Effects of nifedipine derivatives on smooth muscle cells and neuromuscular transmission in the rabbit mesenteric artery

Nimodipine inhibits intestinal and aortic smooth muscle contraction by regulating Ca2+-activated Cl- channels

Nimodipine is a clinically used dihydropyridine L-type calcium channel antagonist that effectively inhibits transmembrane Ca2+ influx following the depolarization of smooth muscle cells, but the detailed effect on smooth muscle contraction is not fully understood. Ca2+-activated Cl- channels (CaCCs) in vascular smooth muscle cells (VSMCs) may regulate vascular contractility. We found that nimodipine can inhibit transmembrane protein 16A (TMEM16A) activity in a concentration-dependent manner by cell-based fluorescence-quenching assay and short-circuit current analysis, with an IC50 value of ~5 μM. Short-circuit current analysis also showed that nimodipine prevented Ca2+-activated Cl- current in both HT-29 cells and mouse colonic epithelia accompanied by significantly decreased cytoplasmic Ca2+ concentrations. In the absence of extracellular Ca2+, nimodipine still exhibited an inhibitory effect on TMEM16A/CaCCs. Additionally, the application of nimodipine to CFTR-expressing FRT cells and mouse colonic mucosa resulted in mild activation of CFTR-mediated Cl- currents. Nimodipine inhibited basolateral CCh-activated K+ channel activity with no effect on Na+/K+-ATPase activity. Evaluation of intestinal smooth muscle contraction showed that nimodipine inhibits intestinal smooth muscle contractility and frequency, with an activity pattern that was similar to that of non-specific inhibitors of CaCCs. In aortic smooth muscle, the expression of TMEM16A in thoracic aorta is higher than that in abdominal aorta, corresponding to stronger maximum contractility in thoracic aorta smooth muscle stimulated by phenylephrine (PE) and Eact. Nimodipine completely inhibited the contraction of aortic smooth muscle stimulated by Eact, and partially inhibited the contraction stimulated by PE. In summary, the results indicate that nimodipine effectively inhibits TMEM16A/CaCCs by reduction transmembrane Ca2+ influx and directly interacting with TMEM16A, explaining the mechanisms of nimodipine relaxation of intestinal and aortic smooth muscle contraction and providing new targets for pharmacological applications.

TTX-sensitive voltage-gated Na+ channels are expressed in mesenteric artery smooth muscle cells

The presence and properties of voltage-gated Na+ channels in mesenteric artery smooth muscle cells (SMCs) were studied using whole cell patch-clamp recording. SMCs from mouse and rat mesenteric arteries were enzymatically dissociated using two dissociation protocols with different enzyme combinations. Na+ and Ca2+ channel currents were present in myocytes isolated with collagenase and elastase. In contrast, Na+ currents were not detected, but Ca2+ currents were present in cells isolated with papain and collagenase. Ca2+ currents were blocked by nifedipine. The Na+ current was insensitive to nifedipine, sensitive to changes in the extracellular Na+ concentration, and blocked by tetrodotoxin with an IC50 at 4.3 nM. The Na+ conductance was half maximally activated at -16 mV, and steady-state inactivation was half-maximal at -53 mV. These values are similar to those reported in various SMC types. In the presence of 1 microM batrachotoxin, the Na+ conductance-voltage relationship was shifted by 27 mV in the hyperpolarizing direction, inactivation was almost completely eliminated, and the deactivation rate was decreased. The present study indicates that TTX-sensitive, voltage-gated Na+ channels are present in SMCs from the rat and mouse mesenteric artery. The presence of these channels in freshly isolated SMC depends critically on the enzymatic dissociation conditions. This could resolve controversy about the presence of Na+ channels in arterial smooth muscle.

Effects of calcium channel blockers on rabbit corneal endothelial function

The effects of calcium channel antagonists and agents that alter intracellular Ca2+ mobilization on corneal endothelial function have been examined. All experiments, except where specifically designated, were performed in the continuous presence of extracellular Ca2+. Verapamil (at 50 microM) increased the swelling rate of corneas bathed in normal Ringer solution whereas nifedipine and diltiazem (both up to 100 microM) were without effect. The nifedipine analog nisoldipine caused corneal swelling at 10 microM and 50 microM but nimodipine was without effect. When briefly exposed to a Ca(2+)-free solution corneal swelling was enhanced after subsequent exposure to 50 microM verapamil in normal Ringer but not after 50 microM diltiazem in normal Ringer, indicating that Ca2+ entry from the bathing solution into the cell was important and was apparently impeded by verapamil. Cadmium (0.6 and 1 mM) but not nickel (up to 250 microM) caused swelling of corneas bathed in normal Ringer. A Ca2+ channel agonist, BAY-K-8644, alone did not influence corneal thickness but when presented to the endothelium with 50 microM verapamil the swelling rate was much reduced compared to verapamil alone. The agonist, therefore, presumably maintained some Ca2+ channels open in face of the Ca2+ channel blocker. An agent that inhibited the release of intracellular Ca2+ stores (TMB-8) caused an initial corneal swelling over the first 1.5 hr of perfusion but thereafter had no effect on corneal thickness. In the presence of continued extracellular Ca2+ one explanation for the results is that modulation of intracellular Ca2+ by agents that alter plasma membrane transfer of Ca2+ influences apical junction permeability.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of systemic anaphylaxis on the hepatic drug-metabolizing system in rats

Rats were immunized by intraperitoneal injection of ovalbumin emulsified with Freund incomplete adjuvant, and then the effect of an intravenous challenge with ovalbumin on hepatic drug-metabolizing enzyme activities was examined. The cytochrome P-450 content and ethylmorphine N-demethylase, benzphetamine N-demethylase, arylhydrocarbon hydroxylase and 7-ethoxycoumarin O-deethylase activities significantly decreased in rats treated with ovalbumin compared with control groups treated with saline, whereas there was no significant reduction in cytochrome b5, NADPH-cytochrome c reductase and NADH-cytochrome c reductase.

The role of endothelium in the phenylephrine-induced oscillatory responses of rabbit mesenteric arteries

Phenylephrine-induced oscillatory contractions in rabbit mesenteric arteries were investigated in vitro. Adrenergic, cholinergic, or histamine antagonists as well as cyclooxygenase and lipoxygenase inhibitors had no effect on this phenylephrine-induced oscillation. The removal of extracellular calcium ions or treatment with a calcium antagonist reduced the amplitude and frequency of the oscillation. Removal of the endothelium or treatment with inhibitors of the synthesis or the target enzyme of endothelium-derived relaxing factor (EDRF) also reduced the amplitude and frequency of the oscillation. In a perfusion bioassay, the perfusate from an endothelium-intact arterial segment induced oscillation of an endothelium-denuded arterial ring recipient. These results suggest that phenylephrine-induced oscillation is mediated by an endothelium-derived factor such as EDRF and depends on the influx of extracellular calcium ions.

Membrane hyperpolarization inhibits agonist-induced synthesis of inositol 1,4,5-trisphosphate in rabbit mesenteric artery

1. Effects of membrane hyperpolarization induced by pinacidil on Ca2+ mobilization induced by noradrenaline (NA) were investigated by measuring intracellular Ca2+ concentration ([Ca2+]i), isometric tension, membrane potential and production of inositol 1,4,5-trisphosphate (IP3) in smooth muscle cells of the rabbit mesenteric artery. 2. Pinacidil (0.1-10 microM) concentration dependently hyperpolarized the smooth muscle membrane with a reduction in membrane resistance. Glibenclamide (1 microM) blocked the membrane hyperpolarization induced by 1 microM-pinacidil. NA (10 microM) depolarized the smooth muscle membrane with associated oscillations. Pinacidil (1 microM) inhibited this response and glibenclamide (1 microM) prevented the action of pinacidil on both the NA-induced events. 3. In thin smooth muscle strips, 10 microM-NA produced a large phasic and a subsequent small tonic increase in [Ca2+]i with associated oscillations. These changes in [Ca2+]i seemed to be coincident with phasic, tonic and oscillatory contractions, respectively. Pinacidil (0.1-1 microM) inhibited the increases in [Ca2+]i and in tension induced by NA, but not by 128 mM-K+. Glibenclamide inhibited these actions of pinacidil. Pinacidil (1 microM) also inhibited the contraction induced by 10 microM-NA in strips treated with A23187 (which functionally removes cellular Ca2+ storage sites), suggesting that membrane hyperpolarization inhibits Ca2+ influxes activated by NA. 4. In Ca2(+)-free solution containing 2 mM-EGTA, NA (10 microM) transiently increased [Ca2+]i, tension and synthesis of IP3. Pinacidil (over 0.1 microM) inhibited the increases in [Ca2+]i, tension and synthesis of IP3 induced by 10 microM-NA in Ca2(+)-free solution containing 5.9 mM-K+, but not in a similar solution containing 40 or 128 mM-K+. Glibenclamide (1 microM) inhibited these actions of pinacidil. These inhibitory actions of pinacidil were still observed in solutions containing low Na+ or low Cl-. These results suggest that pinacidil inhibits NA-induced Ca2+ release from storage sites through an inhibition of IP3 synthesis resulting from its membrane hyperpolarizing action. 5. In beta-escin-treated skinned strips, NA (10 microM) or IP3 (20 microM) increased Ca2+ in Ca2(+)-free solution containing 50 microM-EGTA and 3 microM-guanosine triphosphate (GTP) after brief application of 0.3 microM-Ca2+, suggesting Ca2+ is released from intracellular storage sites. Heparin (500 micrograms/ml, an inhibitor of the IP3 receptor), but not pinacidil (1 microM) or glibenclamide (1 microM), inhibited the Ca2+ release from storage sites induced by NA or IP3. These results suggest that membrane hyperpolarization is essential for the inhibitory action of pinacidil on the NA-induced Ca2(+)-releasing mechanism.(ABSTRACT TRUNCATED AT 400 WORDS)

Actions of FRC-8653 on smooth muscle cells of the rabbit mesenteric artery

In ring preparations of the rabbit mesenteric artery, the Ca-contraction, but not the noradrenaline (NA)-contraction, was inhibited by FRC-8653 (10(-9)-10(-5) M) in a concentration-dependent manner, although with a potency 50-100 times weaker than that of nicardipine. The actions of FRC-8653 on Ca-contraction appeared more slowly (over 1 hr) than those of nicardipine. FRC-8653 (up to 10(-5) M) and nicardipine (up to 10(-7) M) did not change the resting membrane potential of smooth muscles. The amplitude of the evoked excitatory junction potential (e.j.p.) was inhibited by FRC-8653, but not by nicardipine, with no alteration in the facilitation process of the e.j.p.s. The inhibition by FRC-8653 of the e.j.p. appeared rapidly and was reversible. FRC-8653 inhibited the membrane depolarization of smooth muscles elicited by ATP, but not by NA or high [K+]o solution. ATP-induced contractions were also inhibited by FRC-8653. The amplitudes of action potentials evoked by current stimuli were decreased by FRC-8653, with no significant change in the electrotonic potentials. Thus, FRC-8653 has properties similar to those of the dihydropyridine Ca-antagonists, but differs from them in that 1) its inhibitory actions on Ca influx appear slowly and 2) sympathetic transmission is inhibited, possibly by inhibition of the postjunctional events for e.j.p. generation.

Pharmacological properties of voltage-dependent calcium channels in functional microvessels isolated from rat brain

Voltage-operated calcium channels were studied in rat intracerebral microvessels. The contractile reactivity to KCl-depolarization was assessed by the measurement of internal diameter of superfused microvessels. Dihydropyridine receptor sites associated with calcium channels were identified and characterized using 3H(+)PN 200-110 [isopropyl-4-(2,1,3-benzodiazol-4-yl)-1,4-dihydro-2,6-dimethyl-5- -methoxycarbonyl-pyridine-3-carboxylate]. Depolarization induced by high-KCl solution produced a marked reduction of the internal diameter of cerebral microvessels which was associated with the appearance of rhythmic activity. The vessel contraction was reversible and abolished by nimodipine. Binding studies with 3H(+)PN 200-110 revealed the existence of a single class of specific, stereoselective and voltage-dependent binding sites which bound (+)PN 200-110 with a KD of 88 +/- 6.6 pmol l-1 at 37 degrees C in microvessels incubated in NaCl medium. When microvessels were incubated in KCl-medium, the apparent KD value was reduced to 35 +/- 2 pmol l-1. Bmax was not significantly changed. The effect of KCl was not related to concomitant changes in the Na concentration. The potency of various dihydropyridine derivatives in inhibiting 3H(+)PN 200-110 binding was in agreement with their pharmacological potency in smooth muscle preparations. The effect of PN 200-110 and of nimodipine was stereoselective. Ki values of PN 200-110 and of nimodipine were increased in depolarized preparations, while nifedipine's potency was unchanged. Verapamil was only a partial inhibitor of 3H(+)PN 200-110 binding. The effect of diltiazem was stereoselective: the (+)-cis isomer enhanced the binding and the (-)-cis isomer of diltiazem poorly inhibited the binding of PN 200-110. Results showed that isolated cerebral microvessels possess functional voltage-operated calcium channels, which contain potential-modulated receptors for dihydropyridine calcium entry blockers with characteristics similar to those described in other tissues.

Dihydropyridine inhibition of single calcium channels and contraction in rabbit mesenteric artery depends on voltage

1. The effects of membrane potential and the dihydropyridine calcium channel inhibitor, nisoldipine, on single calcium channels in the presence of Bay K 8644 and contraction in the presence and absence of Bay K 8644 were examined in the rabbit mesenteric artery. 2. Membrane depolarization decreased the peak average single calcium channel current that could be elicited by a test pulse to 0 mV. The steady-state inactivation relationship could be described by the Boltzmann equation, [1 + exp[Vm-V0.5)/k)]-1, with a steepness factor, k, of 7.1 mV. Nisoldipine shifted the steady-state inactivation curve to more negative potentials by increasing the fraction of test pulses without openings. 3. The degree of nisoldipine inhibition of average single calcium channel currents increased with membrane depolarization. Depolarization of the holding potential from -100 to -55 mV decreased the concentration of nisoldipine needed for 50% inhibition (Kapp) from 12.1 to 1.9 nM in the presence of 1 microM-Bay K 8644. 4. Membrane depolarization by external potassium (K+) of the intact artery in the presence of nisoldipine decreased contractions evoked by depolarizing test pulses. The relationship between membrane potential and contraction could be empirically described by the Boltzmann equation, with a steepness factor, k, of 7.1 mV. Increasing the nisoldipine concentration from 0.25 to 2.0 nM shifted the mid-point of this relationship from -20.5 to -33.0 mV, without affecting the steepness factor. 5. Nisoldipine inhibition of contraction increased with membrane depolarization. Membrane depolarization from -68.6 to -30.0 mV decreased the Kapp of nisoldipine for contractions from 3.02 to 0.69 nM. Bay K 8644 (1 microM) elevated Kapp about 9.3-fold at 5 mM-K+. In the presence of Bay K 8644, membrane depolarization from -68.6 to -30.0 mV reduced Kapp from 28.4 to 4.0 nM. 6. In the presence of nisoldipine, the effect of membrane depolarization on the time course of development of inhibition was examined. In 3 nM-nisoldipine, after membrane depolarization with 20 mM-K+, the time course of development of inhibition of force could be described by a single exponential with a time constant of 16.5 min. Membrane depolarization to a more positive potential accelerated the development of inhibition. 7. The results were interpreted by a model in which nisoldipine binds with higher affinity to the inactivated state than to the resting state of calcium channels in the mesenteric artery. The approach presented here can be used to estimate the properties of steady-state calcium channel inactivation and dihydropyridine interactions in smooth muscle cells in the intact artery under physiological conditions.(ABSTRACT TRUNCATED AT 400 WORDS)

Two alpha-adrenergic contractions with different Ca2+ activation mechanisms in response to field nerve stimulation in the circular smooth muscle of guinea-pig vas deferens

1. Two noradrenergic components of contraction were induced by field nerve stimulation of the circular smooth muscle of guinea-pig vas deferens. One component occurred during the stimulation (former-response) and the other component occurred after stimulation had ceased (after-response), with a distinct tension fall between these two responses. 2. The effects of stimulation frequency and duration upon these components were examined. 3. Prazosin (0.05-0.5 microM) suppressed both responses, whereas nifedipine (0.02 microM) suppressed only the after-response. 4. It is suggested that in this tissue there are two neurogenic alpha-adrenergic contractions which possess different Ca2+ activation mechanisms. The former-response is due to an increase in cytoplasmic Ca2+ concentration through a pathway independent of action potentials, while the after-response is attributed to action potentials arising as a secondary response to the released noradrenaline.

Role of sarcoplasmic reticulum in arterial contraction: comparison of ryanodines's effect in a conduit and a muscular artery

Ryanodine interferes with sarcoplasmic reticulum function in various types of muscle; in vascular smooth muscle, it can inhibit contractions that depend on sarcoplasmic reticulum calcium release, probably by depleting the sarcoplasmic reticulum calcium store. We tested ryanodine and calcium channel blockers (verapamil, diltiazem, and nitrendipine) on small rings of rat thoracic aorta (RA) and bovine tail artery (BTA) to determine the relative contributions of sarcoplasmic reticulum calcium release and gated calcium entry to contractions induced by norepinephrine, caffeine, and 100 mM K depolarization. Ryanodine blocked caffeine contractions in both tissues and attenuated norepinephrine responses (by 52% in RA, 14% in BTA) but minimally altered potassium contractions. Calcium channel blockers almost completely abolished potassium contractions and reduced norepinephrine contractions (by 45% in RA, 82% in BTA) but hardly affected caffeine responses. The blocking effects of ryanodine and calcium channel antagonists on the norepinephrine responses were additive. Ryanodine had no effect on baseline tension in the standard media; however, when calcium extrusion via Na-Ca exchange was inhibited by low external sodium (0-calcium, low-sodium solution), tension increased progressively after introduction of ryanodine. This indicates that the sarcoplasmic reticulum calcium released by ryanodine then accumulated in the cytosol and activated contraction; restoration of external sodium caused prompt relaxation. The smaller effects of caffeine and ryanodine in BTA indicate that sarcoplasmic reticulum plays a less important role in calcium control in this tissue, with gated calcium entry dominating. These functional findings are correlated with electron-microscopic evidence that BTA has about 60% less sarcoplasmic reticulum than does RA. Ryanodine appears to be a useful tool for determining the functional relevance of sarcoplasmic reticulum for contraction in different arterial smooth muscles.

Characterization of [3H]nifedipine binding to intact vascular smooth muscle cells

Specific binding of the dihydropyridine Ca2+ antagonist [3H]nifedipine to dispersed smooth muscle cells of the porcine coronary artery was investigated and the findings were compared with the binding to microsomes of smooth muscles. Specific binding to intact cells was saturable and reversible. The dissociation constant was 1.93 +/- 0.42 nM and the maximal binding capacity was 59.6 +/- 12.4 fmol/10(6) cells, as assessed by Scatchard analysis of the equilibrium binding at 25 degrees C. The Kd value with intact cells was slightly higher than that observed with microsomes. Specific binding of [3H]nifedipine to intact cells was completely displaced by unlabeled dihydropyridine derivatives. Among other Ca2+ antagonists, verapamil and d-cis-diltiazem partially and flunarizine completely inhibited the binding. In the case of microsomes, d-cis-diltiazem stimulated the binding of [3H]nifedipine. These results suggest that there may be multiple binding sites for different subclasses of Ca2+ antagonists. Polyvalent cations had no effect on the binding to intact cells. In the case of ethylene glycol-bis(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid (EGTA)-treated microsomes, the addition of CaCl2 and BaCl2 increased the Bmax, but the Kd value remained unchanged. MnCl2 and CdCl2 had stimulatory or inhibitory effects, depending on the concentrations, whereas LaCl3 had no effect. The effect of membrane depolarization on the binding was also examined. When the intact cells were incubated in high [K+]o solution for 60 min, the Kd was lowered to 1.4 nM from the control value of 2.0 nM, thereby indicating that [3H]nifedipine binds to Ca2+ channels, with a higher affinity, at depolarized states.

The pharmacology of nisoldipine

Nisoldipine is a calcium antagonist that specifically blocks the slow or voltage-dependent calcium channel up to the highest concentrations. This mode of action has been confirmed in pharmacological studies on isolated organs, electrophysiological and binding studies, and by the measurement of transmembrane calcium transport. As with other dihydropyridine calcium antagonists, an interaction with intracellular calcium reservoirs and calmodulin seems to be of minor importance. The drug exhibits higher potency, longer duration of action, and a higher binding affinity in vitro and in vivo than nifedipine. In contrast to its vasodilating and spasmolytic activity, its negative inotropic effect occurs in vitro only after higher concentrations than after nifedipine. In whole animals a secondary positive inotropic effect occurs regularly owing to sympathetic counter-regulation. The influence of nisoldipine on cardiac stimulus formation and conduction is also very slight in anesthetized animals, and is completely eliminated in awake animals and humans by counter-regulation up to very high doses. The cardiac anti-ischemic action of nisoldipine has been demonstrated in various ischemia models and is probably based predominantly on its afterload-reducing properties in addition to its spasmolytic effect on the coronary arteries. Various other suspected effects, for which there are isolated indications, e.g., inhibition of thromboxane synthesis, preload reduction, interaction with the transport of adenosine, and normalization of the sarcolemmal Na+, K(+)-ATPase activity, are probably of subordinate importance. Its antihypertensive effect is explained primarily by lowering of the peripheral resistance. There are, however, some indications that nisoldipine exerts certain effects over and above pure vasodilation. The prevention of postischemic calcium overloading in the renal tubule epithelium and the natriuretic effect are probably of importance in the therapeutic action. Clinically, nisoldipine was found more potent and prolonged in its action in comparison with nifedipine. In comparative studies, nisoldipine, 10 mg once a day, was found equieffective with nifedipine 10 mg three times or 20 mg twice a day in angina or hypertension, respectively.

Contractile response and electrophysiological properties in enzymatically dispersed smooth muscle cells of rat vas deferens

Electrophysiological studies were performed on single smooth muscle cells isolated from the vas deferens of the rat. The tissue was preincubated in Ca-free modified Tyrode's solution for 1 h and then transferred to a high-K solution for 1 h. It was next minced and treated with the enzyme solution composed of 600-800 unit/ml collagenase and 40 unit/ml elastase. The procedure yielded about 50% spindle shaped Ca-tolerant cells (100-250 microns in length and about 10 microns in diameter). These cells could contract during the superfusion with the solutions containing 10(-8) to 10(-3) M norepinephrine (NE) or adenosine triphosphate (ATP). The cells isolated from the epididymal portion were more sensitive to norepinephrine than were those from the prostatic part. Their basic electrical properties were studied using tight-seal suction electrode technique. The cells had resting potentials around -40 mV and their input resistance was about 0.8 G omega. Action potentials could be evoked by application of depolarizing current. During whole cell voltage clamp, an inward current followed by an outward current was recorded when 800 ms pulses from a holding potential of -60 mV to test potentials positive than -40 mV were applied. The transient outward current generally recorded in other smooth muscle cells was not seen in these cells. The amplitude of the inward current was Ca dependent and sensitive to a Ca antagonist, nicardipine, indicating that Ca ion is the main carrier of this component of the current. When the pipette was filled with Cs-containing solution, the outward current was abolished.(ABSTRACT TRUNCATED AT 250 WORDS)

Evidence that intraluminal pressure affects high potassium- and serotonin-induced contractions differently in the bovine middle cerebral artery: an in vitro study

The effects of changing intraluminal pressure on contractions induced by 70 mM potassium (K+) and 10(-7), 10(-6), and 10(-5) M serotonin (5-HT) were studied in vitro in bovine middle cerebral arteries. Changes in vessel outside diameter in whole-mounted cylindrical sections of artery were detected with a photoelectric infrared device. High K+-or 5-HT (10(-5)M)-induced contractions peaked at 25 mm Hg and were significantly correlated with increasing intraluminal pressure between 25 and 175 mm Hg. Contractions induced with lower concentrations of 5-HT (10(-6), 10(-7) M), norepinephrine, and histamine peaked at 75 mm Hg but were not significantly correlated with rising pressure. Phentolamine (2 X 10(-6) M) added to the extraluminal bath had negligible influence on pressure's ability to affect K+- and 5-HT-induced contractions differently. Reducing bath temperature to 27 degrees C reduced the K+ response at each pressure, but similar temperature changes had little affect on the 5-HT-induced contractions. The K+ response became less sensitive to increasing pressure at low temperatures. Nifedipine (10(-7) M) almost totally eliminated K+-induced contractions, while significantly reducing the responses to all concentrations of 5-HT. The 5-HT responses appeared more sensitive to increasing intraluminal pressure in the presence of nifedipine. Maximum Ca++-induced contractions in the presence of 10(-5) M 5-HT and high K+ occurred at 25 mm Hg, while Ca++-induced contractions and Ca++-induced contractions in the presence of 10(-7) 5-HT or K+ plus 5-HT were maximum at 75 mm Hg.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of bunazosin on electrical responses of smooth muscle cells of the guinea-pig mesenteric artery and vein to perivascular nerve stimulation and to noradrenaline

The noradrenaline-induced depolarization of smooth muscle cell membrane was blocked by bunazosin in the mesenteric artery but not in the mesenteric vein. Bunazosin enhanced the excitatory junction potential (e.j.p.) evoked in the mesenteric artery but did not modulate the slow depolarization evoked in the mesenteric vein. Application of noradrenaline decreased the amplitude of e.j.p. enhanced by bunazosin but not by yohimbine. It was concluded that bunazosin is a highly selective alpha 1-adrenoceptor blocker in vascular tissues.

Actions of the novel thromboxane A2 antagonists, ONO-1270 and ONO-3708, on smooth muscle cells of the guinea-pig basilar artery

The effects of the novel thromboxane A2 (TXA2) antagonists, ONO-1270 and ONO-3708, on the electrical and mechanical responses evoked by various agents, and in particular 9,11-epithio-11,12-methano-thromboxane A2 (STA2), were investigated in the guinea-pig artery. STA2 (up to 0.3 microM), and ONO-1270 and ONO-3708 (up to 1.0 microM) did not modify the membrane potential in smooth muscle cells. Perivascular nerve stimulation induced an excitatory junction potential (e.j.p.), and with frequencies over 0.25 Hz, depression of e.j.ps occurred. STA2 (0.1 microM) and both ONO-1270 and ONO-3708 had no effect on these electrical events. STA2 (over 0.1 microM) produced phasic and tonic contractile responses, in a concentration dependent manner. Both ONO-1270 and ONO-3708 competitively inhibited the phasic contraction induced by STA2 as estimated from parallel shifts in the dose-response curve, and from the Lineweaver-Burk and Schild plots (the PA2 values were 8.22 for ONO-1270 and 8.70 for ONO-3708), but both agents inhibited non-competitively the PGF2 alpha-induced contraction. ONO-1270 and ONO-3708 (up to 0.1 microM) had no effect on contractions induced by K+ and caffeine, but did slightly inhibited contractions induced by 5-hydroxytryptamine (5-HT). Following application of indomethacin, neither agent modified the 5-HT-induced contraction. In Ca2+-free solution, 10 nM STA2 produced a phasic but not a tonic contractile response. ONO-1270 and ONO-3708 (over 1 nM) inhibited this phasic contractile response.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of flunarizine on electrical and mechanical responses of smooth muscle cells in basilar and ear arteries of the rabbit

The effects of flunarizine on electrical and mechanical responses of smooth muscle tissues of the rabbit basilar and ear arteries to transmural stimulation, high-potassium solution (high-K), 5-hydroxytryptamine and noradrenaline were studied. In the basilar artery, 10(-6) M flunarizine (69 min application) blocked spike potentials generated by outward current stimuli or transmural stimulation without change in the resting-membrane potential or membrane resistance. The spike potentials generated in the ear artery were attenuated by a long exposure (up to 2 h) to a high concentration of flunarizine (10(-6) M). Membrane depolarizations produced by high-K, noradrenaline or 5-hydroxytryptamine were not blocked by flunarizine. Flunarizine inhibited smooth muscle contractions produced by transmural stimuli, high-K, noradrenaline or 5-hydroxytryptamine in both arteries, however the inhibition developed slowly, and the ear artery required a longer period of incubation with flunarizine than the basilar artery. The inhibitory effects of flunarizine on basilar artery were more marked against transmural stimulation or high-K induced contractions than against agonist-induced contractions.

Single nisoldipine-sensitive calcium channels in smooth muscle cells isolated from rabbit mesenteric artery

Single smooth muscle cells were enzymatically isolated from the rabbit mesenteric artery. At physiological levels of external Ca, these cells were relaxed and contracted on exposure to norepinephrine, caffeine, or high levels of potassium. The patch-clamp technique was used to measure unitary currents through single channels in the isolated cells. Single channels were selective for divalent cations and exhibited two conductance levels, 8 pS and 15 pS. Both types of channels were voltage-dependent, and channel activity occurred at potentials positive to -40 mV. The activity of both channel types was almost completely inhibited by 50 nM nisoldipine. These channels appear to be the pathways for voltage-dependent Ca influx in vascular smooth muscle and may be the targets of the clinically used dihydropyridines.

Effects of diltiazem on electrical responses evoked spontaneously or by electrical stimulation in the antrum smooth muscle cells of the guinea-pig stomach

In circular smooth muscle cells of the guinea-pig stomach (antrum), diltiazem (10(-6)-10(-5)M) blocked the overshooting spike potential generated either spontaneously or by electrical stimulation in the presence of 2 mM tetraethylammonium chloride, but did not block the slow wave and the abortive spike potential. The membrane was depolarized by high concentrations of diltiazem (more than 3 X 10(-6)M), and this depolarization was associated with an increase in the membrane resistance. The interval between slow waves was shortened to about 0.90 times the control (14.7s) by 10(-5)M diltiazem. Transmural nerve stimulation evoked an inhibitory junction potential (i.j.p.) and enhanced the subsequently generated slow wave. Tetrodotoxin (3 X 10(-7)M) blocked both responses but atropine (10(-6)M) blocked only the latter. Diltiazem (more than 10(-6)M) increased the amplitude of the i.j.p. and depressed the enhancement of the slow wave produced by transmural nerve stimulation, presumably due to depolarization of the membrane. The latency for the i.j.p. remained the same in the presence of diltiazem (10(-5)M). It is concluded that in the guinea-pig stomach, diltiazem blocks Ca-influx during the generation of the overshooting spike potential, but not the Ca-influx related to generation of the abortive spike potential or the slow wave. The cholinergic excitatory and the non-adrenergic, non-cholinergic inhibitory transmission may not be much affected by diltiazem.

Regional differences in the actions of verapamil and isosorbide dinitrate on rabbit and dog vascular smooth muscle

The effects of verapamil and isosorbide dinitrate (ISDN) alone or together on mechanical responses of the rabbit coronary artery and mesenteric vein, and the dog coronary artery have been investigated. Verapamil, in concentrations exceeding 10 nM consistently inhibited and at 10 microM, blocked the contraction evoked by excess concentrations of K in these tissues, but agonist-induced contraction was not modified by the presence or absence of Ca. In concentrations greater than 1 microM, verapamil depolarized the membrane by inhibiting the K-conductance of the membrane. ISDN had little effect on the rabbit coronary artery in concentrations below 10 microM. In contrast, in the rabbit mesenteric vein and dog coronary artery, ISDN in concentrations over 1 microM inhibited the contraction evoked by excess concentrations of K or by agonists. Species differences were apparent in the actions of ISDN on vascular tissues. When verapamil and ISDN were simultaneously applied to the rabbit mesenteric vein and dog coronary artery, the K-induced contraction was markedly inhibited by an amount exceeding that produced by the sum of the contractions evoked by the individual application of both agents. An enhanced vasodilating action induced by simultaneous applications of both agents indicates a possible clinical benefit for anti-anginal treatment.

Nisoldipine-induced relaxation in intact and skinned smooth muscles of rabbit coronary arteries

In smooth muscles of the rabbit coronary artery, nisoldipine inhibited the phasic and tonic responses of the contraction induced by 128 mM K (the IC50 values were 4 X 10(-8) M and 1 X 10(-13) M, respectively). This agent also inhibited the tonic response of the acetylcholine (ACh) (10(-5) M)-induced contraction (the IC50 value was 3 X 10(-10) M), but only slightly inhibited the phasic response (in 10(-7) M, 0.86 times the control). Nisoldipine (less than 10(-7) M) had no effect on the K-induced depolarization of the membrane at any given concentration. This drug (5 X 10(-8) M) did inhibit the oscillatory potential changes and spike potential evoked on the ACh-induced slow depolarization. After depletion of stored Ca from the polarized muscles (5.9 mM K), muscle cells accumulated Ca by application of 2.6 mM Ca without generation of contraction, i.e. a subsequently applied 20 mM caffeine produced the contraction in Ca-free solution. Nisoldipine (less than 10(-7) M) had little effect on this accumulation of Ca. The rate of rise and time to reach the maximum amplitude of the 128 mM K- or ACh-induced contraction (in 2.6 mM Ca) depended on the amount of stored Ca in cells. Nisoldipine (10(-8) M) consistently inhibited the Ca-induced contraction evoked in depolarized muscles (128 mM K), regardless of the amount of stored Ca. However, this agent (10(-8) M) did not inhibit the Ca release from storage sites evoked by activation of the muscarinic receptor. After prolonged superfusion (over 120 min) with Na- and Ca-free solution (guanethidine and atropine were present), application of 2.6 mM Ca produced contraction which was inhibited by 10(-8) M nisoldipine, while the depolarization induced by application of these solutions was not inhibited by nisoldipine. In saponin-skinned muscles, nisoldipine had no effect on the contractile proteins, as estimated from the pCa-tension relationship, or on the Ca accumulation into the Ca release from the Ca storage sites, as estimated from the caffeine-induced contraction. It is concluded that nisoldipine possesses a selective inhibitory action on voltage-dependent Ca influx, when the Ca channel is activated by depolarization.