New aspects for the treatment of cardiac diseases based on the diversity of functional controls on cardiac muscles: diversity in the excitation-contraction mechanisms of the heart

The waveform of the myocardial action potential (AP) triggering contraction differs among the species, developmental stage, and pathological state. The species difference in heart rate, which inversely correlates with body size, originates in the ion-channel mechanisms responsible for diastolic depolarization of the sinoatrial node. In some cases, such as the chronically AV-blocked dog and 11- to 13-day chick embryo, the repolarization reserve is decreased making the heart useful for drug evaluation. The degree of dependence of contraction on sarcoplasmic reticulum (SR) function increases during development. The large SR dependence and short AP of the adult mouse and rat support their rapid contraction under high heart rate. The function of the Na(+)/Ca(2+) exchanger is affected by AP waveform and ion concentrations; its major role is Ca(2+) extrusion, but under pathological conditions such as ischemia-reperfusion, it allows Ca(2+) influx and leads to myocardial injury, including loss of mitochondrial function. The role of mitochondria in ATP supply is less in the fetus where glycolysis plays a greater role. The pharmacological properties of the myocardium are affected by all of these factors and also by autonomic innervation and the hormonal status. Such comprehensive understanding is indispensable for the development of novel therapeutic strategies.

Species difference in the contribution of T-type calcium current to cardiac pacemaking as revealed by r(-)-efonidipine

The contribution of the T-type Ca2+ current to cardiac pacemaking was examined in isolated right atrial tissue from the mouse, guinea pig, and rabbit using a specific blocker, R(-)-efonidipine. At 10(-6) M, R(-)-efonidipine produced negative chronotropy, which was prominent in the mouse and small but significant in the guinea pig. No effect was observed in the rabbit. Microelectrode recordings revealed that R(-)-efonidipine significantly prolongs the pacemaker (phase 4) depolarization of the sinoatrial-node action potential in the mouse and guinea pig. These results provide the first pharmacological evidence that the contribution of T-type Ca2+ current to cardiac pacemaking differs among experimental animal species.

Intracellular mechanisms and receptor types for endothelin-1-induced positive and negative inotropy in mouse ventricular myocardium

We examined the intracellular mechanisms for endothelin-1-induced positive and negative inotropic components that coexist in the mouse ventricular myocardium using isolated ventricular tissue and myocytes from 4-week-old mice. In the presence of SEA0400, a specific inhibitor of the Na+-Ca2+ exchanger, endothelin-1 produced positive inotropy. Endothelin-1, when applied to cardiomyocytes in the presence of SEA0400, did not change the peak amplitude of the Ca2+ transient but increased intracellular pH and Ca2+ sensitivity of contractile proteins. On the other hand, in the presence of dimethylamiloride (DMA), a specific inhibitor of the Na+-H+ exchanger, endothelin-1 produced negative inotropy. In cardiomyocytes, in the presence of DMA, endothelin-1 produced a decrease in peak amplitude of the Ca2+ transient. In the presence of both DMA and SEA0400, endothelin-1 produced neither positive nor negative inotropy. Positive inotropy was blocked by BQ-123 and negative inotropy by BQ-788. These results suggested that endothelin-1-induced positive inotropy is mediated by ET(A) receptors, activation of the Na+-H+ exchanger and an increase in intracellular pH and Ca2+ sensitivity and that the negative inotropy is mediated by ET(B) receptors, activation of the Na+-Ca2+ exchanger and decrease in Ca2+ transient amplitude.

Endocardial endothelium-dependent positive inotropy by Ca2+ pump inhibitors: possible involvement of store-operated Ca2+ entry

Positive inotropy by sarcoplasmic/endoplasmic reticulum Ca(2+) pump inhibitors was found and its mechanisms were analyzed pharmacologically. Thapsigargin and cyclopiazonic acid produced positive inotropy in isolated mouse left atria. The responses were inhibited by pretreatment of the endocardial surface with Triton X-100 or by indomethacin, which suggests that the inotropic responses were mediated by prostaglandin(s) released from the endocardial endothelium as well as acetylcholine-induced positive inotropy. The thapsigargin- and acetylcholine-induced positive inotropy was significantly inhibited by Gd(3+), La(3+) and lavendustin A, a tyrosine kinase inhibitor, but not by Ni(2+) and LOE908, a non-selective cation channel inhibitor. Gd(3+) and lavendustin A had no effect on the exogenously applied PGF(2)alpha-induced positive inotropy. In addition, acetylcholine did not induce any positive inotropy when applied after the application of thapsigargin. These results strongly suggest that thapsigargin- as well as acetylcholine-induced prostaglandin release from endocardial endothelium is mediated by store-operated Ca(2+) entry through Gd(3+)-sensitive channels and activation of tyrosine kinase.

Analysis of arrhythmogenic profile in a canine model of chronic atrioventricular block by comparing in vitro effects of the class III antiarrhythmic drug nifekalant on the ventricular action potential indices between normal heart and atrioventricular block heart

The chronic atrioventricular block dog is a useful model for predicting the future onset of drug-induced long QT syndrome in clinical practice. To better understand the arrhythmogenic profile of this model, we recorded the action potentials of the isolated ventricular tissues in the presence and absence of the class III antiarrhythmic drug nifekalant. The action potential durations of the Purkinje fiber and free wall of the right ventricle were longer in the chronic atrioventricular block dogs than in the dogs with normal sinus rhythm. Nifekalant in concentrations of 1 and 10 microM prolonged the action potential durations of Purkinje fiber and the free wall in a concentration-dependent manner. The extent of prolongation was greater in the chronic atrioventricular block dogs than in the normal dogs. However, increase of temporal dispersion of ventricular repolarization including early afterdepolarization was not detected by nifekalant in either group of dogs, indicating lack of potential to trigger arrhythmias in vitro. These results suggest that the ventricular repolarization delay in the chronic atrioventricular block model by nifekalant may largely depend on the decreased myocardial repolarization reserve, whereas the trigger for lethal arrhythmia was not generated in the in vitro condition in contrast to the in vivo experiment.

Muscarinic receptor subtypes mediating positive and negative inotropy in the developing chick ventricle

The inotropic response to muscarinic receptor stimulation of isolated chick ventricular myocardium was examined at various developmental stages, and the receptor subtype involved was pharmacologically characterized. In embryonic chick ventricles, carbachol (CCh) produced positive inotropy at micromolar concentrations. In hatched chick ventricles, CCh produced negative inotropy at nanomolar concentrations. Neither positive nor negative inotropy was observed in the 19 - 21-day-old embryos. Both positive and negative inotropy were also observed with acetylcholine and oxotremoline-M. The CCh-induced positive inotropy in 7 - 9-day-old embryonic ventricles and the negative inotropy in 1 - 3-day-old hatched chick ventricles were antagonized by muscarinic receptor antagonists; pA(2) values for the positive and negative responses of pirenzepine were 7.5 and 7.2, those of AF-DX116 (11-[(2-[(diethylamino)methyl]-1-piperidinyl)acetyl]-5,11-dihydro-6H-pyrido[2,3-b][1,4] benzodiazepine-6-one) were 6.8 and 6.9, those of 4-diphenylacetoxy-N-methylpiperidine (4-DAMP) were 9.0 and 8.5, and those of himbacine were 7.0 and 8.0, respectively. CCh had no effect on action potential configuration. In conclusion, the positive inotropy is most likely mediated by muscarinic M(1) receptors and the negative inotropy is mostly likely mediated by muscarinic M(4) receptors.

Involvement of Intracellular Ca2+ in the Regulatory Volume Decrease After Hyposmotic Swelling in MDCK Cells

We examined the source of Ca(2+) involved in the volume regulation of Madin-Darby canine kidney (MDCK) cells with confocal microscopy and fluoroprobes. Hyposmosis induced a transient increase in cell volume, as well as cytoplasmic Ca(2+), which peaked at 3 to 5 min and gradually decreased to reach the initial value within about 30 min. This late decrease in cell volume, as well as the transient rise in cytoplasmic Ca(2+), was reduced in Ca(2+)-free solution and was abolished by pretreatment with thapsigargin. In conclusion, Ca(2+) released from the intracellular store contributes to the regulatory volume decrease following hyposmotic swelling in MDCK cells.

Involvement of the Na+/Ca2+ Exchanger in Ouabain-Induced Inotropy and Arrhythmogenesis in Guinea-Pig Myocardium as Revealed by SEA0400

Involvement of the Na+/Ca2+ exchanger in ouabain-induced inotropy and arrhythmogenesis was examined with a specific inhibitor, SEA0400. In right ventricular papillary muscle isolated from guinea-pig ventricle, 1 microM SEA0400, which specifically inhibits the Na+/Ca2+ exchanger by 80%, reduced the ouabain (1 microM)-induced positive inotropy by 40%, but had no effect on the inotropy induced by 100 microM isobutyl methylxantine. SEA0400 significantly inhibited the contracture induced by low Na+ solution. In HEK293 cells expressing the Na+/Ca2+ exchanger, 1 microM ouabain induced an increase in intracellular Ca2+, which was inhibited by SEA0400. The arrhythmic contractions induced by 3 microM ouabain were significantly reduced by SEA0400. These results provide pharmacological evidence that the Na+/Ca2+ exchanger is involved in ouabain-induced inotropy and arrhythmogenesis.

Enhancement of methoxamine-induced contractile responses of rat ventricular muscle in streptozotocin-induced diabetes is associated with alpha1A adrenoceptor upregulation

AIM

To clarify the time-related changes in cardiac function and the mechanism underlying the cardiac dysfunction present in diabetes mellitus, we studied mechanical responses induced by alpha(1)- and beta-adrenoceptors, the Ca(2+)-entry promoter Bay K 8644- and ryanodine (an agent known to inhibit Ca(2+) release from the sarcoplasmic reticulum) in papillary muscles from streptozotocin (STZ)-induced diabetic and age-matched control rats.

METHODS

Male Wistar rats received a single injection of STZ (60 mg kg(-1)) via the tail vein to induce diabetes. For the mechanical studies, papillary muscle preparations were suspended in an organ bath and isometric contractions were measured in 1-, 4-, and 10-week STZ-induced diabetic and age-matched control rats.

RESULTS

In 1-week diabetic rats, the contractions induced by isoproterenol, methoxamine and Bay K 8644 were unchanged (vs. age-matched controls). In 4-week diabetic rats, (a) the isoproterenol- and Bay K 8644-induced contractions were impaired, (b) sensitivity to ryanodine was reduced, whereas (c) the methoxamine-induced contraction was unchanged. In 10-week diabetic rats, the isoproterenol- and Bay K 8644-induced contractile responses were impaired and the sensitivity to ryanodine was reduced, but in sharp contrast the methoxamine-induced contraction was enhanced. Both the mRNA level for the alpha(1A) adrenoceptor (but not the alpha(1B) or alpha(1D) mRNAs) and alpha(1A) adrenoceptor protein were increased in 10-week diabetic rats (vs. age-matched controls).

CONCLUSION

These results suggest that impairments of beta-adrenergic and Ca(2+)-handling mechanisms occur early in the development of cardiomyopathy in STZ-induced diabetic rats, and that this is followed by augmentation of alpha(1A) adrenoceptor-mediated inotropy due to alpha(1A) adrenoceptor upregulation.

Reduction by SEA0400 of myocardial ischemia-induced cytoplasmic and mitochondrial Ca2+ overload

The cardioprotective effects of SEA0400, a novel Na(+)-Ca(2+) exchanger inhibitor, were examined in isolated guinea pig myocardial tissue and ventricular myocytes. In a coronary-perfused right ventricular tissue preparation, SEA0400 had no cardiosuppressive effect during normoxia and experimental ischemia, but enhanced the recovery of contractile force during reperfusion. SEA0400 had no effect on tissue ATP content during normoxia, but attenuated its decrease during ischemia. Treatment of ventricular myocytes with an ischemia mimetic solution (high K(+), glucose free, pH 6.0, gassed with N(2)) resulted in the depolarization of the mitochondrial membrane potential and an increase in cytoplasmic and mitochondrial Ca(2+) concentration, which had a similar time course. SEA0400 significantly delayed these changes. These results suggest that SEA0400 maintains mitochondrial function and tissue ATP content during ischemia through the inhibition of cytoplasmic and mitochondrial Ca(2+) overload.

Kv channels contribute to nitric oxide- and atrial natriuretic peptide-induced relaxation of a rat conduit artery

The role of K(+) channels in nitric oxide (NO)-induced vasorelaxation has been largely investigated in resistance vessels where iberiotoxin-sensitive MaxiK channels play a predominant role. However, the nature of the K(+) channel(s) involved in the relaxation triggered by NO-releasing compounds [nitroglycerin, NTG; NOR 3 [(+/-)-(E)-4-ethyl-2-[(E)-hydroxyimino]-5-nitro-3-hexenamide]] or atrial natriuretic peptide (ANP) in the conduit vessel aorta has remained elusive. We now demonstrate that, in rat aorta, the relaxation due to these vasorelaxants is not affected by the MaxiK channel blocker iberiotoxin (10(-7)-10(-6) M) as was the control vascular bed used (mesenteric artery). The inability of iberiotoxin to prevent NO/ANP-induced aortic relaxations was not due to lower expression of MaxiK in aorta or due to the predominance of iberiotoxin-resistant channels in this conduit vessel. Aortic relaxations were strongly diminished by 4-aminopyridine (4-AP) (> or =5 x 10(-3) M) or by tetraethylammonium (>2 x 10(-3) M) at concentrations known to inhibit voltage-dependent K(+) (K(v)) 2-type channels but not by other K(+) channel inhibitors, glibenclamide, apamin, charybdotoxin, tertiapin, or E-4031 N-[4-[[1-[2-(6-methyl-2-pyridinyl)ethyl]-4-piperidinyl-]carbonyl]phenyl]methanesulfonamide dihydrochloride). Consistent with a role of K(v)2-type channels, K(v) currents in A7r5 aortic myocytes were stimulated by NTG and inhibited by > or =5 x 10(-3) M 4-AP. Furthermore, immunocytochemistry, immunoblot, and real-time polymerase chain reaction analyses confirmed the presence of K(v)2.1 channels in aorta. K(v)2.1 transcripts were approximately 100-fold more abundant than K(v)2.2. Our results support low-affinity 4-AP-sensitive K(v) channels, assembled at least partially by K(v)2.1 subunit, as downstream effectors of NO/ANP-signaling cascade regulating aortic vasorelaxation and further demonstrate vessel-specific K(+) channel involvement in NO/ANP-induced relaxation.

Quantitative Fluorescence Measurement of Cardiac Na+/Ca2+ Exchanger Inhibition by Kinetic Analysis in Stably Transfected HEK293 Cells

We developed a method to quantitatively evaluate the potency of Na+/Ca2+ exchanger (NCX) inhibitors with fluorescence microscopy in NCX1-transfected HEK 293 cells. The reverse mode and forward mode NCX activities were measured as the ascending slope of the early phase increase in cytoplasmic Ca2+ concentration after change to low Na+ extracellular solution and the descending rate (inverse of the exponential time constant) on return to normal solution, respectively. Both modes of NCX were inhibited by SEA0400 (2-[4-[(2,5-difluorophenyl)methoxy]phenoxy]-5-ethoxyaniline) and KB-R7943 (2-[2-[4-(4-nitrobenzyloxy)phenyl]ethyl]isothiourea methanesulfonate), and the concentration-inhibition relationships for both inhibitors were in good agreement with those previously reported in voltage clamped cardiomyocytes.

Blockade by NIP-142, an Antiarrhythmic Agent, of Carbachol-Induced Atrial Action Potential Shortening and GIRK1/4 Channel

Mechanisms for the atria-specific action potential-prolonging action of NIP-142 ((3R*,4S*)-4-cyclopropylamino-3,4-dihydro-2,2-dimethyl-6-(4-methoxyphenylacetylamino)-7-nitro-2H-1-benzopyran-3-ol), a benzopyran compound that terminates experimental atrial arrhythmia, was examined. In isolated guinea-pig atrial tissue, NIP-142 reversed the shortening of action potential duration induced by either carbachol or adenosine. These effects were mimicked by tertiapin, but not by E-4031. NIP-142 concentration-dependently blocked the human G protein-coupled inwardly rectifying potassium channel current (GIRK1/4 channel current) expressed in HEK-293 cells with an EC50 value of 0.64 microM. At higher concentrations, NIP-142 blocked the human ether a go-go related gene (HERG) channel current with an EC50 value of 44 microM. In isolated guinea-pig papillary muscles, NIP-142 had no effect on the negative inotropic effect of carbachol under beta-adrenergic stimulation, indicating lack of effect on the muscarinic receptor and Gi protein. These results suggest that NIP-142 directly inhibits the acetylcholine-activated potassium current.

Comparison of the inhibitory effects of docosahexaenoic acid (DHA) on U46619- and phenylephrine-induced contractions in guinea-pig aorta

Inhibitory effects of docosahexaenoic acid (DHA) on the muscle contractions induced by U46619, a thromboxane A2 (TXA2) mimetic, and phenylephrine were compared in guinea-pig aorta. In de-endothelialized guinea-pig aortic ring preparations, DHA at 10 microM strongly inhibited a sustained contraction produced by U46619 (3-100 nM) whereas it did not exhibit an appreciable effect on phenylephrine (3-10 microM)-induced contraction. The present findings indicate that DHA inhibits more selectively TXA2 receptor (TP receptor)-mediated vascular contraction than alpha-adrenoceptor-mediated response. Selective inhibition by DHA of TP receptor-mediated contraction of blood vessels seems underlie in part the mechanisms by which this polyunsaturated fatty acid exerts its circulatory-protective effects.

Unique excitation–contraction characteristics of mouse myocardium as revealed by SEA0400, a specific inhibitor of Na+–Ca2+ exchanger

The functional role of the sodium-calcium exchanger in mouse ventricular myocardium was evaluated with a newly developed specific inhibitor, SEA0400. Contractile force and action potential configuration were measured in isolated ventricular tissue preparations, and cell shortening and Ca2+ transients were measured in indo-1-loaded isolated ventricular cardiomyocytes. SEA0400 increased the contractile force, cell shortening and Ca2+ transient amplitude, and shortened the late plateau phase of the action potential. alpha-adrenergic stimulation by phenylephrine produced a sustained decrease in contractile force, cell shortening and Ca2+ transient amplitude, which were all inhibited by SEA0400. Increasing the contraction frequency resulted in a decrease in contractile force in the absence of drugs (negative staircase phenomenon). This frequency-dependent decrease was attenuated by SEA0400 and enhanced by phenylephrine. Phenylephrine increased the Ca2+ sensitivity of contractile proteins in isolated ventricular cardiomyocytes, while SEA0400 had no effect. These results provide the first pharmacological evidence in the mouse ventricular myocardium that inward current generated by Ca2+ extrusion through the sodium-calcium exchanger during the Ca2+ transient contributes to the action potential late plateau, that alpha-adrenoceptor-mediated negative inotropy is produced by enhanced Ca2+ extrusion through the sodium-calcium exchanger, and that the negative staircase phenomenon can be explained by increased Ca2+ extrusion through the sodium-calcium exchanger at higher contraction frequencies.

Atria selective prolongation by NIP-142, an antiarrhythmic agent, of refractory period and action potential duration in guinea pig myocardium

NIP-142 is a novel benzopyran compound that was shown to prolong the atrial effective refractory period and terminate experimental atrial fibrillation in the dog. In the present study, we examined the effects of NIP-142 on isolated guinea pig myocardium and on the G-protein-coupled inwardly rectifying potassium channel current (acetylcholine-activated potassium current; I(KACh)) expressed in Xenopus oocytes. NIP-142 (10 and 100 microM) concentration-dependently prolonged the refractory period and action potential duration in the atrium but not in the ventricle. E-4031 and 4-aminopyridine prolonged action potential duration in both left atrium and right ventricle. Prolongation by NIP-142 of the atrial action potential duration was observed at stimulation frequencies between 0.5 and 5 Hz. In contrast, the prolongation by E-4031 was not observed at higher frequencies. Tertiapin, a blocker of I(KACh), prolonged action potential duration in the atrium but not in the ventricle. NIP-142 completely reversed the carbachol-induced shortening of atrial action potential duration. NIP-142 (1 to 100 microM), as well as tertiapin (0.1 to 100 nM), concentration-dependently blocked I(KACh) expressed in Xenopus oocytes; the blockade by NIP-142 was not affected by membrane voltage. In conclusion, NIP-142 was shown to prolong atrial refractory period and action potential duration through blockade of I(KACh) which may possibly explain its previously described antiarrhythmic activity. NIP-142 has pharmacological properties that are different from classical class III antiarrhythmic agents such as atria specificity and lack of reverse frequency dependence, and thus appears promising for the treatment of supraventricular arrhythmia.

Simultaneous real-time detection of initiator- and effector-caspase activation by double fluorescence resonance energy transfer analysis

Fluorescence resonance energy transfer (FRET) with green fluorescent protein (GFP) variants has become widely used for biochemical research. In order to expand the choice of fluorescent range in FRET analysis, we designed various color versions of the FRET-based probes for caspase activity, in which the substrate sequence of the caspase was sandwiched by donor and acceptor fluorescent proteins, and studied the potential of these color versions as fluorescent indicators. Six color versions were constructed by a combination of cyan fluorescent protein (CFP), GFP, yellow fluorescent protein (YFP), and DsRed. Real-time monitoring in single cells revealed that all probes could detect caspase activation during tumor necrosis factor (TNF)-alpha-induced cell death as a fluorescent change. GFP-DsRed and YFP-DsRed were as sensitive as CFP-YFP, and CFP-DsRed also showed a large fluorescent change. By using two probes, CFP-DsRed and YFP-DsRed, we carried out simultaneous multi-FRET analysis and revealed that the initiator- and effector-caspases were activated almost simultaneously in TNF-alpha-induced cell death. These findings may give experimental bases for the development of novel techniques to analyze multi-events simultaneously in single cells by using FRET probes in combination.

MaxiK channel-triggered negative feedback system is preserved in the urinary bladder smooth muscle from streptozotocin-induced diabetic rats

MaxiK channel, the large-conductance Ca2+-sensitive K+ channel, facilitates a negative feedback mechanism to oppose excitation and contraction in various types of smooth muscles including urinary bladder smooth muscle (UBSM). In this study, we investigated how the contribution of MaxiK channel to the regulation of basal UBSM mechanical activity is altered in streptozotocin-induced diabetic rats. Although the urinary bladder preparations from both control and diabetic rats were almost quiescent in their basal mechanical activities, they generated spontaneous rhythmic contractions in response to a MaxiK channel blocker, iberiotoxin (IbTx). The effect of IbTx on the mechanical activity was significantly greater in diabetic rat than in control animal. Similarly, the basal mechanical activity was increased with apamin, an inhibitor for some types of small conductance Ca2+-sensitive K+ channels, and this effect was more pronounced for diabetic rat. However, in both control and diabetic animals, IbTx action was stronger than that of apamin. Diabetes also enhanced the responses to BayK 8644, an L-type Ca2+ channel agonist. The extent of this enhancement in diabetic bladder vs. control was, however, almost the same as that attained with IbTx. Expression levels for MaxiK channel as well as apamin-sensitive K+ channels and L-type Ca2+ channel were not altered by diabetes, when determined as their corresponding mRNA levels. These results indicate that diabetes can potentially increase the basal UBSM mechanical activity. However, in diabetic UBSM, the main negative-feedback system triggered by MaxiK channel is still preserved enough to counteract the possible enhancement of this smooth muscle mechanical activity.

Cardioprotection without cardiosuppression by SEA0400, a novel inhibitor of Na+-Ca2+ exchanger, during ischemia and reperfusion in guinea-pig myocardium

The effect of SEA0400, a novel Na+-Ca2+ exchanger inhibitor, on mechanical and electrophysiological parameters of coronary-perfused guinea-pig right ventricular tissue preparation was examined during no-flow ischemia and reperfusion. Contractile force and action potential duration were decreased during no-flow ischemia, while the resting tension was increased. Upon reperfusion, transient arrhythmias were observed and contractile force returned to less than 50% of preischemic values. SEA0400 (1 microM) had no effect on the decline in contractile force during the no-flow ischemia, but abolished the rise in resting tension. SEA0400 significantly improved the recovery of contractile force after reperfusion to about 80% of the preischemic value. SEA0400 had no effect on the action potential under normal conditions and during ischemia, but significantly improved the recovery of action potential duration after reperfusion. Enhancement of the recovery of contractile force during reperfusion by SEA0400 was also observed when the drug was applied only before and during the ischemic period and when the drug was applied only during reperfusion. The present results indicate that inhibition of Na+-Ca2+ exchanger either during ischemia or during reperfusion exerts cardioprotective effects and enhances the recovery of myocardial contractile function.

Role of Nitric Oxide in Ca2+Sensitivity of the Slowly Activating Delayed Rectifier K+Current in Cardiac Myocytes

Sarcolemmal Ca2+ entry is a vital step for contraction of cardiomyocytes, but Ca2+ overload is harmful and may trigger arrhythmias and/or apoptosis. To maintain the amount of Ca2+ entry within an appropriate range, cardiomyocytes have feedback systems that tightly regulate ion channel activities in response to the changes in intracellular Ca2+ concentration ([Ca2+]i), thereby regulating Ca2+ entry. In guinea pig ventricular myocytes, Ca2+ ionophore, A23187, induced suppression of the L-type Ca2+ currents (I(Ca,L)) and enhancement of the slowly activating delayed rectifier K(+) currents (I(Ks)). At a low stimulation rate, I(Ca,L) suppression and I(Ks) enhancement contributed to the A23187-induced APD shortening with a similar magnitude, whereas at a high stimulation rate, I(Ks) enhancement dominantly contributed to APD shortening. I(Ks) enhancement induced by A23187 was attributable to actions of nitric oxide (NO), because they were inhibited by an inhibitor of NO synthase (NOS) and by a NO scavenger. A23187-induced alterations of APD and I(Ks) were strongly suppressed by a NOS3 inhibitor, but barely affected by a NOS1 inhibitor, suggesting that NOS3 was responsible for NO release in this phenomenon. Inhibition of calmodulin (CaM), but not Akt, blocked the enhancement of I(Ks) by A23187. Thus, CaM-dependent NOS3 activation confers the selective Ca2+-sensitivity on I(Ks). Ca2+-induced I(Ks) enhancement and resultant APD shortening potentially act as a physiological regulatory mechanism of Ca2+ recycling, because they were observed at a physiological range of [Ca2+]i in cardiac myocytes and are induced by physiologically relevant Ca2+ loading, such as digitalis application and rise in extracellular Ca2+ concentration.

Pathophysiological Significance of T-type Ca2+ Channels: T-type Ca2+ Channels and Drug Development

T-type Ca(2+) channels are present in cardiovascular, neuronal, and endocrine systems; and they are now receiving attention as novel therapeutic targets. Many drugs and compounds non-specificaly block T-type Ca(2+) channels. Certain dihydropyridine compounds, such as efonidipine, have blocking activity on both L-type and T-type Ca(2+) channels which possibly underlies their excellent clinical profiles such as minimum reflex tachycardia and renal protection. Selective inhibitors of T-type Ca(2+) channels, such as non-hydrolyzable mibefradil and R(-)-efonidipine, are powerful pharmacological tools for further studies and may lead to the development of novel therapeutic strategies.

Lack of action potential-prolonging effect of terfenadine on rabbit myocardial tissue preparations

The effects of terfenadine, an antiallergic drug also known for its QT-prolonging and arrhythmogenic activities, on the action potential of isolated myocardial tissue preparations from rabbits were examined with microelectrode techniques. In the Purkinje fibers and atrium, terfenadine concentration dependently decreased the maximum rate of rise (+.V(max)) without affecting other action potential parameters. In the ventricle, terfenadine had little effect on action potential configuration. In the sinoatrial node, terfenadine 20 microM prolonged cycle length mainly through inhibition of +.V(max). Terfenadine 1 microM completely inhibited the human ether a go-go-related gene (HERG) channel current expressed in HEK293 cells in the same experimental solution as in microelectrode experiments. The lack of terfenadine effect on the action potential duration suggests that there are drugs for which the HERG channel inhibitory action underlying in vivo QT prolongation cannot be evaluated based on their action potential-prolonging activity in isolated myocardial tissue preparations.

MaxiK channel mediates beta2-adrenoceptor-activated relaxation to isoprenaline through cAMP-dependent and -independent mechanisms in guinea-pig tracheal smooth muscle

We examined the contribution of large-conductance, Ca(2+)-sensitive K+ (MaxiK) channel to beta2-adrenoceptor-activated relaxation to isoprenaline in guinea-pig tracheal smooth muscle focusing on the role for cAMP in the coupling between beta2-adrenoceptor and MaxiK channel. Isoprenaline-elicited relaxation was confirmed to be mediated through beta2-type of adrenoceptor since the response was antagonized in a competitive fashion by a beta2-selective adrenoceptor antagonist butoxamine with a pA2 value of 6.56. Isoprenaline-induced relaxation was significantly potentiated by a selective inhibitor of cyclic AMP-specific phosphodiesterase, Ro-20-1724 (0.1-1 microM). cAMP-dependent mediation of MaxiK channel in the relaxant response to isoprenaline was evidenced since the potentiated response to isoprenaline by the presence of Ro-20-1724 (1 microM) was inhibited by the channel selective blocker, iberiotoxin (IbTx, 100 nM). This concept was supported by the finding that the relaxation to a membrane permeable cAMP analogue, 8-bromo-cAMP (1 mM), was susceptible to the inhibition by IbTx. On the other hand, isoprenaline-induced relaxation was not practically diminished by an adenylyl cyclase inhibitor SQ 22,536 (100 microM). However, isoprenaline-induced relaxation in the presence of SQ 22,536 was suppressed by IbTx. Characteristics of isoprenaline-induced relaxant response, i.e., impervious to SQ 22,536 but susceptible to IbTx, were practically mimicked by cholera toxin (CTX, 5 microg/ml), an activator of adenylyl cyclase coupled-heterotrimeric guanine nucleotide-binding regulatory protein Gs. These findings indicate that in guinea-pig tracheal smooth muscle: 1) MaxiK channel substantially mediates beta2-adrenoceptor-activated relaxation; 2) both cAMP-dependent and -independent mechanisms underlie the functional coupling between beta2-adrenoceptor and MaxiK channel to induce muscle relaxation; and 3) direct regulation of MaxiK channel by Gs operates in cAMP-independent coupling between beta2-adrenoceptor and this ion channel.

β1-Subunit of MaxiK Channel in Smooth Muscle: a Key Molecule Which Tunes Muscle Mechanical Activity

The MaxiK channel is the large-conductance, voltage-dependent, and Ca(2+)-activated K(+) channel. This channel is almost ubiquitously distributed among mammalian tissues including smooth muscles. The ability of MaxiK to work as a rheostat fine tuning membrane potential and intracellular Ca(2+) enables it to mediate opposite functions: it facilitates contraction, but also acts as a negative feedback mechanism to restore tone after a contraction cycle. MaxiK activation mediates relaxations to a variety of physiological substances, whereas its inhibition plays a significant role in contractile responses. At the molecular level, MaxiK is a protein complex formed by at least two integral dissimilar membrane subunits, the pore-forming alpha-subunit and a regulatory beta-subunit. In smooth muscles, beta1 is the predominant subunit and most MaxiK seem to be assembled of alpha- and beta1-subunits. The presence of the beta1-subunit confers MaxiK with higher Ca(2+)/voltage sensitivity, which makes this channel an efficient tuner of smooth muscle functions in physiological conditions. The enhanced smooth muscle mechanical activities in mice lacking the beta1-subunit gene support the principal role of this channel molecular component in tissue and whole animal functions. In this review, we discuss MaxiK channel roles as a tuner of smooth muscle contractility, especially focusing attention on the modulatory beta1-subunit.

The R(−)-Enantiomer of Efonidipine Blocks T-type but Not L-type Calcium Current in Guinea Pig Ventricular Myocardium

In guinea pig ventricular cardiomyocytes, the R(-)-enantiomer of efonidipine concentration-dependently blocked T-type Ca2+ current with 85% inhibition at 1 microM. In contrast, R(-)-efonidipine (1 microM) had no effect on the L-type Ca2+ current and Ca2+ transient in cardiomyocytes and contractile force in papillary muscles. Thus, R(-)-efonidipine is a highly selective blocker of the T-type Ca2+ current in native myocardia.

Dementia associated mental and behavioural disturbances in elderly people in the community: findings from the first Nakayama study

OBJECTIVE

To determine the prevalence of mental and behavioural disturbances associated with dementia in elderly people living in the Japanese community of Nakayama.

METHODS

A door to door three phase population survey was carried out on all persons aged 65 years and older living at home. The study included a psychiatric interview, neurological and neuropsychological examination, and cranial computed tomography. Participants with dementia were rated on the neuropsychiatric inventory.

RESULTS

Of 1438 inhabitants, 1162 (81.0%) completed the protocol. The prevalence of dementia was 4.8%. Of the 60 participants with dementia (Alzheimer's disease 35%, vascular dementia 47%, and dementia from other causes 17%), 53 (88.3%) had shown one or more mental and behavioural disturbances. Apathy/indifference (56.7%), followed by agitation/aggression (35%), aberrant motor behaviour (31.7%), and irritability (31.7%) were the common symptoms. More productive (positive) symptoms such as delusions and aberrant motor behaviour were found in the Alzheimer group than in the vascular dementia group.

CONCLUSIONS

A wide range of dementia associated mental and behavioural disturbances developed in the majority of community dwelling individuals with dementia. The findings suggest that a screening programme focusing on identifying these symptoms should be included in the physician's diagnostic tools for dementia.

Effect of Cilnidipine on L- and T-type Calcium Currents in Guinea Pig Ventricle and Action Potential in Rabbit Sinoatrial Node

Cilnidipine, a dihydropyridine Ca(2+) channel antagonist, is known to have inhibitory effects on both L- and N-type Ca(2+) currents. In the present study, we examined the effect of cilnidipine on myocardial L- and T-type Ca(2+) currents and sinoatrial node action potential configuration. In voltage clamped guinea pig ventricular myocytes, cilnidipine concentration-dependently decreased L- and T-type Ca(2+) currents. In rabbit sinoatrial node tissue, cilnidipine increased cycle length through reduction of phase 4 depolarization slope. In conclusion, cilnidipine has inhibitory effects on T-type Ca(2+) current, which may contribute to its negative chronotropic potency.

The mouse sino-atrial node expresses both the type 2 and type 3 Ca(2+) release channels/ryanodine receptors

Ca(2+) released from intracellular Ca(2+) stores is shown to be involved in pacemaker activity in the sino-atrial (SA)-node. However, little is known about the molecular identity of the Ca(2+) release channel/ryanodine receptor (RYR) involved in pacemaker activity. We examined the mRNA distribution of three different RYR isoforms (RYR1, RYR2, and RYR3) in the mouse SA-node. RNase protection assay and in situ hybridization revealed that RYR2 mRNA expresses widely in the heart including the SA-node, while RYR3 mRNA expression is limited to the SA-node and to the right atrium. Thus, not only RYR2 but also RYR3 may participate in pacemaker activity.

[Molecular mechanisms for cAMP-independent activation of MaxiK channel: G protein direct channel regulation and contribution to the tuning of vascular smooth muscle tone]

MaxiK channel, the large conductance Ca(2+)-sensitive K+ channel, is expressed abundantly in vascular smooth muscles and plays a key role in the tuning of their excitability and contractility. The present study was carried out to elucidate the contribution of MaxiK channel to prostacyclin receptor (IP receptor)-mediated vascular relaxation with a special reference to the role of cAMP. An IP agonist, beraprost, induced a strong relaxation in de-endothelialized guinea pig thoracic aorta, which was almost abolished by a MaxiK channel selective blocker, iberiotoxin (IbTx). Beraprost produced a 30-fold rise in tissue cAMP contents. In addition, beraprost-induced relaxation potentiated in the presence of Ro-20-1724 (a selective inhibitor of cAMP-specific phosphodiesterase) was completely counteracted by IbTx. However, beraprost-induced relaxation was not affected by SQ22,536, an adenylyl cyclase inhibitor, which abolished this IP agonist-induced elevation of cAMP contents. SQ22,536-insensitive relaxant component was significantly inhibited by IbTx. Cholera toxin, a Gs activator, qualitatively mimicked the effects of beraprost. Furthermore, MaxiK channel currents in aortic myocytes were increased by beraprost in a GTP-dependent manner. These results indicate that both cAMP-dependent and -independent pathways contribute to MaxiK channel-mediated vascular relaxation following IP receptor stimulation. Direct regulation by Gs seems to partly account for MaxiK channel-mediated, cAMP-independent mechanism.

Evidence for a significant role of a Gs-triggered mechanism unrelated to the activation of adenylyl cyclase in the cyclic AMP-independent relaxant response of guinea-pig tracheal smooth muscle

Cyclic AMP is a key molecule in the regulation of airway smooth muscle tone. Increased cyclic AMP leads to relaxation of this smooth muscle and its inhibition results in the muscle contraction. A constitutive role for cyclic AMP in the contraction and relaxation of airway muscle is supported by the observations that direct activators of adenylyl cyclase, such as forskolin and membrane-permeable cyclic AMP analogues, relax this smooth muscle potently. This traditional view of the role for cyclic AMP is the basis for the idea that relaxation of airway smooth muscle mediated through adenylyl cyclase-linked, G(s)-coupled receptors, including the beta(2)-adrenoceptor, is achieved mainly by the elevation of cyclic AMP content [cyclic AMP-dependent mechanism(s)]. However, recent pharmacological and biochemical evidence raises a fundamental question concerning the role of cyclic AMP; can G(s)-coupled receptor-mediated relaxation of tracheal smooth muscle be attributed exclusively to cyclic AMP-dependent mechanism(s)? In the present study, we show that cholera toxin (CTX, 5 microg/ml), an activator of the heterotrimeric guanine-nucleotide-binding protein G(s), relaxes guinea-pig tracheal smooth muscle. CTX also elevates tissue cyclic AMP content by about 30-fold and this is practically abolished by an adenylyl cyclase inhibitor, SQ 22,536 (100 microM). However, unexpectedly, the relaxant response to CTX is not affected by SQ 22,536. These results firstly show that activation of G(s) is able to produce a relaxation in tracheal smooth muscle independently of the elevation of cyclic AMP. G(s)-triggered, cyclic AMP-unrelated cellular mechanism(s) seem(s) to play a substantial role in smooth muscle relaxation mediated through adenylyl cyclase-linked receptors. This mechanism may account in part for the cyclic AMP-independent relaxant response of tracheal smooth muscle.

Phospholipase C inhibitors suppress spontaneous mechanical activity of guinea pig urinary bladder smooth muscle

Urinary bladder smooth muscle (UBSM) exhibits spontaneous rhythmic contraction. This spontaneous mechanical activity is generated in the presence of neuronal blockade and thus is myogenic in origin. The spontaneous myogenic contraction of UBSM may be the fundamental determinant of the physiological functions of the urinary bladder to store and excrete urine. Although the mechanisms by which UBSM generates spontaneous contraction have not been completely ascertained, its induction has been suggested to be intimately associated with smooth muscle cell action potentials to enhance extracellular Ca(2+) influx through voltage-gated L-type Ca(2+) channels. However, the alteration of membrane electrical activity does not seem to be the exclusive trigger mechanism for the generation of the spontaneous contraction. In the present study, we show that spontaneous mechanical activity of guinea pig UBSM is substantially diminished by an inhibitor of phospholipase C (PLC), U-73122, but is not affected by its inactive form, U-73343. Significant attenuation of the mechanical activity can be also obtained with another PLC inhibitor 2-nitro-4-carboxyphenyl-N,N-diphenylcarbamate. Our present findings suggest a significant role for the activation of PLC and subsequent inositol 1,4,5-trisphosphate-induced Ca(2+) release mechanism as an alternative triggering system for inducing spontaneous mechanical activity of UBSM. The present results support the idea that the action potential is not the sole pacemaker mechanism by which spontaneous contraction is induced in UBSM.

Inhibition of agonist-induced positive inotropy by a selective Rho-associated kinase inhibitor, Y-27632

We examined the effect of Y-27632 ((+)-(R)-trans-4-(1-aminoethyl)-N-(4-pyridyl)cyclo-hexanecarboxamide), a selective Rho-associated kinase (ROCK) inhibitor, on agonist-induced inotropy in isolated mouse left atria. Endothelin-1, angiotensin-II, and prostaglandin F(2)(alpha) (PGF(2)(alpha)) produced positive inotropy, which was significantly attenuated by Y-27632 (100 microM). On the other hand, isoproterenol-induced positive inotropy was not attenuated by the drug. These results provide the first evidence that the Rho/ROCK pathway is involved in endothelin-1-, angiotensin-II-, and PGF(2)(alpha)-induced positive inotropy, but not in beta-adrenoceptor-mediated positive inotropy.

Pharmacological evidence for involvement of phospholipase D, protein kinase C, and sodium-calcium exchanger in alpha-adrenoceptor-mediated negative inotropy in adult mouse ventricle

The intracellular signalling pathway for alpha-adrenoceptor-mediated negative inotropy was studied pharmacologically in isolated adult mouse ventricle. The negative inotropy was inhibited by GF-109203X, a nonselective protein kinase C inhibitor. Phorbol 12-myristate 13-acetate also produced sustained negative inotropy, which was inhibited by KB-R7943, a Na(+)/Ca(2+) exchanger inhibitor. The alpha-adrenoceptor-mediated negative inotropy was augmented by RHC-80267, a diacylglycerol lipase inhibitor, but was inhibited either by C(2)-ceramide, a phospholipase D inhibitor, and high concentration of propranolol (50 micro M), which inhibits phosphatidate phosphohydrolase. The inotropy was not affected by U-73122, a phospholipase C inhibitor. Lavendustin-A, a tyrosine kinase inhibitor, also inhibited the negative inotropy. These findings suggest that alpha-adrenoceptor-mediated negative inotropy in adult mouse ventricle is mediated by activation of tyrosine kinase, the phospholipase D-phosphatidate phosphohydrolase pathway, and protein kinase C.

Possible involvement of prostaglandins F(2alpha) and D(2) in acetylcholine-induced positive inotropy in isolated mouse left atria

The inotropic action of prostaglandins PGF(2alpha), PGD(2) and PGE(2) on isolated mouse left atria was characterized and compared with the positive inotropic action of acetylcholine, which has previously been shown to be mediated by prostaglandins released from the endocardial endothelium. PGF(2alpha), PGD(2) and PGE(2) produced positive inotropic responses; the time course of the change in contractile force induced by PGF(2alpha) and PGD(2) was about the same as that by acetylcholine, while that by PGE(2) was slower. Fluprostenol and sulprostone, FP and EP receptor agonists, respectively, had positive inotropic effects while BW-245C, a DP receptor agonist, had no effect. AH-6809, a DP receptor antagonist, had no inhibitory effect on the positive inotropic response to PGD(2). Dimethylamiloride, an inhibitor of Na(+)/H(+) exchange, inhibited the positive inotropic response to PGF(2alpha), PGD(2) and acetylcholine, but not PGE(2). Fluorometric pH measurement with carboxy-SNARF-1-loaded atrial myocytes revealed no change in intracellular pH on application of PGF(2alpha). PGF(2alpha) and PGD(2) significantly prolonged the duration of the atrial action potential while PGE(2) had no significant effect. These findings suggest that prostaglandins induce positive inotropic response in mouse atria through FP and EP receptor stimulation and that the former mechanism mediates in part the positive inotropic response to acetylcholine.

Elucidation with the protease alpha-chymotrypsin of the inhibitory modulating action of endogenous neuropeptide Y over sympathetic neurotransmission in rat vas deferens

The physiological role of endogenous neuropeptide Y (NPY) in sympathetic neurotransmission was examined in rat and guinea pig vas deferens (VD), using alpha-chymotrypsin (alpha-CT). NPY-like immunoreactivity was detected in the longitudinal muscle layer of VD densely in rats but sparsely in guinea pigs, and it disappeared following surgical denervation. Under blockade of the prejunctional alpha(2)-adrenergic autoinhibition, alpha-CT potentiated the phasic contraction in rat, but not guinea pig, VD induced by trains of transmural nerve stimulation (TNS) in a frequency-dependent manner, which was reproducible during repeated applications and not affected by pretreatment with capsaicin. In contrast, alpha-CT did not potentiate the twitch response or contractions induced respectively by a single pulse TNS or by direct electrical stimulation to the smooth muscle. Exogenously applied NPY suppressed the twitch response, which was cancelled by alpha-CT, and excitatory junction potentials, although it affected neither spontaneous junction potentials nor the direct electrical stimulation-induced contraction. These observations provided further evidence to support that NPY is released endogenously by TNS at high frequency, acting prejunctionally to suppress sympathetic neurotransmission. Thus, the protease alpha-CT proved itself to be a useful tool to reveal a functional role of endogenously released peptides.

Bk(Ca) channel activity enhances with muscle stretch in guinea-pig urinary bladder smooth muscle

Possible effects of muscle stretch on the spontaneous rhythmic contractile activity and the extent of contribution of voltage-dependent and Ca2+ -activated K+ (BK(Ca)) channels were examined mechanically in guinea-pig urinary bladder smooth muscle (UBSM). The amplitude of the spontaneous mechanical activity showed stretch-dependency. Iberiotoxin, a selective blocker of BK(Ca) channels, potently increased contraction amplitude, and this effect was more prominent when muscle length was more than twice of its initial length. BK(Ca) channels seem activated more strongly to counteract enhanced spontaneous mechanical activity with UBSM stretch.

Optical Bioimaging: From Living Tissue to a Single Molecule: Atrio-Ventricular Difference in Myocardial Excitation-Contraction Coupling — Sequential Versus Simultaneous Activation of SR Ca2+ Release Units —

Rapid-scanning cofocal microscopy has been applied to the analysis of early phase Ca(2+) transients in ventricular and atrial cardiomyocytes. On electrical stimulation of ventricular myocytes, Ca(2+) concentration begins to rise earliest at the Z-line level and becomes uniform throughout the cytoplasm within about 10 ms after the onset of the action potential; transsarcolemmal Ca(2+) influx triggers Ca(2+) release from release sites on the junctional sarcoplasmic reticulum (SR) coupled to T-tubules at the Z-line throughout the cytoplasm. In atrial myocytes lacking the T-tubular network, transsarcolemmal Ca(2+) influx during an action potential triggers SR Ca(2+) release only at subsarcolemmal region. SR Ca(2+) release then spreads towards the central region of the cell through a propagated Ca(2+)-induced-Ca(2+) release mechanism. The atrio-ventricular difference in excitation-contraction coupling mechanisms underlies some of the atrio-ventricular difference in response to physiological and pharmacological stimuli.

Pharmacological evidence that tetraethylammonium-sensitive, iberiotoxin-insensitive K+ channels function as a negative feedback element for sympathetic neurotransmission by suppressing omega-conotoxin-GVIA-insensitive Ca2+ channels in the relaxation of rabbit facial vein

The facial vein isolated from various species relaxes in response to electrical field stimulation (EFS). EFS-elicited relaxation of the facial vein is mediated through the release of noradrenaline (NA) from sympathetic nerve endings and the subsequent activation of smooth muscle beta-adrenoceptors. The release of NA from sympathetic nerve endings in arterial tissues requires transmembrane Ca2+ influx, mediated predominantly by voltage-gated N-type Ca2+ channels. The present pharmaco-mechanical study was undertaken to determine whether the N-type channel is the exclusive pre-junctional Ca2+ channel mediating NA release from sympathetic nerve endings in the rabbit facial vein. Possible roles of K+ channels in the sympathetic neurotransmission were also examined, especially focusing on the contribution of voltage-dependent, Ca2+-activated K+ (BKCa) channels. An isolated ring preparation of the rabbit facial vein exhibited intrinsic myogenic tone which lasted for several hours when stretched. EFS produced frequency-dependent (0.25-2 Hz) relaxation in this preparation. EFS-elicited relaxation was abolished by tetrodotoxin (TTX, 1 microM), guanethidine (5 microM) or propranolol (1 microM), indicating that NA released from sympathetic nerve endings was mediating the relaxant response. NA-mediated neurogenic relaxation was almost eliminated by omega-conotoxin-GVIA (1 microM), an N-type Ca2+ channel blocker. On the other hand, tetraethylammonium (TEA, 2 mM) strongly potentiated EFS-elicited relaxation without affecting the relaxation induced by exogenously applied NA. This potentiation by TEA was not profoundly diminished by omega-conotoxin-GVIA (1 microM) alone or omega-conotoxin-GVIA (1 microM) plus omega-agatoxin IVA (10 nM, P-type channel blocker), but was almost abolished by omega-conotoxin-GVIA (1 microM) plus omega-agatoxin IVA (10 nM) plus omega-conotoxin-MVIIC (3 microM, N-, P- and Q-type channel blocker). The potentiating effect of TEA was not mimicked by iberiotoxin (100 nM) or charybdotoxin (3 microM), both of which block BKCa channels. These findings suggest that pre-junctional N-type Ca2+ channels play the predominant role in the sympathetic nerve transmission in the rabbit facial vein, as in peripheral arterial vascular beds. In addition, Ca2+ channels resistant to 1 microM omega-conotoxin-GVIA, most probably Q-type channels, appear to be present at the sympathetic nerve endings in the rabbit facial vein and contribute substantially to the regulation of NA release from the nerve endings. Prejunctional K+ channels, sensitive to TEA but pharmacologically distinct from iberiotoxin-sensitive BKCa channels, seem to be functionally coupled intimately with the omega-conotoxin-GVIA-resistant Ca2+ channels, and thus function as a negative feedback element in sympathetic neurotransmission in the rabbit facial vein.