Factors affecting rabbit mesenteric artery smooth muscle sensitivity to calcium antagonists

The sensitivity of rabbit isolated superior mesenteric artery to Ca++ antagonists was examined under various conditions. Relaxation dose-response curves for D600 or nifedipine were generated, and IC50 values were calculated. In the first series of experiments, D600 or nifedipine IC50 was found to be 20-25-fold greater for norepinephrine (NE, 5 microM) contraction than for 80 nM K+ contraction. Even when the tissues were depolarized with 80 mM K+ before NE contraction, D600 or nifedipine IC50 still remained significantly greater compared with 80 mM K+ alone and remained closer to that during NE alone. Also a protocol was designed to study NE-induced phasic contraction in EGTA-physiological salt solution (a functional indicator of intracellular Ca++ release) as well as NE-induced sustained contraction after readdition of Ca++. The effects of varying [K+]ex (0-80 nM range) on NE-induced [Ca++]i release as well as on the D600 IC50 for NE contraction was studied. Increasing [K+]ex was found to enhance NE-sensitive [Ca++]i release and lower the D600 IC50 for NE contraction. Thus, conditions causing an increase in the ability of NE to cause [Ca++]i release were associated with an increase in the sensitivity of NE contraction to D600. These data provide functional evidence that the receptor-agonist sensitive Ca++ influx process in vascular smooth muscle is not solely regulated by changes in membrane potential. Additional mechanisms, such as a modulatory role of [Ca++]i release, in this process are implicated.(ABSTRACT TRUNCATED AT 250 WORDS)     

Mode of vasorelaxing action of 5-[3-[[2-(3,4-dimethoxyphenyl)-ethyl]amino]-1-oxopropyl]-2,3,4,5- tetrahydro-1,5-benzothiazepine fumarate (KT-362), a new intracellular calcium antagonist

In rabbit aorta, pretreatment with KT-362 (KT; 10(-6) and 10(-5) M) inhibited contractile responses to norepinephrine (NE; 3 X 10(-9)-10(-5) M) and methoxamine (10(-7)-10(-4) M) but failed to affect responses to potassium (10-70 mM). KT (10(-5) M) partially inhibited Ca++-induced contractions in K+-depolarized aorta pre-equilibrated in a Ca++-free medium. After incubation of tissues for 30 min in a Ca++-free medium containing EGTA (0.2 mM), residual responses to NE and methoxamine were inhibited by KT (10(-6)-10(-4) M) and nitroglycerin (10(-5) M), but not by nifedipine, verapamil or diltiazem (all 10(-5) M). The inhibitory action of a combined treatment with KT and nitroglycerin (both 10(-5) M) on the residual response to NE was also much greater than that of either agent alone. In a Ca++-free medium, the residual caffeine-induced contraction of rabbit iliac artery was inhibited by KT (10(-5)-10(-4) M) but not by nifedipine (10(-5) M). The inhibitory action of KT on the residual responses to methoxamine and caffeine in a Ca+-free medium was much greater than that of nitroglycerin. In a Ca++-free medium with low EGTA (0.01 mM), D600 (10(-5) M) and NE (3 X 10(-7) M), the addition of Ca++ (2 mM) resulted in a tonic contraction.(ABSTRACT TRUNCATED AT 250 WORDS)     

An unusual Ca(2+) entry pathway activated by protein kinase C in dog splenic artery.

The characteristics of the Ca(2+) entry pathways that are activated by protein kinase C (PKC) in canine splenic artery were investigated. Phorbol 12, 13-dibutyrate (PDB) contracted tissues and increased Ca(2+) influx. PDB-induced contraction was reduced by preincubation of tissues in Ca(2+)-free Krebs' solution (1 mM EGTA) but was unaffected when Ca(2+)-free solution was applied after contraction was initiated with PDB. In contrast, (45)Ca influx and contraction induced by PDB were resistant to nifedipine, Cd(2+), Gd(3+), La(3+), or Ni(2+) whether added before or during exposure to PDB. Indeed, Cd(2+) reduced (45)Ca(2+) efflux and potentiated Ca(2+) influx, but not PDB-induced contraction. Norepinephrine (NE)-induced contractions were inhibited by preincubation in Ca(2+)-free Krebs' solution (1 mM EGTA). Nifedipine (10 microM) led to a small reduction in the NE-induced contraction but was without effect on (45)Ca(2+) influx. Pretreatment for 16 min with Cd(2+), Gd(3+), or La(3+) (each 1 mM) reduced or abolished NE-induced contraction and Ca(2+) influx. Application of these cations after exposure to NE did not affect (45)Ca(2+) influx but reduced tension. The Q(10) for the increase in (45)Ca(2+) influx was approximately 2 for high K(+) and NE, but 4 for PDB. The results suggest that stimulation of PKC in dog splenic artery activates a Ca(2+) entry pathway that is resistant to di- and trivalent cations. The inhibition of Ca(2+) influx by preincubating with cations during short-term exposure to NE may represent an action on Ca(2+) turnover that precedes activation of PKC.     

Effects of YM-12617, an alpha adrenoceptor blocking agent, on electrical and mechanical properties of the guinea-pig mesenteric and pulmonary arteries

The effects of YM-12617 on the electrophysiological properties of smooth muscle membranes and prejunctional nerve terminals and contractions evoked by different procedures were studied using guinea-pig mesenteric and pulmonary arteries. In concentrations over 1 nM, YM-12617 inhibited the depolarization induced by norepinephrine in both muscle tissues. Yohimbine had no effect whereas prazosin inhibited the norepinephrine-induced depolarization to a lesser extent than YM-12617. When YM-12617, in concentrations over 1 microM, was applied to the mesenteric artery the amplitude of the first excitatory junction potential evoked by a train stimulation of perivascular nerves was inhibited, but the facilitation of excitatory junction potentials evoked by frequencies over 0.1 Hz was enhanced. As a consequence, the amplitude of the excitatory junction potentials after completion of the facilitation exceeded the control, as was expected to occur with a typical alpha-2 adrenoceptor blocker. YM-12617 inhibited the contraction evoked by exogenously applied norepinephrine or perivascular nerve stimulation, with a higher potency than seen with prazosin, but this agent had no effect on the contraction evoked by excess concentrations of K+ or by direct muscle stimulation. These results indicate that YM-12617 possesses a more potent alpha-1 adrenoceptor blocking action than does prazosin, and is more selective for alpha-1 than for alpha-2 adrenoceptors.     

Calcium entry blocker activity of cyproheptadine in isolated cardiovascular preparations

Cyproheptadine was compared with nifedipine, verapamil and diltiazem for calcium entry blocker activity in isolated cardiovascular preparations. Using rat aortic strips, all compounds (10(-7) M) inhibited both the contraction caused by the readdition of calcium (1.0 mM) into regular buffer or buffer containing potassium (130 mM) or norepinephrine (10(-5) M) and the potassium-stimulated uptake of 45Ca. The rank order of potency for these experiments was in general nifedipine greater than cyproheptadine greater than or equal to verapamil greater than diltiazem. The same order of potency also was found for the four compounds in relaxing potassium (40 mM)-contracted aortic strips (IC50 values: nifedipine, 2.6 X 10(-9) M; cyproheptadine, 6.3 X 10(-8) M; verapamil, 7.6 X 10(-8) M; and diltiazem, 2.1 X 10(-7) M), but cyproheptadine was the least potent agent in antagonizing the spontaneous contractions of the rat portal vein (IC50 values: nifedipine, 6.6 X 10(-9) M; verapamil 7.7 X 10(-8) M; diltiazem 9.6 X 10(-8) M; and cyproheptadine 3.9 X 10(-7)M). None of the compounds (10(-7) M) inhibited the contraction to norepinephrine (10(-5) M) in rabbit aortic strips bathed in calcium-free buffer (1 mM ethylene glycol bis(beta-aminoethyl ether)-N, N'-tetraacetic acid). Nifedipine, verapamil and diltiazem were more potent in inhibiting the restoration of contractility by isoproterenol in potassium-depolarized rabbit papillary muscles than decreasing force in normally polarized muscles; cyproheptadine was equipotent when tested in these two preparations. Cyproheptadine was the least potent of the four compounds in lowering perfusion pressure in the perfused canine hindlimb.(ABSTRACT TRUNCATED AT 250 WORDS)     

Mechanisms of the bepridil-induced vasodilation of the rabbit mesenteric artery

Bepridil, at a concentration over 10 microM, increased the membrane resistance and, over 20 microM, depolarized the smooth muscle membrane of the rabbit mesenteric artery. These changes in membrane properties may be due to inhibition of K and possibly Cl conductances, as examined by the current-voltage relationship. The spike potential evoked by current pulses in the presence of 3 mM tetraethylammonium or from the excitatory junction potential (e.j.p.) was somewhat inhibited by bepridil (over 10 microM). Bepridil (over 5 microM) reduced the amplitude of e.j.p. without changing the facilitation process of e.j.ps and, at concentrations beyond 20 microM, it inhibited the facilitation of e.j.ps. Bepridil (over 0.1 microM) inhibited and over 1 microM blocked completely the tonic response of the K-induced contraction. Thus, the voltage-dependent Ca channels activated by K-induced depolarization and during the spike generation differ in nature. Bepridil inhibited the caffeine-induced contraction in intact muscles and the Ca-induced contraction in skinned muscles prepared by saponin to the same extent, but more effectively inhibited the norepinephrine-induced contraction without altering the norepinephrine-induced depolarization. We conclude that bepridil mainly inhibits the voltage-dependent Ca channel (the K depolarization sensitive) in smooth muscle membrane. High concentrations of bepridil (over 10 microM) inhibit the Ca spike, the K and Cl conductances, contractile proteins and Ca release from intracellular stored sites of smooth muscle cells and Ca mobilization at nerve terminals. Actions of bepridil differ from those of other Ca antagonists.     

Specific actions of gallium on norepinephrine-induced tension and associated 45Ca movements in rabbit aortic smooth muscle

Gallium ion (Ga) dose-dependently (60-360 microM) inhibited contractions induced by norepinephrine (NE, 1 microM) in rabbit aortic (and media intimal) strips, but did not affect contractions elicited with high K+ (80 mM) solution. The initial phasic portion of the NE-induced response was either unaffected or only slightly (less than 10%) reduced, but the tonic portion of the response was inhibited completely by higher concentrations (greater than or equal to 300 microM) of Ga . In resting muscles, the equilibrated (90 min) 45Ca uptake was not altered by Ga (360 microM). Also, 45Ca efflux from either high- or low-affinity Ca++ binding sites was unaltered by Ga . The effects of Ga (360 microM) on 45Ca retained after a subsequent 60-min washout at 0.5 degrees C in an isosmotic (80.8 mM) La solution were also examined. High affinity La -resistant 45Ca released by NE (1 microM) was not altered by Ga . Under conditions favoring low affinity Ca++ uptake, 45Ca retention in control and K+-treated muscles was not changed by Ga , but the additional incremental 45Ca uptake associated with NE (in the presence of high K+) was blocked. Thus, Ga appears to have a selective inhibitory action on NE-associated 45Ca uptake without affecting either resting and high K+-induced 45Ca uptake or that 45Ca fraction released by NE. This action may result from a selective blockade by Ga of receptor-linked Ca++ channels in rabbit aortic smooth muscle.     

Alpha and beta adrenoceptor blocking action of carvedilol in the canine mesenteric artery and vein

We observed the effects of carvedilol, a novel beta adrenoceptor blocker, on electrical responses of smooth muscle cells produced by endogenous and exogenous norepinephrine (NE) in isolated canine mesenteric artery and vein. Carvedilol inhibited the NE-induced depolarization in the artery but not in vein, with potencies equivalent to prazosin, i.e., carvedilol blocked alpha 1 adrenoceptors in arterial smooth muscles. Stimulation of perivascular nerves evoked an excitatory junction potential (e.j.p.) and a slow depolarization in these vascular smooth muscles. Carvedilol inhibited the slow depolarization evoked in the artery but not in the vein, with no marked inhibition of the e.j.p.s. High concentrations (10(-5) M) of carvedilol inhibited the e.j.p., slow depolarization, and also the compound action potentials of sympathetic nerve bundles running along the mesenteric vessels, suggesting that these inhibitions were due to local anesthetic actions. The e.j.p. amplitude was increased by isoprenaline and was decreased by NE. The NE and isoprenaline actions were antagonized by yohimbine and propranolol, respectively. Carvedilol inhibited the isoprenaline-actions but not the NE actions on the e.j.p., suggesting that this drug blocked prejunctional beta adrenoceptors but not the alpha 2 adrenoceptors. These results indicate that carvedilol blocks alpha 1 and beta adrenoceptors but not alpha 2 adrenoceptors in vascular tissues.     

Nicardipine actions on smooth muscle cells and neuromuscular transmission in the guinea-pig basilar artery

The effects of nicardipine on smooth muscle cells of the guinea-pig basilar artery were investigated by means of microelectrode and isometric tension recording methods. Nicardipine (1 X 10(-8) to 3 X 10(-6) M) did not modify the membrane potential and resistance of smooth muscle cells. The spike evoked by application of outward current pulse in the presence of tetraethylammonium (greater than 1 X 10(-3) M) was inhibited by 1 X 10(-9) M and was almost blocked by 3 X 10(-7) M nicardipine. Perivascular nerve stimulation evoked the excitatory junction potential which was slightly suppressed by 3 X 10(-6) M nicardipine. The contractions evoked by excess concentration of [K]0, NaCl-free solution or ATP was abolished and by 5-hydroxytryptamine was markedly inhibited in Ca-free ethylene glycol bis(beta-aminoethyl ether)N,N'-tetracetic acid-containing solution, but that induced by caffeine was only slightly inhibited. Nicardipine (greater than 3 X 10(-10) M) markedly inhibited the K-induced contraction noncompetitively as estimated from the Lineweaver-Burk's plot. The ATP-induced contractions were slightly inhibited by nicardipine (greater than 1 X 10(-8) M) to a lesser extent than the K-induced contraction. On the other hand, nicardipine (1 X 10(-6) M) had no effect on the NaCl-free-or 5-hydroxytryptamine-induced contraction. When nicardipine (1 X 10(-6) M) was applied during 2.5 mM Ca loading to muscle cells after depletion of the stored Ca, the subsequently generated caffeine-induced contraction was slightly inhibited due to inhibition of the passive Ca influx. These results indicate that nicardipine possesses a selective inhibitory action for the Ca channel, but the inhibition is limited to particular Ca influxes such as the voltage-dependent one, but not the receptor operated and NaCl-free-induced Ca influxes. This agent acts predominantly on the postjunctional muscle rather than the prejunctional nerve terminal.     

Agonist actions of Bay K 8644, a dihydropyridine derivative, on the voltage-dependent calcium influx in smooth muscle cells of the rabbit mesenteric artery

Actions of methyl-1,4-dihydro-2,6-dimethyl-3-nitro-4-(2- trifluoromethylphenyl)-pyridine-5-carboxylate (Bay K 8644) on the mechanical response evoked in intact and skinned mesenteric artery of the rabbit were investigated. The data were compared to that of nisoldipine, another dihydropyridine derivative Bay K 8644 increased the amplitudes of both the phasic and tonic components of the K+-induced contraction which is due to an increase in the voltage-dependent influx of Ca ion. Bay K 8644 antagonized competitively the actions of nisoldipine (a Ca antagonist) on the tonic but not on the phasic component of the K+-induced contraction. The contractions caused by high concentrations of norepinephrine were enhanced to a greater extent by Bay K 8644 than that evoked by lower concentrations of norepinephrine. Bay K 8644 had no effect on Ca++ extrusion from cells, which was estimated from the change in amplitudes of the norepinephrine-induced contractions in Na+- and Ca++-free solutions. This agent had no effect on the contractile proteins and Ca storage sites, as estimated from the Ca++- or caffeine-induced contraction observed in skinned muscles. The results suggested that Bay K 8644 acts primarily on the voltage-dependent Ca++ channel, presumably the same site at which other dihydropyridine derivatives (Ca antagonists) act, and that the influx of Ca++ is accelerated.     

Postsynaptic alpha adrenoceptors in baboon cerebral and mesenteric arteries

Postsynaptic alpha adrenergic mechanisms were compared in cerebral and mesenteric arteries isolated from the baboon. The contractile response to norepinephrine (NE) of the cerebral artery was potent and similar to that of the mesenteric artery. The EC50 was 3.1 (2.0-5.0) X 10(-7) M for the cerebral artery and 2.6 (1.5-4.8) X 10(-7) M for the mesenteric artery. The maximum contraction expressed as a force developed/cross-sectional area did not differ between the two arteries, whereas that expressed as percentage of 30 mM KCl-induced contraction in the cerebral artery (118 +/- 9%) was less than in the mesenteric artery (145 +/- 6%). Phenylephrine produced a contraction in a manner similar to NE, although the EC50 values in both arteries were 2 to 3 times as large as those for NE. Clonidine produced a moderate contraction in the mesenteric artery (35 +/- 8% of the KCl-induced contraction) but no contraction in the cerebral artery. NE-induced contraction in the cerebral artery was inhibited more prominently by prazosin, a selective alpha-1 adrenoceptor antagonist, than that in the mesenteric artery; the pA2 value for prazosin in the cerebral artery was higher (9.70) than that in the mesenteric artery (8.95). In contrast, there was no difference in pA2 values for either phentolamine or yohimbine between the two arteries. Clonidine-induced contraction in the mesenteric artery was attenuated by prazosin rather than yohimbine at the same concentration used. Thus, it may be concluded that contractile processes related to postsynaptic alpha-1 adrenoceptor stimulation are predominantly operative in baboon cerebral and mesenteric arteries, the cerebral artery being more susceptible to the inhibitory effect of prazosin.     

Ca++ utilization in the constriction of rat aorta to full and partial alpha-1 adrenoceptor agonists

l-Norepinephrine and l-phenylephrine were full agonists and cirazoline, SKF d-89748, Sgd 101/75 and SKF l-89748 were partial agonists in contracting rat isolated aortic rings. Clonidine, l-amidephrine and St 587 were found ineffective. Nifedipine (10(-8)-10(-6) M) abolished the contractions to Sgd 101/75 and to high K+ with similar potency but only partially inhibited the contractions to the other alpha adrenoceptor agonists. Norepinephrine, phenylephrine, cirazoline and Sgd 101/75 were full agonists in stimulating 45Ca++ influx, which amounted to 50% of the maximal influx produced by high K+. SKF d- and l-89748 behaved as partial agonists, whereas St 587, amidephrine and clonidine were virtually inactive. Nifedipine was equally effective in blocking the influx of 45Ca++ produced by K+ and the alpha adrenoceptor agonists. Norepinephrine stimulated 45Ca++ efflux to an extent similar to that for high K+. In the following order of decreasing efficacy, phenylephrine, cirazoline and SKF d- and l-89748 caused significant stimulation of 45Ca++ efflux. Sgd 101/75, amidephrine, clonidine and St 587 were without effect. However, Sgd 101/75 (10(-5) M) antagonized the 45Ca++ efflux of norepinephrine. Nifedipine (3 X 10(-7) M) completely suppressed the K+-induced 45Ca++ efflux but only partly affected the 45Ca++ efflux caused by the alpha adrenoceptor stimulants. A highly significant (r = 0.975) linear relationship was found between the nifedipine-resistant contractile response and the 45Ca++ efflux obtained in the presence of nifedipine. The data suggest that the stimulation of alpha-1 adrenoceptors in rat aorta can activate two distinct processes of Ca++ utilization for contraction.(ABSTRACT TRUNCATED AT 250 WORDS)     

Potassium-induced contraction in the lamb proximal urethra: involvement of norepinephrine and different calcium entry pathways.

The purpose of this work was to investigate the mechanisms involved in the peculiar biphasic response of the lamb urethral smooth muscle to high K+ solutions. The relative amplitude of the phasic and tonic components of the contraction and its reproducibility were dependent on the concentration of K+ used. Only concentrations higher than 80 mM (i.e., 120 mM) showed a tonic component greater in amplitude than the phasic one and manifested a tachyphylactic effect. Phentolamine (10(-6) M), prazosin (10(-6) M) and chemical denervation with 6-hydroxydopamine significantly inhibited the tonic component of the K+ (120 mM)-induced contraction, modifying its morphology. Reproducible contractions to K+ (120 mM) could be obtained in the presence of prazosin (10(-6) M) or cocaine (10(-6) M). The preparations were also shown to accumulate [3H]noradrenaline and release it upon depolarization with K+ (60 and 120 mM). Calcium removal inhibited the K+ (120 mM)-induced contraction. After addition of calcium (0.5-5 mM) the contractile activity was restored. Nifedipine (10(-6) M) and verapamil (10(-6) M) but not sodium nitroprusside (10(-6) M) significantly blocked the contractile response for calcium as well as the phasic component of the K+ contraction in calcium-containing medium. In preparations treated with prazosin (10(-6) M) the tonic component of the K+ (120 mM) contraction was more sensitive to nifedipine and removal of extracellular calcium than the phasic one.(ABSTRACT TRUNCATED AT 250 WORDS)     

Inhibitory effects of a synthetic atrial peptide on contractions and 45Ca fluxes in vascular smooth muscle

The mechanism of a synthetic atrial peptide (APII)-induced inhibition of smooth muscle contractility was investigated by studying its effects on tension development and 45Ca fluxes in isolated rabbit aorta. APII (10(-9) to 10(-7) M) produced a dose-dependent relaxation of contractions produced by alpha adrenoceptor activation with norepinephrine (NE; 10(-6) M). APII was a potent relaxant of NE contraction with an IC50 = 1.1 X 10(-8) M, with 10(-7) M APII causing a 97% relaxation. APII also produced a dose-dependent inhibition of NE contraction when added to the resting muscle before the exposure to NE. The relaxing effects of APII were found to be endothelium independent. In contrast, APII was only marginally effective in relaxing high-K+ contraction, with 10(-7) M APII causing only 17% relaxation. Furthermore, when a NE contraction was obtained on top of a high-K+ contraction, APII was still capable of relaxing the NE component. APII was similarly more effective in inhibiting NE-stimulated 45Ca influx than high-K+-stimulated 45Ca influx, indicating selective action of APII on the receptor-operated Ca++ channels. This was in contrast to D600, a well known Ca++ antagonist, which had a more selective inhibitory effect on the potential-operated Ca++ channels. The data presented indicate that APII is a potent relaxant of contractions produced by receptor-agonists involving 45Ca influx through receptor-operated Ca++ channels. APII may also prove to be a very useful tool to further distinguish and define receptor-operated Ca++ channels and potential-operated Ca++ channels in vascular smooth muscle.     

Actions of nifedipine on calcium fluxes and contraction in isolated rat arteries

Experiments were performed on isolated rat aorta and superior mesenteric artery in order to study the action of nifedipine on norepinephrine and K-depolarization-evoked contractions and transmembrane calcium fluxes. Concentration-dependent contractions were obtained with norepinephrine in physiological solution and with Ca++ in K-depolarizing solution. Nifedipine caused a concentration-dependent depression of the maximum response. When aorta was depolarized by 40 mM KCI (instead of usual 100 mM KCI concentration), high concentrations of Ca++ evoked a relaxation that was also blocked by nifedipine. The action of nifedipine has been examined on Ca influx and efflux in arteries stimulated by norepinephrine and K-depolarization. Norepinephrine-evoked Ca influx, but not Ca efflux, was reduced by nifedipine. Concentration inhibitory curves for Ca influx and contraction could be superimposed. K-depolarization-dependent Ca entry and Ca efflux were blocked by nifedipine at concentrations lower than those required to antagonize norepinephrine actions. The results suggest that the action of nifedipine on artery contractility can be related to blockade of calcium entry through channels opened during depolarization or receptor-response coupling.     

Mechanism of cyclosporin potentiation of vasoconstriction of the isolated rat mesenteric arterial bed: role of extracellular calcium

Hypertension is a serious side-effect of the clinical use of Cyclosporin A (CsA). One notion is that alterations of vascular reactivity contribute to this hypertension. In this study we used the isolated rat mesenteric vascular bed to test the specific hypothesis that CsA modifies vascular calcium regulation to potentiate vascular contractility. Mesenteric vessels from CsA-treated rats (10 mg/kg/day i.m. for 7 days) exhibited significantly greater vasoconstrictor responses to exogenous norepinephrine (NE) and potassium-induced depolarization than those of vehicle-treated animals. Similarly, in vitro CsA (8.3 X 10(-8) to 8.3 X 10(-6) M) augmented in a concentration-dependent manner both the sensitivity and the maximum response to NE and potassium. This effect of CsA on vasoconstriction was critically dependent on the presence of external calcium [( Ca++]o). The degree of vasoconstriction potentiation correlated significantly with the [Ca++]o which was used during exposure of the mesenteric bed to CsA. Reapplication of external calcium in the presence of CsA to "calcium-depleted" preparations increased significantly the amplitude of subsequent NE responses in a calcium-free medium. Thus, CsA-potentiated NE responses, once established, were not reversed by removing external calcium; however, attenuation occurred with the intracellular calcium antagonist dantrolene. Verapamil and nifedipine blocked potassium-elicited responses, but failed to prevent the CsA effect on NE-induced vasoconstriction. We conclude that CsA potentiation of vasoconstriction depends on extracellular calcium, and results from an enhanced transmembrane calcium transport. We speculate that CsA also increases the filling of intracellular stores of releasable calcium. These effects lead to greater calcium influx and greater intracellular calcium release upon stimulation.     

Fusobacterium necrophorum haemolysin stimulates motility of ileal longitudinal smooth muscle of the guinea-pig

Fusobacterium necrophorum haemolysin (0.5-3.1 mg protein/mL) dose-dependently induced contractions of the isolated ileal longitudinal smooth muscle of the guinea-pig. The haemolysin (3.1 mg protein/mL) -induced maximum contraction of 75% of the response to 60 mM K+ declined within 17 min and the muscles then demonstrated rhythmic contractions. Tetrodotoxin (3.1 x 10(-6) M) had no effect on the contraction due to the haemolysin. After incubation in Ca(2+)-free medium, the ileal response to the haemolysin was lost. Verapamil, a Ca2+ channel blocker, dose-dependently inhibited the contraction to the haemolysin. The rabbit anti-serum against F. necrophorum haemolysin inhibited the haemolysin-induced contraction of ileal muscle. The bacterial haemagglutinin and the lipopolysaccharide had no effect on the response of ileal muscle. These findings suggest that the haemolysin-induced direct stimulation of ileal motility dependant on Ca2+ influx will increase the probability of contact of F. necrophorum and ileal mucosa and could increase the chances of colonization for F. necrophorum.     

Effects of Bay k 8644 and nifedipine on isolated dog cerebral, coronary and mesenteric arteries

Vasoconstrictor effects of Bay k 8644, a dihydropyridine Ca++ agonist, and vasorelaxant effects of nifedipine were investigated in helical strips of dog cerebral (basilar, posterior cerebral and middle cerebral) and peripheral (coronary and mesenteric) arteries. The addition of Bay k 8644 produced a dose-dependent contraction in the absence of any contractile agent in the basilar artery with a pD2 value of 8.53. Similar sensitivity to Bay k 8644 was observed in the posterior cerebral, middle cerebral or coronary artery. Bay k 8644 was much less effective in producing a contraction in the mesenteric artery. An elevation of the concentration of extracellular K+ eliminated the difference between the responses to Bay k 8644 in the basilar and mesenteric artery. Contractile responses of the basilar artery to Bay k 8644 were antagonized competitively by nifedipine (pA2 = 8.17), but non-competitively by diltiazem. The pA2 values for nifedipine antagonism of Bay k 8644 responses with the elevated K+ were the same between the basilar and mesenteric arteries. Increased sensitivity to exogenously added K+ also was observed in cerebral and coronary arteries when compared with the mesenteric artery. The addition of nifedipine to an unstimulated strip produced a dose-dependent relaxation in cerebral and coronary arteries, but not in the mesenteric artery. When the cerebral and peripheral arteries were contracted with K+ to the same magnitude, nifedipine produced similar relaxations among these arteries. Nifedipine was less efficacious in antagonizing the contractile response to Bay k 8644 compared with the contractile response to K+ in cerebral arteries. These results suggest that 1) the voltage-dependent Ca++ channels in the cerebral and coronary arteries are in different states of activation from those in the mesenteric artery, 2) Bay k 8644 contracts the cerebral and coronary arteries by acting primarily on the same site, presumably dihydropyridine receptors of the voltage-dependent Ca++ channels at which nifedipine acts, 3) the dihydropyridine receptors were the same between the basilar and mesenteric arteries and 4) there may be a difference in the state of the Ca++ channel in the arteries between the stimulation with Bay k 8644 and K+-depolarization.     

The influence of ketamine on inotropic and chronotropic responsiveness of heart muscle.

The influence of ketamine on the inotropic and chronotropic responsiveness of heart muscle was examined in spontaneously beating right atrial preparations and in electrically driven left atrial preparations of guinea pigs. Ketamine (2.63 X 10(-5) to 4.2 X 10(-4) M) decreased heart rate of right atria and decreased contractile tension and its maximum rate of increase in both right and left atrial preparations (right atria greater than left atria). Ketamine did not prevent the heart rate increase produced by norepinephrine (NE; 1 X 10(-8) to 1 X 10(-4) M) in right atria; however, the maximum heart rate was consistently lower in ketamine-treated than in control muscles even after exposure to NE. Although contractile tension was decreased by ketamine, the maximum inotropic response to NE was consistently greater in ketamine-treated atria than in control atria. An inhibitor of the slow Ca++ current in heart muscle, D600, depressed the contractile effects of NE but did not prevent the positive inotropic interaction of ketamine and NE. Ketamine similarly enhanced the inotropic responses to norepinephrine (1 X 10(-6) M), epinephrine (1 X 10(-6) M), isoproterenol (1 X 10(-7) M) and dibutyryl cyclic adenosine 3':5'-monophosphate (AMP; 4 X 10(-3) M) in left atria electrically paced at a constant frequency of contraction of 1 Hz; however, ketamine inhibited the positive inotropic response to increased frequency of stimulation (0.1-3.0 Hz) and to ouabain (3 X 10(-7) M). These findings demonstrate that ketamine can exert a selective positive inotropic influence in heart muscle independent of heart rate or direct or reflexogenic autonomic nervous system changes, and suggest that this activity could in some way be associated with an alteration of the intracellular disposition of cyclic AMP.     

Eperisone, an antispastic agent, possesses vasodilating actions on the guinea-pig basilar artery

The effects of eperisone, an antispasmodic, on the electrical and mechanical properties of smooth muscle cells of the guinea-pig basilar artery were investigated. Eperisone (below 50 microM) did not modify the membrane potential, membrane resistance, excitatory junction potential evoked by perivascular nerve stimulation or the K- and 5-hydroxytryptamine-induced depolarizations. However, in concentrations over 10 microM, this agent reversibly blocked the spontaneously generated and evoked action potentials in the presence of 5 to 8 mM tetraethylammonium. The contractions evoked by high concentrations of K, 5-hydroxytryptamine or direct muscle stimulation in the presence of tetraethylammonium were dose dependently inhibited by eperisone, in concentrations over 1 microM. In Ca-free 2 mM ethylene glycol bis(beta-aminoethyl ether)-N,N'-tetraacetic acid-containing solution, 5-hydroxytryptamine produced only a single contraction. Eperisone inhibited this contraction to the same extent as that observed in the presence of Ca. These results indicate that eperisone possesses the property of a Ca antagonist on smooth muscle tissues of the guinea-pig basilar artery, in addition to the action of antispastic agent, i.e., this agent blocks the voltage-dependent influx of Ca at the smooth muscle membrane, but not at the nerve terminals, and inhibits the action of Ca in cells through inhibition of the contractile protein.