心肌α1—肾上腺素受体激动对豚鼠心室乳头肌的影响

The alpha-adrenoceptor agonist phenylephrine (5.0 x 10(-6) mol/L) was used to stimulate myocardial alpha-adrenoceptors of the guinea-pig ventricular papillary muscle, and changes of transmembrane action potential and contractile force of the muscle were observed. The alpha 1-adrenoceptor blocker prazosin (5.0 x 10(-7) mol/L) and the alpha 2-adrenoceptor blocker yohimbine (5.0 x 10(-7) mol/L) were used to determine which subtype of alpha-adrenoceptor is responsible for the effects. The beta-adrenoceptor blocker propranolol (1.0 x 10(-6) mol/L) was used throughout the experiment. The results show that the myocardial alpha 1-adrenoceptor stimulation (1) increases the contractile force of the guinea-pig ventricular papillary muscle, (2) prolongs the time to peak contractile force while the duration of relaxation is not altered, (3) prolongs the fast response action potential duration, and (4) increases the maximal rate of depolarization during the phase 0 of the slow response action potential. It is suggested that the electrophysiological and positive inotropic effects of myocardial alpha 1-adrenoceptor stimulation might be due to the activation of the slow inward current and an increase in Ca2+ influx.     

Alterations in alpha 1-adrenoceptor stimulation of isolated atria from experimental diabetic rats

This purpose of this investigation was to determine the influence of experimental diabetes (3 months) on the responsiveness of rat isolated atria to alpha 1-adrenoceptor stimulation by phenylephrine. Diabetes was chemically induced with streptozotocin (65 mg/kg i.v.) in 42- to 43-day-old, nonfasted male Sprague-Dawley derived rats. Chronotropic (right atria) and inotropic (left atria) indices were recorded in response to alpha 1-adrenoceptor stimulation by phenylephrine. These experiments were performed in the presence of beta-adrenoceptor antagonism (timolol). Isolated right atria from diabetic rats demonstrated a greater increase in heart rate in response to phenylephrine than did corresponding control atria. Left atria were supersensitive (decrease in EC50 values) and hyperresponsive to alpha 1-adrenoceptor stimulation by phenylephrine when compared with stimulation of control left atria. Diabetic left atria in response to phenylephrine were observed to exchange more radioactive calcium (45Ca2+) than control left atria, whereas both diabetic and control left atria exchanged the same amount of 45Ca2+ during basal contractile conditions. Phenylephrine had no effect on 45Ca2+ efflux from either diabetic or control atria. These results indicate that 3 months of uncontrolled experimental diabetes in the rat produces an enhancement of alpha 1-adrenoceptor activation of isolated atria, and that there is an alteration in Ca2+ mobilization which may contribute to the enhanced receptor activation.     

Evidence against the role of alpha 1-adrenoceptor reserve in buffering the inhibitory effect of nifedipine on the contractility of canine vascular smooth muscle

The relationship between the postsynaptic alpha 1-adrenoceptor reserve and the sensitivity of vasoconstriction induced by alpha-adrenoceptor agonists to the dihydropyridine Ca2+ entry blocker nifedipine was investigated in isolated muscle strips of dog mesenteric artery (DMA) and saphenous vein (DSV). The amplitudes of the contractile responses of DMA induced by phenylephrine were the same as those in DSV in the presence and in the absence of extracellular Ca2+. The use of 3 x 10(-9) M phenoxybenzamine to irreversibly block the alpha 1-adrenoceptors revealed a marked difference in the size of the alpha 1-adrenoceptor reserve between DMA (40%) and DSV (7%). In spite of a larger receptor reserve, the contractile responses induced by phenylephrine in DMA were more sensitive to nifedipine compared with those in DSV. These results suggest that the postsynaptic alpha 1-adrenoceptor reserve in vascular smooth muscle, at least in DMA and DSV, does not play an important role in buffering the inhibitory effect of nifedipine on the contractile response to a full agonist of alpha 1-adrenoceptors. Other factors, such as the difference in the membrane depolarizing effect, the ability to utilize intracellular Ca2+ for contraction, and the possible existence of alpha 1-adrenoceptor subtypes, may contribute to the different inhibitory effects of nifedipine on these blood vessels.     

Does in vitro activation of postsynaptic alpha 2-adrenoceptor utilize intracellular Ca2+ for contraction in dog saphenous vein?

Dog saphenous vein spiral strips were employed to determine whether an intracellular source of Ca2+ is used for contraction upon activation of the alpha 2-adrenoceptor by B-HT 920 in Ca2+-free Krebs solution containing 50 microM EGTA. The studies were carried out in parallel with the activation of the alpha 1-adrenoceptor by phenylephrine (Phe) under the condition that B-HT 920 (10(-5) M) and Phe (2 x 10(-6) M) gave rise to a similar level of responses in Ca2+-containing Krebs solution. A similar level of responses to these agonists at equieffective concentrations in Ca2+-free medium were also observed. Such responses to Phe and B-HT 920 were inhibited by 10(-7) M rauwolscine and 10(-7) M prazosin, respectively, and were not affected by 10(-7) M nifedipine or 5 mM Mn2+. The responses to B-HT 920 (10(-5) M) and submaximal concentration of Phe (2 x 10(-6) M) in Ca2+-free medium were additive. However, if the vascular strips were first contracted maximally with 10(-4) M Phe in Ca2+-free medium to deplete the intracellular Ca store, subsequent addition of B-HT 920 failed to induce additional response. Our results strongly suggest that activation of alpha 2-adrenoceptor in dog saphenous vein in Ca2+-free medium indeed utilizes intracellular Ca2+ for contraction. We also found that the failure of earlier studies to demonstrate the contractile effects of B-HT 920 in dog saphenous vein was due to experimental artifacts derived from the use of high concentration of EGTA and artificial pH-buffering reagent.     

Postsynaptic alpha-adrenoceptor characterization and Ca2+ channel antagonist and activator actions in rat tail arteries from normotensive and hypertensive animals

Postsynaptic alpha-adrenoceptors in the rat tail artery have been examined by determining the pA2 values for antagonists against several alpha-adrenoceptor agonists. In this tissue the alpha-adrenoceptor agonists all produce concentration-dependent mechanical responses with the following rank order of potency: clonidine greater than norepinephrine greater than phenylephrine greater than UK 14304 greater than B-HT 920. Antagonism by prazosin and yohimbine of phenylephrine, norepinephrine, and clonidine responses does not reveal the anticipated discrimination between alpha 1- and alpha 2-adrenoceptors. Thus, pA2 values for prazosin (9.1-9.5), yohimbine (7.2-7.4), and corynanthine (7.0-7.1) and idazoxan (7.6) do not show large differences between these receptor agonists and suggests the predominance of alpha 1-adrenoceptor mediated contractile responses in this preparation. Significant differences between antagonist activities (pA2 values) in Wistar Kyoto (WKY) and spontaneously hypertensive rats (SHR) artery preparations have not been observed. The sensitivity sequence of alpha-adrenoceptor agonist-induced responses to nifedipine and D 600 is B-HT 920 greater than clonidine greater than phenylephrine greater than norepinephrine. Dependence of agonist response upon extracellular Ca2+ parallels the sensitivity to Ca2+ channel antagonists. Sensitivity to D 600 of phenylephrine responses increased with decreasing concentration of phenylephrine or with receptor blockade by phenoxybenzamine: sensitivity of responses to B-HT 920 was not affected by these procedures. Tail artery strips from WKY and SHR do not exhibit major differences in sensitivity to D 600 or to Ca2+ depletion. Bay k 8644, a Ca2+ channel activator, produces concentration-dependent mechanical responses in the tail artery in the presence of modestly elevated K+ concentrations (10-15 mM): these actions of elevated K+ can be mimicked by both alpha 1- and alpha 2-adrenoceptor agonists including methoxamine, St 587, UK 14304, and clonidine. These studies do not provide clear evidence for the existence of discrete postsynaptic alpha 1- and alpha 2-adrenoceptor populations in rat tail artery as indicated by pA2 values or Ca2+ dependence of response.     

Effects of thioridazine on mechanical responses of human vas deferens induced by noradrenaline or potassium.

Thioridazine (0.1-10 mumol l-1) inhibited shortening of specimens of human vasa deferentia induced by noradrenaline (100 mumol l-1) or high extracellular potassium (136 mmol l-1). Thioridazine did not inhibit the lengthening response. In Ca(2+)-free media with EGTA (0.5 mmol l-1) similar results were obtained with responses to noradrenaline, but exposure to potassium elicited small contractions that were potentiated by thioridazine. Both shortening and lengthening responses to noradrenaline were antagonized by the alpha-adrenoceptor blockers prazosin (1-10 mumol l-1) and phentolamine (1-10 mumol l-1) and by the Ca2+ antagonists verapamil (10 mumol l-1) and diltiazem (10 mumol l-1). Responses to potassium were virtually abolished by the Ca2+ antagonists. These results show that thioridazine specifically inhibits longitudinal muscle of the human vas deferens and that its action cannot be entirely accounted for by a blockade of voltage-gated Ca2+ channels.     

45Ca displacement related to pharmacologically induced prolonged action potentials in Nitella flexilis

Nitella cells were loaded with 45Ca2+ to an activity of 2 X 10(5) cpm. Insertion of two glass-capillary electrodes into each of six cells released varying amounts of Ca2+ in the order of 1 mumol per cell, but hyperpolarizing and depolarizing pulses up to 500 ms in duration caused no measurable loss (less than 57 pmol) of Ca2+ even when the latter elicited action potentials. Addition of 10 mumol of Ba2+ or tetraethylammonium (TEA) caused losses up to 1200 pmol of Ca2+ from the cells and prolonged the action potentials by a factor of three or more. Subsequent addition of Ba2+ or TEA to treated cells caused no further losses of Ca. Because prolonged action potentials can apparently only be elicited after the chelation or displacement of Ca2+, we propose that, as in many animal cells, the K+ channels responsible for the normal brief repolarizing phase of the action potential are controlled by Ca2+ in these electrically excitable plant cells.     

The alpha 1- and alpha 2-adrenoceptor and muscarinic responses of normal and axotomized bullfrog sympathetic ganglia

When neurones in bullfrog paravertebral sympathetic ganglia are studied by means of the sucrose-gap technique, muscarinic agonists produce a biphasic response (an initial hyperpolarization of ganglionic C cells followed by a depolarization of ganglionic B cells). Activation of ganglionic alpha 2-adrenoceptors promotes hyperpolarization. The present experiments with selective alpha 1- and alpha 2-adrenoceptor agonists and antagonists provided evidence for the existence of hitherto undescribed alpha 1-adrenoceptors, which are responsible for the production of depolarizing responses in these ganglia. Fifteen to twenty-five days after cutting postganglionic axons (axotomy), there was a nonselective depression of both alpha 1- and alpha 2-adrenoceptor mechanisms but little change in muscarinic responses. These results argue against the hypothesis that C cells assume all the properties of B cells after axotomy. Since the alpha-selective agonist phenylephrine failed to depolarize axotomized ganglia, it is unlikely that an alpha 1-adrenoceptor mechanism is prominent in axotomized neurones as it is in some immature adrenergic neurones. The data are consistent with the idea that axotomy selectively affects the properties of certain types of cation channels and raise questions as to the mechanisms involved in regulating the expression and maintenance of specific neurotransmitter responses on ganglionic neurones.     

Extracellular signal-regulated kinase1/2 in contraction of vascular smooth muscle

Smooth muscle contractility is regulated by both intracellular Ca2+ concentration ([Ca2+]i) and Ca2+ sensitivity of the contractile apparatus. Extracellular signal-regulated kinases1/2 (ERK1/2) have been implicated in modulating Ca2+ sensitivity of smooth muscle contraction but mechanisms of action remain elusive. This study investigated the roles of ERK1/2 in modulating [Ca2+]i, calcium sensitivity and the 20-kDa myosin light chain (MLC20) phosphorylation during contraction activated by alpha1-adrenoceptor agonist phenylephrine and thromboxane A2 mimetic U46619 in rat tail artery strips. A specific inhibitor for ERK1/2 activation, U0126, inhibited phenylephrine- and U46619-induced contraction, shifting both concentration-response curves rightward. During phenylephrine-stimulated contraction, U0126 exhibited concentration-dependent inhibition towards force but significant decreases in [Ca2+]i were detected only at higher concentration. Both phenylephrine and U46619 induced a transient activation of ERK1/2 which was abolished by U0126 but unaffected by a general tyrosine kinase inhibitor genistein or Rho kinase inhibitor Y27632 at concentrations inhibiting more than 50% force. Interestingly, U0126 had no effect on steady-state MLC20 phosphorylation levels stimulated by both receptor agonists. These results indicated that during contraction of rat tail artery smooth muscle activated by alpha1-adrenoceptor agonist or thromboxane A2 analogue, ERK1/2 increase Ca2+ sensitivity that does not involve the modulation of MLC20 phosphorylation.     

Positive inotropic effect of ryanodine on rabbit ventricular muscle: dependence on the intracellular calcium load

Two types of electrical and mechanical responses to 1 mumol/l ryanodine, depending on the intracellular calcium load, were observed in rabbit papillary muscles. In a normal calcium solution, ryanodine induced a transient decline followed by a stable increase in the developed force (by 20 +/- 5% of the pretreatment level; n = 30) and prolonged the action potential (AP). The positive ryanodine response showed an increased time-to-peak force and was completely suppressed by 2 mumol/l nifedipine, partially blocked by 50 mumol/l tetracaine (Ca2+ release blocker), but greatly potentiated by 20 mmol/l CsCl or (-) Bay R 5414 which prolonged the AP. The prolonged time-to-peak force of the positive ryanodine response was shortened by procedures raising the content of Ca2+ in the sarcoplasmic reticulum (SR). It is suggested that the initial decline in the force amplitude results from Ca2+ leakage from the SR which is further compensated for by an elevation of both the transmembrane Ca2+ entry and intracellular Ca2+ release. In calcium overloaded myocardium, 1 mumol/l ryanodine caused irreversible contracture and dramatic AP shortening, explained by a massive Ca2+ release from the overloaded SR into the cytoplasm. It is concluded that the calcium content in the SR is the main modulator of the electrical and mechanical effects of ryanodine in ventricular myocardium.     

Rho kinase is involved in Ca2+ entry of rat penile small arteries

Tonic physiological activity of RhoA/Rho kinase contributes to the maintenance of penile flaccidity through its involvement in the Ca(2+) sensitization of erectile tissue smooth muscle. The present study hypothesized that Rho kinase is also involved in the modulation of Ca(2+) entry induced by alpha(1)-adrenoceptor stimulation of penile arteries. Rat penile arteries were mounted in microvascular myographs for simultaneous measurements of intracellular Ca(2+) ([Ca(2+)](i)) and force. The Rho-kinase inhibitor Y-27632 markedly reduced norepinephrine-mediated electrically induced contractions and the increases in both [Ca(2+)](i) and tension elicited by the alpha(1)-adrenoceptor agonist phenylephrine (Phe). In contrast, the protein kinase C (PKC) inhibitor Ro-31-8220 reduced tension without altering the Phe-induced increase in [Ca(2+)](i). In the presence of nifedipine, Y-27632 still inhibited the non-L-type Ca(2+) signal and blunted Phe contraction. Y-27632 did not impair the capacitative Ca(2+) entry evoked by store depletion with cyclopiazonic acid but largely reduced the Ba(2+) influx stimulated by Phe in fura-2 AM-loaded arteries. The addition of Y-27632 to arteries depolarized with high KCl markedly reduced tension without changing [Ca(2+)](i). In alpha-toxin-permeabilized penile arteries stimulated with threshold Ca(2+) concentrations, Y-27632 inhibited the sensitization induced by either guanosine 5'-O-(3-thiotriphosphate) (GTPgammaS) or Phe in the presence of GTPgammaS. However, Y-27632 failed to alter contractions induced by a maximal concentration of free Ca(2+). These results suggest that Rho kinase, besides its contribution to the Ca(2+) sensitization of the contractile proteins, is also involved in the regulation of Ca(2+) entry through a nonselective cation channel activated by alpha(1)-adenoceptor stimulation in rat penile arteries.     

Comparison of the electromechanical responsiveness of alpha-1-adrenoceptor stimulation in ventricles of normal and cardiomyopathic hamsters

Alterations in alpha(1)-adrenoceptor (alpha(1)AR) density and related signal transduction proteins were reported in cardiomyopathic hearts in the failing stage. The electromechanical modification of alpha(1)-adrenergic stimulation in the failing heart is unclear. The present study compares the alpha(1)AR-stimulated electromechanical response in failing ventricles of genetically cardiomyopathic BIO 14.6 hamsters (280-320 days old) with that in age-matched normal Syrian hamsters. The action potential was recorded with a conventional microelectrode technique, and twitch force was measured with a transducer. In the presence of propranolol, phenylephrine increased the contraction and prolonged the action potential duration (APD) to similar values in ventricles of both strains, despite a prolonged basal APD in cardiomyopathic ventricles. The positive inotropism stimulated by phenylephrine was inhibited by staurosporine, and was potentiated by 4 beta-phorbol-12,13-dibutyrate (PDBu) in both strains. The maximum positive inotropic effect of phenylephrine in PDBu-treated ventricles of normal hamsters was significantly greater than that in BIO 14.6 hamsters. The effects of phenylephrine on the ventricular force-frequency relationship and on the mechanical restitution in both normal and BIO 14.6 strain hamsters were examined. The uniform negative force-frequency relationship and the altered mechanical restitution reveal a defect of intracellular Ca(2+) handling in cardiomyopathic BIO 14.6 hamsters. alpha(1)-Adrenergic modulation cannot convert the defective properties in the model of the failing heart. Nevertheless, phenylephrine decreased post-rest potentiation in short rest periods, and enhanced post-rest decay after longer resting periods. The results indicate that alpha(1)-adrenergic action enhances a gradual loss of Ca(2+) from the sarcoplasmic reticulum, although its action in prolonging the APD can indirectly increase the influx of Ca(2+).     

Electrophysiological research on the coupling of excitation-contraction during alpha-adrenoreceptor activation in blood vessel smooth muscles

Alpha 1-adrenoceptor blocker--prazosin--was found to inhibit noradrenaline-induced depolarization and concentration in the smooth muscles of the portal rabbit vein, indicating that this reaction was due to alpha 1-adrenoceptor activation. In the pulmonary artery both alpha 1 and alpha 2-adrenoceptors appear to be involved in noradrenaline excitatory action, as the effect was not completely inhibited by prazosin. The results suggest that hypotensive action of prazosin is related to the cessation of Ca2+ ion influx through alpha 1-operated calcium channels. The decrease in Ca2+ influx through voltage-dependent calcium channels due to prazosin-evoked elimination of depolarization can also contribute to this effect.     

Effect of phenylephrine on pyruvate dehydrogenase activity in rat hepatocytes and its interaction with insulin and glucagon

In isolated rat hepatocytes phenylephrine promotes a rapid increase in the amount of pyruvate dehydrogenase present in its active form (PDHa). This action is mediated by alpha 1-adrenergic receptors and is not observed in Ca2+-depleted hepatocytes. It is mimicked by the Ca2+ ionophore A23187. No changes in metabolites known to affect PDH activity are measured 3 min after addition of phenylephrine. Glucagon also increases PDHa, its action is additive to that of phenylephrine. The action of phenylephrine on PDHa could be explained by an increase in mitochondrial free Ca2+.     

Effects of flavonoids on vascular smooth muscle of the isolated rat thoracic aorta

The potency, structure-activity relationship, and mechanism of vasorelaxation of a series of flavonoids, representing different subclasses (flavonols: fisetin, rutin, quercetin; flavones: chrysin, flavone, baicalein; flavanones: naringenin, naringin; isoflavones: diadzein and flavanes: epigallo catechin gallate), were examined in the isolated rat aorta. Most of the flavonoids tested showed concentration dependent relaxant effects against K+ (80 mM) and phenylephrine (PE, 0.1 microM)-induced contractions with a greater inhibition of the responses to the alpha1-adrenoceptor agonist. The relaxant effects of most of the flavonoids involve in part the release of nitric oxide and prostaglandins from the endothelium as pretreatment with L-NAME and indomethacin attenuated the responses. In addition, the relaxant action of the flavonoids includes inhibition of Ca+2 influx and release of Ca+2 from intracellular stores. A structure-activity relationship amongst the flavonoids was suggested.     

Mechanism of the negative inotropic effects of alpha 1-adrenoceptor agonists on mouse myocardium

This study was done to identify the mechanism of the alpha1-adrenoceptor (AR) mediated negative inotropic effects of phenylephrine (PE) on adult mouse myocardium. As reported by others, we also found that the nonselective alpha1AR agonist PE produced a negative inotropic effect on ventricular strips from adult mice that was inhibited by the alpha1AAR antagonist 5-methylurapidil (5MU) but not by the alpha1BAR antagonist chloroethylclonidine (CEC) or the alpha1DAR antagonist BMY 7378. The selective alpha1AAR agonist A61603 also produced a negative inotropic effect, which was antagonized by 5MU. Phorbol 12,13-dibutyrate (activator of all PKC isoforms) mimicked the negative inotropic responses to PE and A61603. The negative inotropic effects of PE were inhibited by bisindolylmaleimide (inhibitor of all PKC isoforms) but not by G? 6976 (inhibitor of Ca2+-dependent PKC). Rottlerin, an inhibitor of Ca2+-independent PKCdelta, antagonized the negative inotropic effects of PE and A61603. PE and A61603 increased the translocation of PKCdelta, which was prevented by rottlerin. These data suggest that the alpha1AR-mediated negative inotropy on adult mouse myocardium is signaled by Ca2+-independent PKCdelta.     

Effect of potassium channel blocking agents on the actions of phenylephrine in rabbit taenia caeci

The effects of tetraethylammonium, apamin, 4-aminopyridine and holding potential on the phenylephrine-evoked outward currents in dispersed smooth muscle cells of the rabbit taenia caeci were analyzed using the whole cell patch clamp method. Phenylephrine (10 mumol/l) under the double sucrose gap condition, substantially hyperpolarized the smooth muscle membrane and reduced the input membrane resistance. This concentration of phenylephrine enhanced the frequency and amplitude of spontaneous transient outward currents (s.t.o.c.s) and elicited a low amplitude sustained outward current which were voltage and temperature dependent. In addition, phenylephrine (10 mumol/l) reduced the outward current evoked by voltage steps. Tetratehylammonium (1-5 mmol/l) attenuated the depolarization-evoked outward current, blocked the appearance of s.t.o.c.s, and fully abolished the phenylehrine induced changes in membrane currents. Apamin (0.1-10 mumol/l) only slightly affected the evoked outward current and s.t.o.c.s. However apamin did not change the phenylephrine-induced outward currents. Pretreatment with 4-aminopyridine (0.5-2 mmol/l) did not reduce the phenylephrine-induced sustained outward current and s.t.o.c.s but prevented the phenylephrine induced reduction of the depolarization-evoked outward current. These results are in favour of assumption that the phenylephrine induced hyperpolarization and reduction in the input membrane resistance are consequences of an enhanced potassium current via tetraethylammonium-sensitive, apamin and 4-aminopyridine resistant potassium channels.     

Vasa recta pericytes express a strong inward rectifier K+ conductance

Strong inward rectifier potassium channels are expressed by some vascular smooth muscle cells and facilitate K+-induced hyperpolarization. Using whole cell patch clamp of isolated descending vasa recta (DVR), we tested whether strong inward rectifier K+ currents are present in smooth muscle and pericytes. Increasing extracellular K+ from 5 to 50 and 140 mmol/l induced inward rectifying currents. Those currents were Ba2+ sensitive and reversed at the K+ equilibrium potential imposed by the electrode and extracellular buffers. Ba2+ binding constants in symmetrical K+ varied between 0.24 and 24 micromol/l at -150 and -20 mV, respectively. Ba2+ blockade was time and voltage dependent. Extracellular Cs+ also blocked the inward currents with binding constants between 268 and 4,938 micromol/l at -150 and -50 mV, respectively. Ba2+ (30 micromol/l) and ouabain (1 mmol/l) depolarized pericytes by an average of 11 and 24 mV, respectively. Elevation of extracellular K+ from 5 to 10 mmol/l hyperpolarized pericytes by 6 mV. That hyperpolarization was reversed by Ba2+ (30 micromol/l). We conclude that strong inward rectifier K+ channels and Na+-K+-ATPase contribute to resting potential and that KIR channels can mediate K+-induced hyperpolarization of DVR pericytes.     

The effect of neomycin on contractile activity of the canine cervical lymphatic vessel induced by various agents

The effects of neomycin on isometric contractions induced by norepinephrine (NE), alpha 1-adrenoceptor agonist phenylephrine (PE), KCl, and an activator of GTP-binding proteins (G-proteins) NaF were studied in the isolated canine cervical lymphatic vessel (CLV). Incubation of the CLV with 0.3 or 1.3 mmol/l neomycin for 30-180 min did not affect significantly either the basal vascular tone or the response to NE, and potentiated the response to KCl by 24 +/- 6% (p less than 0.05). On the other hand, neomycin (1.3 mmol/l) treatment reduced by 22 +/- 6% (p less than 0.05) the contractions induced by PE and completely (by 96 +/- 3%, p less than 0.001) suppressed the effects of NaF. Upon the combined action of NaF and NE, neomycin reduced only NaF-component of the total response. Verapamil (100 mumol/l) had no effect on the magnitude of NaF-induced tension and partially inhibited NE- and PE-induced contractions (by 20 +/- 4% (p less than 0.05) and 53 +/- 5% (p less than 0.01), respectively). Indomethacin (10 mumol/l) did not influence significantly the contractions evoked by NE, KCl, and NaF either under control conditions or in the presence of neomycin. These data suggest that the phosphoinositides may considerably contribute to the CLV contractions evoked by NaF, but do not play a substantial role in the responses of the vessel to alpha-adrenergic stimulation and KCl.     

Calcium handling in vasoconstriction to stimulation of alpha 1- and alpha 2-adrenoceptors

Vasoconstriction to stimulation of postsynaptic alpha 1- and alpha 2-adrenoceptors involves different mechanisms of Ca2+ mobilization. Alpha 2-adrenoceptor-mediated vasoconstriction in vivo as well as in vitro is invariably and effectively antagonized by Ca2+ channel blockers, such as nifedipine or verapamil, and is therefore primarily carried by influx of extracellular Ca2+. On the other hand, alpha 1-adrenoceptor stimulation has been linked to both influx of extracellular Ca2+ and release of Ca2+ from intracellular stores. The sensitivity of alpha 1-adrenoceptor-mediated vasoconstriction to blockade by Ca2+ channel antagonists depends on how much both mechanisms of Ca2+ mobilization contribute to the contraction process, and varies between vascular tissues and alpha 1-adrenoceptor agonists. The experimental evidence for the differential utilization of Ca2+ in vasoconstriction to alpha 1- and alpha 2-adrenoceptor stimulation is reviewed.