential, thereby reducing the effects of excitatory agents.     

Comparison of the in vitro effects of K+ channel modulators on detrusor and portal vein strips from guinea pigs.

The effects of K+ channel openers and blockers on smooth muscles of vascular and nonvascular origin from guinea pigs have been investigated. Cromakalim, pinacidil, nicorandil and minoxidil sulfate all abolished the spontaneous myogenic activity of the guinea pig portal vein and the KCl-evoked activity of detrusor strips with the same rank order of ptoency. Whereas both apamin and charybdotoxin stimulated myogenic activity of the detrusor strips, they produced insignificant effects on spontaneously active portal vein strips and failed to antagonize the mechanoinhibitory effects of cromakalim in the two tissues. Glibenclamide, on the other hand, only stimulated the myogenic activity of portal vein strips but antagonized the mechanoinhibitory effects of cromakalim, pinacidil, nicorandil and minoxidil sulfate in both tissues. Rubidium, at millimolar concentrations, stimulated the myogenic activity, and antagonized the actions of cromakalim in both tissues. The data indicate that there are definite functional dissimilarities as exhibited by the differential response of the two tissues to K+ channel modulators. These findings may be exploited in the design of new drugs with tissue selectivity.     

Effects of several potassium channel openers and glibenclamide on the uterus of the rat.

1. The ability of several potassium (K+) channel openers to inhibit spasm of the uterus of the nonpregnant rat and their susceptibility to antagonism by glibenclamide was assessed in vitro and in vivo. 2. In the isolated uterus exposed to oxytocin (0.2 nM), cromakalim, RP 49356 and pinacidil were of similar potency (mean pD2 = 6.4, 6.0 and 6.2 respectively) while minoxidil sulphate was of lower potency (pD2 = 4.7). Glibenclamide antagonized cromakalim and RP 49356 with the interactions consistent with competitive antagonism (mean pA2 of 6.57 and 7.00 respectively). Glibenclamide also antagonized pinacidil (pA2 = 6.22) but the slope of the Schild plot was significantly greater than -1. Neither salbutamol nor minoxidil sulphate was antagonized by glibenclamide (10 microM). 3. Cromakalim (1 and 10 microM), RP 49356 (1 and 10 microM), pinacidil (1 microM) and minoxidil sulphate (100 microM) suppressed spasm evoked by low (less than 40 mM) but not high (greater than or equal to 40 mM) KCl concentrations. Glibenclamide (10 microM) prevented cromakalim (10 microM)-, RP 49356 (10 microM)- and pinacidil (10 microM)-induced suppression of KCl (20 mM)-evoked spasm. Pinacidil (10 and 100 microM), cromakalim (100 microM) and salbutamol (0.01-1 microM) inhibited spasm evoked by all concentrations of KCl (10-80 mM). Suppression of spasm evoked by KCl (10-80 mM) by cromakalim (100 microM) and pinacidil (100 microM) was insensitive to glibenclamide (10 microM). 4. Cromakalim (0.1 mg kg-1) and RP 49356 (0.1 mg kg-1), given by i.v. bolus injection, inhibited uterine contractions, produced a fall in blood pressure and a slight tachycardia in the conscious ovariectomized rat.(ABSTRACT TRUNCATED AT 250 WORDS)     

Action of polygodial on agonist-induced contractions of the rat portal vein in vitro

This study investigated the vasorelaxant action of the sesquiterpene polygodial, isolated from the bark of Drymis winteri, on rat portal vein in vitro, contracted by various agonists. Polygodial (21-342 microM) preincubated 20 min before, produced graded antagonism of the contractile responses caused by bradykinin, endothelin-1, noradrenaline, the stable analogue of thromboxane A2 U46619, substance P, neurokinin B, and senktide (an NK3-selective agonist). Polygodial, at the same concentration, also produced graded inhibition of the contractile response induced by potassium chloride and by phorbol ester. At the median inhibitory concentration (IC50) level, polygodial was approximately 114- to 177-fold more active in inhibiting mediated contractions to senktide and phorbol ester. When assessed in the tonic contraction induced by endothelin-1 (0.5 nM) or by phorbol (3 microM), polygodial (0.1-100 microM) produced concentration-dependent relaxation, with maximal inhibition (E(max)) of 62 +/- 2% and 100%, respectively. Finally, polygodial (0.1-100 microM) inhibited the rhythmic spontaneous contractions of the rat portal vein (E(max) of 75 +/- 2%). Taken together, these results suggest that the vasorelaxant actions caused by polygodial in rat portal vein are, at least in part, associated with inhibition of calcium influx through voltage-sensitive channels and interaction with protein kinase C-dependent mechanisms. In addition, these data confirm and extend our previous suggestion that polygodial preferentially antagonizes tachykinin-mediated contraction, especially the NK3-mediated responses.     

Biphasic effects of NMDA on the motility of the rat portal vein

The effect of NMDA on the motility of the rat portal vein was studied in an isolated preparation. NMDA induced a concentration-dependent (10(-7) - 10(-4) M) increase of the contraction frequency (maximum increase, 148+/-6% of control at NMDA 10(-4) M). The NMDA-induced excitatory response was prevented by the competitive NMDA receptor antagonists (+/-)-2-Amino-5-phosphonopentanoic acid (AP-5, 5x10(-4) M) or (RS)-3-(2-carboxypiperazine-4-yl) propyl-1-phosphonic acid (CPP, 10(-4) M). Tetrodotoxin (TTX, 10(-6) M) or atropine (10(-4) M) abolished the NMDA-induced increase of the portal vein motility and reversed the excitatory effect to a concentration-dependent inhibition (maximum inhibition, 52+/-8 and 29+/-7% of controls, respectively, at NMDA 10(-3) M). Removal of the endothelium abolished the NMDA-induced inhibitory response. Sodium nitroprusside concentration-dependently (10(-7) - 10(-5) M) inhibited the portal vein motility, while L-N(G)-nitro-arginine methyl ester (L-NAME, 10(-4) M) reversed the inhibitory effect of NMDA (in the presence of TTX), restoring the portal vein spontaneous activity to control values. These results show that NMDA modulates the portal vein motility in a biphasic manner: via indirect activation, through prejunctional NMDA receptors presumably located on intrinsic excitatory neuronal afferences, or via direct inhibition, through endothelial NMDA receptors activating the nitric oxide pathway. Overall these findings support the hypothesis of the existence of a peripheral glutamatergic innervation modulating the contractile activity of the rat portal vein. British Journal of Pharmacology (2000) 129, 156 - 162     

Effects of 4-aminopyridine on mechanical activity and noradrenaline release in the rat portal vein in vitro.

4-Aminopyridine (4-AP) increased the spontaneous mechanical activity of the isolated rat portal vein. Since denervation and adrenergic receptor blockade failed to prevent this effect of 4-AP it is suggested that the drug enhances the electrical excitability of the muscle membrane. 4-AP significantly increased the response of the muscle to electrical nerve stimulation in most experiments but had little effect on the response to applied noradrenaline (NA). Both spontaneous and evoked release of 3H-activity, following preincubation in 3H-noradrenaline, were increased in the presence of 4-AP (10(-3) M). The present results with 4-AP can be explained by its known ability to block the transient potassium conductance which accompanies the action potential in excitable tissues.     

Orchidectomy enhances the effects of phenylephrine in rat isolated portal vein

1. Orchidectomy results in long‐term testosterone deprivation similar to that observed in male clinical pathologies, such as hypogonadism and age‐related reductions in plasma testosterone concentrations. Although the vascular effects of these sorts of hormone deprivations are known in arteries, they have not been studied to the same extent in veins. 2. The aim of the present study was to determine the effect of orchidectomy, with or without subsequent testosterone replacement (started 23 days after orchidectomy; 10 mg/kg, i.m., testosterone propionate once every 5 days for 3 weeks), on responses of rat isolated portal veins and vena cavae to exogenous phenylephrine (PE). Isolated vessels were mounted in an organ bath and concentration–response curves constructed to PE (10^?10–10^?4 mol/L), endothelin (ET; 10^?10–10^?5 mol/L) and KCl (10^?2–1.2 × 10^?1 mol/L; as a control). 3. Orchidectomy had no effect on contractile responses of either the portal vein or vena cava to KCl. However, orchidectomy enhanced the maximum response (R_max) of the portal vein, but not the vena cava, to PE. Testosterone replacement had no effect on these responses. The effects of orchidectomy on the R_max to PE in portal veins were not altered by the nitric oxide synthase inhibitor N^G‐nitro‐l‐ arginine methyl ester (10^?4 mol/L) alone or combined with 10^?5 mol/L indomethacin (a non‐selective cyclo‐oxygenase inhibitor), but they were abolished following treatment of isolated vessels with the ET_A and ET_B receptor antagonists BQ‐123 and BQ‐788 (both at 10^?6 mol/L). Orchidectomy did not alter portal vein responses to the application of exogenous ET. 4. The results of the present study indicate that orchidectomy‐induced decreases in plasma testosterone can increase the venoconstrictor effects of PE on the portal vein and that this effect involves activation of both ET_A and ET_B receptors by locally produced ET.     

Comparison of the effects of thapsigargin and BAY K 8644 on spontaneous mechanical activity in rat portal vein and contractile responses of rat cardiac muscle.

The effect of thapsigargin, 10(-9)-10(-6) M, and Bay K 8644, 10(-9)-10(-7) M, was studied on isolated portal veins and cardiac muscles from rats. In rat portal veins thapsigargin induced a concentration dependent increase in the amplitude of the spontaneous mechanical activity without increasing the frequency of spontaneous activity. Thapsigargin was less effective than Bay K 8644 in increasing the amplitude of the mechanical activity. In contrast to thapsigargin Bay K 8644, 10(-6) M increases the frequency of the mechanical activity. Atropine, 10(-6) M, and phentolamine, 10(-6) M, had no effect on the thapsigargin and Bay K 8644 induced increase in mechanical activity. Nitrendipine, 10(-6) M, totally abolished the mechanical response in preparations stimulated by thapsigargin and Bay K 8644. In rat atrial and papillary muscles Bay K 8644 increases the frequency in right atrium and tension in both atrial and papillary muscles. Thapsigargin was without effect on the frequency and tension in the cardial preparations. In conclusion, thapsigargin increases the amplitude of spontaneous activity in rat portal veins. In contrast to Bay K 8644 thapsigargin was less effective in increasing the amplitude and had no effect on the frequency of spontaneous activity; furthermore, thapsigargin was without effect on cardiac muscles. The results support the view that an endoplasmatic Ca2(+)-pump sensitive to thapsigargin is of importance for spontaneous activity in portal veins while such pump is of minor importance for contractile activity in cardiac muscles.     

Comparison of effects of a new dihydropyridine, Bay K 8644, and nifedipine on spontaneous mechanical activity in rat portal vein.

In isolated portal veins from rats, Bay K 8644 (methyl-1, 4-dihydro-2, 6-dimethyl-3-nitro-4 (2-trifluoromethyl-phenyl) pyridine-5-carboxylate) increased the spontaneous mechanical activity in low but not in high concentrations. The Bay K 8644-induced increase in spontaneous mechanical activity was abolished in Ca-free medium and restored by readdition of Ca. Nifedipine abolished the augmenting effect of Bay K 8644 on the spontaneous mechanical activity; this effect of nifedipine could be eliminated by further increasing the concentration of Bay K 8644. The results are consistent with the conclusion that in rat portal vein, Bay K 8644 increases the entry of extracellular Ca by a mechanism antagonistic to that of nifedipine and in high concentration has a Ca-entry blocking effect.     

Effects of buphenin on the rat portal vein

The vasodilator and tocolytic substance buphenin (10 mumol/l) stimulated the spontaneous phasic activity of some (8 out of 18) isolated rat portal vein preparations; 0,1-1 mmol/l buphenin diminished or abolished the activity in all preparations. The isotonic and isometric tonic contractions of portal vein in response to adrenaline, noradrenaline and phenylephrine (0,1-1 mumol/l) disappeared almost completely after addition of buphenin in equimolar concentrations, whereas acetylcholine contractions persisted. The beta-adrenergic blocking agents propranolol and dichloisoprenaline (10 mumol/l) only slightly antagonized the inhibitory effect of buphenin on the contractile responses to catecholamines. It is concluded that buphenin exerts dual action upon rat portal vein: the drug partially stimulates the beta-receptors and partially blocks the alpha-adrenergic receptors.     

Interactions between apelin and angiotensin II on rat portal vein.

Apelin (AP), an endogenous ligand of the APJ receptor, is involved in the regulation of cardiovascular homeostasis. Regardless the multiple similarities between AP and angiotensin II (Ang II), their roles seem to be different. We studied both the interactions between Apelin 13 (AP13) and Ang II and to what extent, if any, nitric oxide (NO) is involved. The experiments were performed in endothelium-denuded or endothelium-intact rat portal vein in the presence of 10 microM N(G)-nitro L-arginine methyl ester or 10 microM aminoguanidine. AP13 did not modify the isolated rat portal vein tone by itself, but inhibited the Ang II-induced contractions acting mainly by a NO-dependent mechanism. Our results sustain the hypothesis that AP13 could increase the activity of both constitutive and inducible NO synthase on either endothelium intact or endothelium denuded rat portal vein rings.     

Comparison of the effects of fenamates on Ca-activated chloride and potassium currents in rabbit portal vein smooth muscle cells.

1. The perforated patch and conventional whole-cell recording techniques were used to study the action of flufenamic, mefenamic and niflumic acid on calcium-activated chloride and potassium currents in rabbit portal vein smooth muscle cells. 2. In K-conditions at a holding potential of -77 mV flufenamic acid and mefenamic acid decreased the amplitude of spontaneous transient inward currents (STICs, calcium-activated chloride currents, ICl(Ca)) in a concentration-dependent manner. The potency sequence was niflumic > flufenamic > mefenamic acid. 3. At -77 mV 1 x 10(-5) M flufenamic acid increased the STIC exponential decay time constant (tau). At higher concentrations the STIC decay was described by 2 exponentials with an initial decay (tau f) faster than the control tau value and a second exponential (tau s) which had a time constant slower than the control tau value. Low concentrations of mefenamic acid had no effect or decreased the tau value whereas in higher concentrations biphasic currents were recorded. 4. In K-free conditions the inhibitory effect of both flufenamic and mefenamic acid on STIC amplitude was greater at +50 mV compared to -50 mV, showing that the effect of these agents was voltage-dependent. 5. In cells held at 0 mV in K-containing conditions the fenamates reduced both the frequency and amplitude of spontaneous transient outward currents (STOCs, calcium-activated potassium currents, IK(Ca)). The concentration range to produce these effects was higher than that to decrease STIC amplitude and the potency sequence was flufenamic > niflumic > or = mefenamic acid. 6. All these compounds in concentrations greater than 5 x 10(-5) M evoked a 'noisy' potassium current at 0 mV which reached a maximum after approximately 3 min. This current was readily reversible on washout of the drug and could be elicited several times in the same cell. The current-voltage relationship of the fenamate-evoked current exhibited pronounced outward rectification characteristic of IK(Ca). 7. The current evoked by 2 x 10(-4) M flufenamic acid and 5 x 10(-4) M niflumic acid was not affected by 1 x 10(-5) M glibenclamide but was markedly inhibited by 1 x 10(-3) M tetraethylammonium. Furthermore, large currents were activated by flufenamic and niflumic acid in the presence of caffeine and cyclopiazonic acid (an inhibitor of the sarcoplasmic reticulum Ca-ATPase) to deplete intracellular Ca-stores. 8. Conventional whole-cell recording was performed with pipette solutions in which the ability to buffer changes in intracellular calcium was varied by altering the concentration of the calcium chelator (2-aminophenoxy)-ethane-N,N,N',N'-tetraacetic acid (BAPTA). Flufenamic acid (2 x 10(-4) M) and niflumic acid (5 x 10(-4) M) both evoked large outward currents when recordings were made with either 1 x 10(-4) M or 1 x 10(-2) M BAPTA. Furthermore, bathing the cells in nominally calcium-free extracellular solution did not reduce the amplitude of the evoked currents. 9. It is concluded that both flufenamic and mefenamic acid inhibit ICl(Ca) by a mechanism similar to niflumic acid, possibly open channel blockade. Furthermore, at concentrations greater than 5 x 10(-5) M all three fenamates inhibited STOC activity and evoked directly an outward current which resembled IK(Ca).     

Rabbit versus rat urinary bladder: effects of in vitro hypoxia.

PURPOSE: Studies indicate that bladder hypoxia may be an etiological factor for lower urinary tract dysfunction. Rat and rabbit are two species of experimental animals used frequently to study lower urinary tract function and dysfunction. The objective of this study was to compare directly effects of in vitro hypoxia on contractile responses of rat and rabbit urinary bladder to different forms of stimulation. METHODS: Sexually mature male New Zealand White rabbits and Sprague-Dawley rats were compared. Each bladder was excised while the animal was anesthetized, and longitudinal bladder strips were cut, then mounted in organ baths. A tension of 2 g was placed on all strips. Effects of 1, 2, 3 and 4 h hypoxia followed by 1 h of reoxygenation on contractile responses of bladder strips to field stimulation (FS), carbachol (100 micromol/l), ATP (1 mmol/l) and KCl (120 mmol/l) were determined. RESULTS: Contractility, per unit tissue mass, of rat bladder strips was significantly greater than that of rabbit bladder strips in response to FS (all frequencies), carbachol, KCl and ATP. Hypoxia (followed by reoxygenation) resulted in time-dependent progressive reduction in contractile responses of bladder strips to all stimuli. Rat bladder was significantly more sensitive to hypoxia than rabbit bladder in response to FS and carbachol. Hypoxia induced similar effects on rat and rabbit bladder responses to ATP and KCl. CONCLUSION: Rat bladder neurogenic and cholinergic responses are significantly more sensitive to hypoxia than are those of rabbit bladder, which may be due to the rat bladder's greater contractile force generation and previously reported higher Ca2+-ATPase activity.     

Ammonium ion increases the tone of rat portal vein.

1. The effect of ammonium ion on vascular tone was investigated using the portal vein isolated from rat. 2. Ammonium chloride at 10-90 mM induced a contractile response. 3. Spontaneous twitch contraction of portal strips was augmented by ammonium chloride at 10-60 mM. 4. Ammonium chloride-induced contraction was abolished in calcium-free solution or in the presence of 1 microM nifedipine. 5. Methylamine (60 mM) also induced a contractile response and augmented the spontaneous twitch contraction in rat portal vein. 6. After withdrawal of ammonium chloride or methylamine from the organ bath solution, the spontaneous twitch contraction was strongly inhibited. 7. These results suggest that ammonium compounds increase vascular tone by causing influx of extracellular calcium through the voltage-dependent calcium channel and intracellular alkalinization is involved in this process.     

The pharmacokinetics of pyridostigmine and 3-hydroxy-N-methylpyridinium in the rat: dose-dependent effects after portal vein administration.

1 The elimination kinectis of [14C]-pyridostigmine iodine and [14-C-methyl]-3-hydroxypyridinium bromide (3-OH NMP) have been studied in the rat. 2 For pyridostigmine, at a given dose level, the fraction of the dose eliminated unchanged was reduced and the metabolite fraction was increased after portal vein administration when compared to jugular vein administration. This indicates that pyridostigmine is subject to metabolism during the first passage through the liver. 3 When doses of pyridostigmine 1.25 mumol/kg and higher were injected via the portal vein, the proportion excreted in urine as unchanged drug remained constant; in contrast, the percentage of the dose eliminated as the metabolite was significantly reduced. This indicates that a dose-dependent process is involved in the urinary excretion of 3-OH NMP. 4 This conclusion was supported by studies involving the portal and systemic venous injection of 3-OH NMP at different dose levels. After 4 h, approximately85% of the lowest dose was eliminated unchanged in ug this period. The proportion of the dose eliminated in urine was not related to the route of administration. 5 After the injection of pyridostigmine into the jugular vein, the initial rate of drug excretion fell rapidly for approximately 10 min; in contrast, after injection into the portal vein, the rate of excretion of the drug rose to a maximum at 30 minutes. This suggests that the hepatoportal system behaves as a distinct region during the distribution of this drug.     

Some effects of sodium nitroprusside, methoxyverapamil (D600) and nifedipine on rat portal vein.

1 The effects of methoxyverapamil (D600), nifedipine and sodium nitroprusside on noradrenaline-induced electrical and mechanical activity in rat portal vein have been examined. 2 D600 and nifedipine produced a concentration-dependent reduction in mechanical responses to noradrenaline whilst sodium nitroprusside had no effect. The effects of D600 and nifedipine were reversed by increasing the extracellular calcium concentration. 3 The mechano-inhibitory effects of D600 were accompanied by a marked reduction in electrical activity with some evidence of electromechanical uncoupling. 4 The mechano-inhibitory effects of nifedipine were associated with considerable electromechanical uncoupling. 5 It is concluded that in the concentrations used, D600 acts primarily by inhibiting calcium influx with some effect on electromechanical coupling whilst nifedipine interferes mainly with the coupling process. The inactivity of sodium nitroprusside suggests that the excitation-contraction coupling process in rat portal vein is relatively simple and further studies with this tissue seem indicated.     

Effect of frusemide on the reactivity of rat portal vein

Short lengths of rat portal vein were suspended in Krebs bicarbonate solution. Contractile responses to noradrenaline and angiotensin II and the parameters of spontaneous contractions were determined before and after addition of frusemide at 10–⁶ and 10–⁴ g/ml. Frusemide at 10–⁶ g/ml slightly suppressed the amplitude of spontaneous contractions and slightly decreased the responses to noradrenaline and angiotensin II. At 10–⁴ g/ml the drug had a much greater inhibitory effect on endogenous and exogenous stimulation. The concentrations used appear to be similar to those achieved during effective diuretic and hypotensive treatment. It is postulated that frusemide may have a direct inhibitory action on the reactivity of vascular smooth muscle mrticularlv in the capacitance vessels.     

Effects of felodipine on energy turnover in the rat portal vein

The effects of the vasodilating dihydropyridine, felodipine, on tissue concentrations of high-energy phosphates and on oxygen consumption and lactate production in the smooth muscle of the rat portal vein were investigated. Felodipine (100 nM) caused a gradual decrease in the amplitude of the spontaneous phasic contractions in a calcium-containing medium. The mean active force was reduced by about 80% within 15 min. The inhibition of force was associated with reductions in both oxygen consumption and lactate production. No effects of felodipine could be observed in a calcium-free solution. The metabolic rates and force during felodipine inhibition approached those recorded in the calcium-free media. Felodipine (30 nM) did not alter the tissue levels of ATP, ADP, AMP and phosphocreatine. Relaxation by felodipine is thus associated with a decreased energy demand for contraction and, possibly, ionic translocation. The reduced ATP hydrolysis is compensated for by the regeneration of metabolic ATP, thus keeping the cellular levels of high-energy phosphates constant.