Search

Aims Several inherited arrhythmic diseases have been linked to single gene mutations in cardiac ion channels and interacting proteins. However, the mechanisms underlying most arrhythmias, are thought to involve altered regulation of the expression of multiple effectors. In this study, we aimed to examine the role of a transcription factor (TF) belonging to the Iroquois homeobox family, IRX5, in cardiac electrical function. Methods and results Using human cardiac tissues, transcriptomic correlative analyses between IRX5 and genes involved in cardiac electrical activity showed that in human ventricular compartment, IRX5 expression strongly correlated to the expression of major actors of cardiac conduction, including the sodium channel, Nav1.5, and Connexin 40 (Cx40). We then generated human-induced pluripotent stem cells (hiPSCs) derived from two Hamamy syndrome-affected patients carrying distinct homozygous loss-of-function mutations in IRX5 gene. Cardiomyocytes derived from these hiPSCs showed impaired cardiac gene expression programme, including misregulation in the control of Nav1.5 and Cx40 expression. In accordance with the prolonged QRS interval observed in Hamamy syndrome patients, a slower ventricular action potential depolarization due to sodium current reduction was observed on electrophysiological analyses performed on patient-derived cardiomyocytes, confirming the functional role of IRX5 in electrical conduction. Finally, a cardiac TF complex was newly identified, composed by IRX5 and GATA4, in which IRX5 potentiated GATA4-induction of SCN5A expression. Conclusion Altogether, this work unveils a key role for IRX5 in the regulation of human ventricular depolarization and cardiac electrical conduction, providing therefore new insights into our understanding of cardiac diseases. [ABSTRACT FROM AUTHOR]

Background and Purpose: Protoxin II (ProTx II) is a high affinity gating modifier that is thought to selectively block the Nav 1.7 voltage-dependent Na+ channel, a major therapeutic target for the control of pain. We aimed at producing ProTx II analogues entitled with novel functionalities for cell distribution studies and biochemical characterization of its Nav channel targets.Experimental Approach: We took advantage of the high affinity properties of the peptide, combined to its slow off rate, to design a number of new tagged analogues useful for imaging and biochemistry purposes. We used high-throughput automated patch-clamp to identify the analogues best matching the native properties of ProTx II and validated them on various Nav -expressing cells in pull-down and cell distribution studies.Key Results: Two of the produced ProTx II analogues, Biot-ProTx II and ATTO488-ProTx II, best emulate the pharmacological properties of unlabelled ProTx II, whereas other analogues remain high affinity blockers of Nav 1.7. The biotinylated version of ProTx II efficiently works for the pull-down of several Nav isoforms tested in a concentration-dependent manner, whereas the fluorescent ATTO488-ProTx II specifically labels the Nav 1.7 channel over other Nav isoforms tested in various experimental conditions.Conclusions and Implications: The properties of these ProTx II analogues as tools for Nav channel purification and cell distribution studies pave the way for a better understanding of ProTx II channel receptors in pain and their pathophysiological implications in sensory neuronal processing. The new fluorescent ProTx II should also be useful in the design of new drug screening strategies. [ABSTRACT FROM AUTHOR]

The patch-clamp technique and more recently the high throughput patch-clamp technique have contributed to major advances in the characterization of ion channels. However, the whole-cell voltage-clamp technique presents certain limits that need to be considered for robust data generation. One major caveat is that increasing current amplitude profoundly impacts the accuracy of the biophysical analyses of macroscopic ion currents under study. Using mathematical kinetic models of a cardiac voltage-gated sodium channel and a cardiac voltage-gated potassium channel, we demonstrated how large current amplitude and series resistance artefacts induce an undetected alteration in the actual membrane potential and affect the characterization of voltage-dependent activation and inactivation processes. We also computed how dose–response curves are hindered by high current amplitudes. This is of high interest since stable cell lines frequently demonstrating high current amplitudes are used for safety pharmacology using the high throughput patch-clamp technique. It is therefore critical to set experimental limits for current amplitude recordings to prevent inaccuracy in the characterization of channel properties or drug activity, such limits being different from one channel type to another. Based on the predictions generated by the kinetic models, we draw simple guidelines for good practice of whole-cell voltage-clamp recordings. [ABSTRACT FROM AUTHOR]

Prokaryotic NaV channels are tetramers and eukaryotic NaV channels consist of a single subunit containing four domains. Each monomer/domain contains six transmembrane segments (S1-S6), S1-S4 being the voltage-sensor domain and S5-S6 the pore domain. A crystal structure of NaVMs, a prokaryotic NaV channel, suggests that the S4-S5 linker (S4-S5L) interacts with the C-terminus of S6 (S6T) to stabilize the gate in the open state. However, in several voltage-gated potassium channels, using specific S4-S5L-mimicking peptides, we previously demonstrated that S4-S5L/S6T interaction stabilizes the gate in the closed state. Here, we used the same strategy on another prokaryotic NaV channel, NaVSp1, to test whether equivalent peptides stabilize the channel in the open or closed state. A NaVSp1-specific S4-S5L peptide, containing the residues supposed to interact with S6T according to the NaVMs structure, induced both an increase in NaVSp1 current density and a negative shift in the activation curve, consistent with S4-S5L stabilizing the open state. Using this approach on a human NaV channel, hNaV1.4, and testing 12 hNaV1.4 S4-S5L peptides, we identified four activating S4-S5L peptides. These results suggest that, in eukaryotic NaV channels, the S4-S5L of DI, DII and DIII domains allosterically modulate the activation gate and stabilize its open state. [ABSTRACT FROM AUTHOR]

Aims The Brugada syndrome (BrS) is an inherited cardiac disorder predisposing to ventricular arrhythmias. Despite considerable efforts, its genetic basis and cellular mechanisms remain largely unknown. The objective of this study was to identify a new susceptibility gene for BrS through familial investigation. Methods and results Whole-exome sequencing performed in a three-generation pedigree with five affected members allowed the identification of one rare non-synonymous substitution (p.R211H) in RRAD , the gene encoding the RAD GTPase, carried by all affected members of the family. Three additional rare missense variants were found in 3/186 unrelated index cases. We detected higher levels of RRAD transcripts in subepicardium than in subendocardium in human heart, and in the right ventricle outflow tract compared to the other cardiac compartments in mice. The p.R211H variant was then subjected to electrophysiological and structural investigations in human cardiomyocytes derived from induced pluripotent stem cells (iPSC-CMs). Cardiomyocytes derived from induced pluripotent stem cells from two affected family members exhibited reduced action potential upstroke velocity, prolonged action potentials and increased incidence of early afterdepolarizations, with decreased Na+ peak current amplitude and increased Na+ persistent current amplitude, as well as abnormal distribution of actin and less focal adhesions, compared with intra-familial control iPSC-CMs Insertion of p.R211H- RRAD variant in control iPSCs by genome editing confirmed these results. In addition, iPSC-CMs from affected patients exhibited a decreased L-type Ca2+ current amplitude. Conclusion This study identified a potential new BrS-susceptibility gene, RRAD. Cardiomyocytes derived from induced pluripotent stem cells expressing RRAD variant recapitulated single-cell electrophysiological features of BrS, including altered Na+ current, as well as cytoskeleton disturbances. Open in new tab Download slide Open in new tab Download slide [ABSTRACT FROM AUTHOR]

Mutations in voltagegated Naα channels have been linked to several channelopathies leading to a wide variety of diseases including cardiac arrhythmias, epilepsy, and myotonia. We have previously demonstrated that voltage- gated Na± channel (Nav)1.5 trafficking-deficient mutant channels could lead to a dominant negative effect by impairing trafficking of the wild-type (WT) channel. We also reported that voltage-gated Na± channels associate as dimers with coupled gating properties. Here, we hypothesized that the dominant negative effect of mutant Na± channels could also occur through coupled gating. This was tested using cell surface biotinylation and single channel recordings to measure the gating probability and coupled gating of the dimers. As previously reported, coexpression of Nav1.5-L325R with WT channels led to a dominant negative effect, as reflected by a 75% reduction in current density. Surprisingly, cell surface biotinylation showed that Nav1.5- L325R mutant is capable of trafficking, with 40% of Nav1.5-L325R reaching the cell surface when expressed alone. Importantly, even though a dominant negative effect on the Na± current is observed when WT and Nav1.5-L325R are expressed together, the total Nav channel cell surface expression was not significantly altered compared with WT channels alone. Thus, the trafficking deficiency could not explain the 75% decrease in inward Na± current. Interestingly, single channel recordings showed that Nav1.5-L325R exerted a dominant negative effect on the WT channel at the gating level. Both coupled gating and gating probability of WT:L325R dimers were drastically impaired. We conclude that dominant negative suppression exerted by Nav1.5 mutants can also be caused by impairing the WT gating probability, a mechanism resulting from the dimerization and coupled gating of voltage-gated Na± channel α-subunits. [ABSTRACT FROM AUTHOR]

Fast opening and closing of voltage-gated sodium channels are crucial for proper propagation of the action potential through excitable tissues. Unlike potassium channels, sodium channel α-subunits are believed to form functional monomers. Yet, an increasing body of literature shows inconsistency with the traditional idea of a single α-subunit functioning as a monomer. Here we demonstrate that sodium channel α-subunits not only physically interact with each other but they actually assemble, function and gate as a dimer. We identify the region involved in the dimerization and demonstrate that 14-3-3 protein mediates the coupled gating. Importantly we show conservation of this mechanism among mammalian sodium channels. Our study not only shifts conventional paradigms in regard to sodium channel assembly, structure, and function but importantly this discovery of the mechanism involved in channel dimerization and biophysical coupling could open the door to new approaches and targets to treat and/or prevent sodium channelopathies. [ABSTRACT FROM AUTHOR]

Mutations in Nav1.4 and Nav1.5 α-subunits have been associated with muscular and cardiac channelopathies, respectively. Despite intense research on the structure and function of these channels, a lot of information is still missing to delineate the various physiological and pathophysiological processes underlying their activity at the molecular level. Nav1.4 and Nav1.5 sequences are similar, suggesting structural and functional homologies between the two orthologous channels. This also suggests that any characteristics described for one channel subunit may shed light on the properties of the counterpart channel subunit. In this review article, after a brief clinical description of the muscular and cardiac channelopathies related to Nav1.4 and Nav1.5 mutations, respectively, we compare the knowledge accumulated in different aspects of the expression and function of Nav1.4 and Nav1.5 α-subunits: the regulation of the two encoding genes (SCN4A and SCN5A), the associated/regulatory proteins and at last, the functional effect of the same missense mutations detected in Nav1.4 and Nav1.5. First, it appears that more is known on Nav1.5 expression and accessory proteins. Because of the high homologies of Nav1.5 binding sites and equivalent Nav1.4 sites, Nav1.5-related results may guide future investigations on Nav1.4. Second, the analysis of the same missense mutations in Nav1.4 and Nav1.5 revealed intriguing similarities regarding their effects on membrane excitability and alteration in channel biophysics. We believe that such comparison may bring new cues to the physiopathology of cardiac and muscular diseases. [ABSTRACT FROM AUTHOR]

Mutations in Nav1.4 and Nav1.5 α-subunits have been associated with muscular and cardiac channelopathies, respectively. Despite intense research on the structure and function of these channels, a lot of information is still missing to delineate the various physiological and pathophysiological processes underlying their activity at the molecular level. Nav1.4 and Nav1.5 sequences are similar, suggesting structural and functional homologies between the two orthologous channels. This also suggests that any characteristics described for one channel subunit may shed light on the properties of the counterpart channel subunit. In this review article, after a brief clinical description of the muscular and cardiac channelopathies related to Nav1.4 and Nav1.5 mutations, respectively, we compare the knowledge accumulated in different aspects of the expression and function of Nav1.4 and Nav1.5 α-subunits: the regulation of the two encoding genes (SCN4A and SCN5A), the associated/regulatory proteins and at last, the functional effect of the same missense mutations detected in Nav1.4 and Nav1.5. First, it appears that more is known on Nav1.5 expression and accessory proteins. Because of the high homologies of Nav1.5 binding sites and equivalent Nav1.4 sites, Nav1.5-related results may guide future investigations on Nav1.4. Second, the analysis of the same missense mutations in Nav1.4 and Nav1.5 revealed intriguing similarities regarding their effects on membrane excitability and alteration in channel biophysics. We believe that such comparison may bring new cues to the physiopathology of cardiac and muscular diseases. [ABSTRACT FROM AUTHOR]

Introduction: Phosphatidylinositol-4,5-bisphosphate (PIP2) is a cofactor necessary for the activity of KCNQ1 channels. Some Long QT mutations of KCNQ1, including R243H, R539W and R555C have been shown to decrease KCNQ1 interaction with PIP2. A previous study suggested that R539W is paradoxically less sensitive to intracellular magnesium inhibition than the WT channel, despite a decreased interaction with PIP2. In the present study, we confirm this peculiar behavior of R539W and suggest a molecular mechanism underlying it. Methods and Results: COS-7 cells were transfected with WT or mutated KCNE1-KCNQ1 channel, and patch-clamp recordings were performed in giant-patch, permeabilized-patch or ruptured-patch configuration. Similar to other channels with a decreased PIP2 affinity, we observed that the R243H and R555C mutations lead to an accelerated current rundown when membrane PIP2 levels are decreasing. As opposed to R243H and R555C mutants, R539W is not more but rather less sensitive to PIP2 decrease than the WT channel. A molecular model of a fragment of the KCNQ1 C-terminus and the membrane bilayer suggested that a potential novel interaction of R539W with cholesterol stabilizes the channel opening and hence prevents rundown upon PIP2 depletion. We then carried out the same rundown experiments under cholesterol depletion and observed an accelerated R539W rundown that is consistent with this model. Conclusions: We show for the first time that a mutation may shift the channel interaction with PIP2 to a preference for cholesterol. This de novo interaction wanes the sensitivity to PIP2 variations, showing that a mutated channel with a decreased affinity to PIP2 could paradoxically present a slowed current rundown compared to the WT channel. This suggests that caution is required when using measurements of current rundown as an indicator to compare WT and mutant channel PIP2 sensitivity. [ABSTRACT FROM AUTHOR]

The channel pore-forming α subunit Kv4.2 is a major constituent of A-type (IA) potassium currents and a key regulator of neuronal membraneexcitability. Multiple mechanisms regulate the properties, subcellular targeting, and cell-surface expression of Kv4.2-encoded channels. In the present study, shotgun proteomic analyses of immunoprecipitated mouse brain Kv4.2 channel complexes unexpectedly identified the voltage-gated Na+ channel accessory subunit Navβ1. Voltage-clamp and current-clamp recordings revealed that knockdown of Nav&ngr;1 decreases IA densities in isolated cortical neurons and that action potential waveforms are prolonged and repetitive firing is increased in Scn1b-null cortical pyramidal neurons lacking Navβ1. Biochemical and voltage-clamp experiments further demonstrated that Navβ1 interacts with and increases the stability of the heterologously expressed Kv4.2 protein, resulting in greater total and cell-surface Kv4.2 protein expression and in larger Kv4.2-encoded current densities. Together, the results presented here identify Navβ1 as a component of native neuronal Kv4.2-encoded IA channel complexes and a novel regulator of IA channel densities and neuronal excitability. [ABSTRACT FROM AUTHOR]

Pathological biomechanical stresses cause cardiac hypertrophy, which is associated with QT prolongation and arrhythmias. Previous studies have demonstrated that repolarizing K current densities are decreased in pressure overload-induced left ventricular hypertrophy, resulting in action potential and QT prolongation. Cardiac hypertrophy also occurs with exercise training, but this physiological hypertrophy is not associated with electrical abnormalities or increased arrhythmia risk, suggesting that repolarizing K currents are upregulated, in parallel with the increase in myocyte size, to maintain normal cardiac function. To explore this hypothesis directly, electrophysiological recordings were obtained from ventricular myocytes isolated from two mouse models of physiological hypertrophy, one produced by swim-training of wild-type mice and the other by cardiac-specific expression of constitutively active phosphoinositide-3-kinase-p110α (caPI3Kα). Whole-cell voltage-clamp recordings revealed that repolarizing K current amplitudes were higher in ventricular myocytes isolated from swim-trained and caPI3Kα, compared with wild-type, animals. The increases in K current amplitudes paralleled the observed cellular hypertrophy, resulting in normalized or increased K current densities. Electrocardiographic parameters, including QT intervals, as well as ventricular action potential waveforms in swim-trained animals/myocytes were indistinguishable from controls, demonstrating preserved electrical function. Additional experiments revealed that inward Ca current amplitudes/densities were also increased in caPI3Kα, compared with WT, left ventricular myocytes. The expression of transcripts encoding K, Ca and other ion channel subunits was increased in swim-trained and caPI3Kα ventricles, in parallel with the increase in myocyte size and with the global increases in total cellular RNA expression. In contrast to pathological hypertrophy, therefore, the functional expression of repolarizing K (and depolarizing Ca) channels is increased with physiological hypertrophy, reflecting upregulation of the underlying ion channel subunit transcripts and resulting in increased current amplitudes and the normalization of current densities and action potential waveforms. Taken together, these results suggest that activation of PI3Kα signalling preserves normal myocardial electrical functioning and could be protective against the increased risk of arrhythmias and sudden death that are prevalent in pathological cardiac hypertrophy. Cardiac hypertrophy is an adaptive response of the heart to increased pressure or volume, and is observed in a variety of cardiac diseases, including in individuals with high blood pressure and/or who have recently experienced a heart attack. This pathological hypertrophy is associated with marked changes in the electrical properties of the heart, predisposing afflicted individuals to life threatening cardiac arrhythmias. Interestingly, hypertrophy is also observed with exercise, particularly in trained athletes, and, in contrast with pathological hypertrophy, physiological hypertrophy is not associated with increased arrhythmia risk. The results presented here demonstrate that ion channel expression is upregulated with exercise and with the activation of metabolic signalling pathways normally triggered by exercise to maintain the normal electrical functioning of the myocardium. These observations suggest that activation of the pathways triggered in physiological hypertrophy may provide a novel therapeutic strategy to reduce or prevent arrhythmias in patients with pathological hypertrophy. [ABSTRACT FROM AUTHOR]

Background: Loss-of-function mutations in SCN5A, the gene encoding Nav1.5 NaP+ channel, are associated with inherited cardiac conduction defects and Brugada syndrome, which both exhibit variable phenotypic penetrance of conduction defects. We investigated the mechanisms of this heterogeneity in a mouse model with heterozygous targeted disruption of Scn5a (Scn5a+/- mice) and compared our results to those obtained in patients with loss-of-function mutations in SCN5A. Methodology/Principal Findings: Based on ECG, 10-week-old Scn5a+/- mice were divided into 2 subgroups, one displaying severe ventricular conduction defects (QRS interval>18 ms) and one a mild phenotype (QRS≤18 ms; QRS in wild-type littermates: 10-18 ms). Phenotypic difference persisted with aging. At 10 weeks, the Na+ channel blocker ajmaline prolonged QRS interval similarly in both groups of Scn5a+/- mice. In contrast, in old mice (>53 weeks), ajmaline effect was larger in the severely affected subgroup. These data matched the clinical observations on patients with SCN5A loss-of-function mutations with either severe or mild conduction defects. Ventricular tachycardia developed in 5/10 old severely affected Scn5a+/- mice but not in mildly affected ones. Correspondingly, symptomatic SCN5A-mutated Brugada patients had more severe conduction defects than asymptomatic patients. Old severely affected Scn5a+/-mice but not mildly affected ones showed extensive cardiac fibrosis. Mildly affected Scn5a+/- mice had similar Nav1.5 mRNA but higher Nav1.5 protein expression, and moderately larger INa current than severely affected Scn5a+/- mice. As a consequence, action potential upstroke velocity was more decreased in severely affected Scn5a+/- mice than in mildly affected ones. Conclusions: Scn5a+/- mice show similar phenotypic heterogeneity as SCN5A-mutated patients. In Scn5a+/- mice, phenotype severity correlates with wild-type Nav1.5 protein expression. [ABSTRACT FROM AUTHOR]

Abstract: Objective: The voltage-gated K+ channel KCNQ1 associates with the small KCNE1 β subunit to underlie the IKs repolarizing current in the heart. Based on sequence homology, the KCNE family is recognized to comprise five members. Controversial data have indicated their participation in several K+ channel protein complexes, including KCNQ1. The expression level and the putative functions of the different KCNE subunits in the human heart still require further investigation. Methods: We have carried out a comparative study of all KCNE subunits with KCNQ1 using the patch-clamp technique in mammalian cells. Real-time RT-PCR absolute quantification was performed on human atrial and ventricular tissue. Results: While KCNQ1/KCNE1 heteromultimer reached high current density with slow gating kinetics and pronounced voltage dependence, KCNQ1/KCNE2 and KCNQ1/KCNE3 complexes produced instantaneous voltage-independent currents with low and high current density, respectively. Co-expression of KCNE4 or KCNE5 with KCNQ1 induced small currents in the physiological range of voltages, with kinetics similar to those of the KCNQ1/KCNE1 complex. However, co-expression of these inhibitory subunits with a disease-associated mutation (S140G-KCNQ1) led to currents that were almost undistinguishable from the KCNQ1/KCNE1 canonical complex. Absolute cDNA quantification revealed a relatively homogeneous distribution of each transcript, except for KCNE4, inside left atria and endo- and epicardia of left ventricular wall with the following abundance: KCNQ1≫KCNE4≥KCNE1>KCNE3>KCNE2>KCNE5. KCNE4 expression was twice as high in atrium compared to ventricle. Conclusions: Our data show that KCNQ1 forms a channel complex with 5 KCNE subunits in a specific manner but only interactions with KCNE1, KCNE2, and KCNE3 may have physiological relevance in the human heart. [Copyright &y& Elsevier]

Abstract: Ion channels are an ensemble of specialized membrane proteins that act in concert to create and modulate the electrical activity of many excitable cells, including cardiac myocytes. Following completion of the sequencing of various genomes, including that of the human, the complete repertoire of ion channel genes has been elucidated for different species. How transcripts issued from this gene collection are expressed and modulated in relation to variable physiological and pathological situations is the subject of functional or physiological genomics. Specialized microarrays (IonChips) comprising probes for the ensemble of ion channel and regulatory genes were developed as an alternative to whole-genome DNA chips. Physiological genomics of cardiac ion channel genes is a growing field that, in combination with genetics, should markedly increase our comprehension of the molecular mechanisms leading to arrhythmias. [Copyright &y& Elsevier]

Even though sequencing of the mammalian genome has led to the discovery of a large number of ionic channel genes, identification of the molecular determinants of cellular electrical properties in different regions of the heart has been rarely obtained. We developed a high-throughput approach capable of simultaneously assessing the expression pattern of ionic channel repertoires from different regions of the mouse heart. By using large-scale real-time RT-PCR, we have profiled 71 channels and related genes in the sinoatrial node (SAN), atrioventricular node (AVN), the atria (A) and ventricles (V). Hearts from 30 adult male C57BL/6 mice were microdissected and RNA was isolated from six pools of five mice each. TaqMan data were analysed using the threshold cycle (Ct) relative quantification method. Cross-contamination of each region was checked with expression of the atrial and ventricular myosin light chains. Two-way hierarchical clustering analysis of the 71 genes successfully classified the six pools from the four distinct regions. In comparison with the A, the SAN and AVN were characterized by higher expression of Navβ1, Navβ3, Cav1.3, Cav3.1 and Cavα2δ2, and lower expression of Kv4.2, Cx40, Cx43 and Kir3.1. In addition, the SAN was characterized by higher expression of HCN1 and HCN4, and lower expression of RYR2, Kir6.2, Cavβ2 and Cavγ4. The AVN was characterized by higher expression of Nav1.1, Nav1.7, Kv1.6, Kvβ1, MinK and Cavγ7. Other gene expression profiles discriminate between the ventricular and the atrial myocardium. The present study provides the first genome-scale regional ionic channel expression profile in the mouse heart. [ABSTRACT FROM AUTHOR]

The sequence of human urotensin II (UII) has been recently established as H-Glu-Thr-Pro-Asp-Cys-Phe-Trp-Lys-Tyr-Cys-Val-OH, and it has been reported that UII is the most potent mammalian vasoconstrictor peptide identified so far. A series of UII analogues was synthesized, and the contractile activity of each compound was studied in vitro using de-endothelialised rat aortic rings. Replacement of each amino acid by an l -alanine or by a d -isomer showed that the N- and C-terminal residues flanking the cyclic region of the amidated peptide were relatively tolerant to substitution. Conversely, replacement of any residue of the cyclic region significantly reduced the contractile activity of the molecule. The octapeptide UII (4-11) was 4 times more potent than UII, indicating that the C-terminal region of the molecule possesses full biological activity. Alanine or d -isomer substitutions in UII (4-11) or in UII (4-11) -NH 2 , respectively, showed a good correlation with the results obtained for UII-NH 2 . Disulfide bridge disruption or replacement of the cysteine residues by their d -enantiomers markedly reduced the vasoconstrictor effect of UII and its analogues. In contrast, acetylation of the N-terminal residue of UII and UII-NH 2 enhanced the potency of the peptide. Finally, monoiodination of the Tyr 6 residue in UII (4-11) increased by 5 fold the potency of the peptide in the aortic ring bioassay. This structure-activity relationship study should provide useful information for the rational design of selective and potent UII receptor agonists and antagonists. [ABSTRACT FROM AUTHOR]