Atrial electrophysiological characteristics of aging.

INTRODUCTION: Advancing age is a known risk factor for developing atrial fibrillation (AF), yet it is unknown which electrophysiological changes contribute to this increased susceptibility. The goal of this study is to investigate conduction disturbances and unipolar voltages (UV) related to aging. METHODS: We included 216 patients (182 male, age: 36-83 years) without a history of AF undergoing elective coronary artery bypass surgery. Five seconds of sinus rhythm were recorded intraoperatively at the right atrium (RA), Bachmann's bundle (BB), the left atrium and the pulmonary vein area (PVA). Conduction delay (CD), -block (CB), -velocity (CV), length of longest CB lines and UV were assessed in all regions. RESULTS: With aging, increasing conduction disturbances were found, particularly at RA and BB (RA: longest CB line rs = .158, p = .021; BB: CB prevalence rs = .206, p = .003; CV rs = -.239, p < .0005). Prevalence of low UV areas (UV <5th percentile) increased with aging at the BB and PVA (BB: rs = .237, p < .0005 and PVA: rs = .228, p = .001). CONCLUSIONS: Aging is accompanied by an increase in conduction disturbances during sinus rhythm and a higher prevalence of low UV areas, particularly at BB and in the RA. These electrophysiological alterations could in part explain the increasing susceptibility to AF development associated with aging.

Classification of sinus rhythm single potential morphology in patients with mitral valve disease.

AIMS: The morphology of unipolar single potentials (SPs) contains information on intra-atrial conduction disorders and possibly the substrate underlying atrial fibrillation (AF). This study examined the impact of AF episodes on features of SP morphology during sinus rhythm (SR) in patients with mitral valve disease. METHODS AND RESULTS: Intraoperative epicardial mapping (interelectrode distance 2 mm) of the right and left atrium (RA, LA), Bachmann's bundle (BB), and pulmonary vein area (PVA) was performed in 67 patients (27 male, 67 ± 11 years) with or without a history of paroxysmal AF (PAF). Unipolar SPs were classified according to their differences in relative R- and S-wave amplitude ratios. A clear predominance of S-waves was observed at BB and the RA in both the no AF and PAF groups (BB 88.8% vs. 85.9%, RA 92.1% vs. 85.1%, respectively). Potential voltages at the RA, BB, and PVA were significantly lower in the PAF group (P < 0.001 for each) and were mainly determined by the size of the S-waves amplitudes. The largest difference in S-wave amplitudes was found at BB; the S-wave amplitude was lower in the PAF group [4.08 (2.45-6.13) mV vs. 2.94 (1.40-4.75) mV; P < 0.001]. In addition, conduction velocity (CV) at BB was lower as well [0.97 (0.70-1.21) m/s vs. 0.89 (0.62-1.16) m/s, P < 0.001]. CONCLUSION: Though excitation of the atria during SR is heterogeneously disrupted, a history of AF is characterized by decreased SP amplitudes at BB due to loss of S-wave amplitudes and decreased CV. This suggests that SP morphology could provide additional information on wavefront propagation.

Case report: peri-device leakage after percutaneous left atrial appendage occlusion: plug, clip, or amputate?

Background: Although peri-device leakage is frequently observed after left atrial appendage occlusion (LAAO), there is no consensus on the optimal management strategy. It is unknown whether additional plugging should be preferred over surgical exclusion of the LAA, as experience with additional plugging is limited. Case summary: In this case report, we demonstrate the clinical implications of additional plugging and surgical exclusion in a 65-year-old male patient with peri-device leakage and recurrent thromboembolic events. After the recurrence of paroxysmal atrial fibrillation (AF) and a transient ischaemic attack despite adequate anticoagulation, the patient was opted for re-do pulmonary vein isolation and LAAO with a Watchman device. Due to multiple ischaemic strokes and recurrent AF in combination with significant peri-device leakage, additional plugging with a second device was performed. Post-procedurally, the patient had another ischaemic stroke and persisting peri-device leakage was observed during follow-up. Due to progressive symptoms of AF and patient's preference to discontinue DOAC, he underwent a Cox MAZE IV procedure, including amputation of the LAA with both devices. Within six months after surgery, the patient experienced two more ischaemic events. In the following two years, the patient remained free of any cerebrovascular accidents or recurrence of AF. Discussion: Additional plugging of peri-device leakage is not always successful in stroke prevention. In combination with recurrent AF, progressive symptoms, contraindication for oral anticoagulation, and patient's preference, surgical LAA exclusion could be preferred over additional plugging.

Unipolar atrial electrogram morphology is affected by age: evidence from high-resolution epicardial mapping.

BACKGROUND: It is unknown which features of unipolar atrial electrogram (U-AEGM) morphology are affected by ageing and whether age-related changes in U-AEGM morphology are equally distributed throughout the right and left atria. PATIENTS AND METHODS: Epicardial high-resolution mapping was performed in patients undergoing coronary artery bypass grafting surgery during sinus rhythm (SR). Mapping areas include the right atrium (RA), left atrium (LA), pulmonary vein area (PVA) and Bachmann's bundle (BB). Patients were categorized into a young (age < 60) and aged (age ≥ 60) group. U-AEGM were classified as single potentials (SPs, one deflection), short double potentials (SDPs, deflection interval ≤ 15ms), long double potentials (LDPs, deflection interval > 15ms) and fractionated potentials (FPs, ≥3 deflections). RESULTS: A total of 213 patients (age: 67 (59-73) years; young group N = 58, aged group N = 155) were included. Only at BB, the proportion of SPs (p = 0.007) was significantly higher in the young group, while the proportion of SDPs (p = 0.051), LDPs (p = 0.004) and FPs (p = 0.006) was higher in the aged group. After adjusting for potential confounders, older age was associated with a reduction in SPs [regression coefficient (ß): -6.33, 95% confident interval (CI): -10.37 to -2.30] at the expense of an increased proportion of SDPs (ß: 2.49, 95% CI: 0.09 to 4.89), LDPs (ß: 1.94, 95% CI: 0.21 to 3.68) and FPs (ß: 1.90, 95% CI: 0.62 to 3.18). CONCLUSIONS: Age-related remodeling particularly affects BB as indicated by the decreased amount of non-SP at this location in the elderly.Key MessagesAgeing preferentially affects the morphology of unipolar atrial electrograms recorded at Bachmann's bundle.At Bachmann's bundle, the proportion of short double-, long double- and fractionated potentials increase during ageing at the expense of a decrease in the proportion of single potentials, reflecting aggravation of abnormalities in conduction.The increase in abnormal unipolar atrial electrograms at Bachmann's bundle during ageing supports the concept that Bachmann's bundle may play an important role in development of age-related arrhythmias such as atrial fibrillation.

Detection of endo-epicardial atrial low-voltage areas using unipolar and omnipolar voltage mapping.

Background: Low-voltage areas (LVA) can be located exclusively at either the endocardium or epicardium. This has only been demonstrated for bipolar voltages, but the value of unipolar and omnipolar voltages recorded from either the endocardium and epicardium in predicting LVAs at the opposite layer remains unknown. The goal of this study was therefore to compare simultaneously recorded endo-epicardial unipolar and omnipolar potentials and to determine whether their voltage characteristics are predictive for opposite LVAs. Methods: Intra-operative simultaneous endo-epicardial mapping (256 electrodes, interelectrode distances 2 mm) was performed during sinus rhythm at the right atrium in 93 patients (67 ± 9 years, 73 male). Cliques of four electrodes (2 × 2 mm) were used to define maximal omnipolar (Vomni,max) and unipolar (Vuni,max) voltages. LVAs were defined as Vomni,max ≤0.5 mV or Vuni,max ≤1.0 mV. Results: The majority of both unipolar and omnipolar LVAs were located at only the endocardium (74.2% and 82.0% respectively) or epicardium (52.7% and 47.6% respectively). Of the endocardial unipolar LVAs, 25.8% were also located at the opposite layer and 47.3% vice-versa. In omnipolar LVAs, 18.0% of the endocardial LVAs were also located at the epicardium and 52.4% vice-versa. The combination of epicardial Vuni,max and Vomni,max was most accurate in identifying dual-layer LVAs (50.4%). Conclusion: Unipolar and omnipolar LVAs are frequently located exclusively at either the endocardium or epicardium. Endo-epicardial LVAs are most accurately identified using combined epicardial unipolar and omnipolar voltages. Therefore, a combined endo-epicardial unipolar and omnipolar mapping approach is favoured as it may be more indicative of possible arrhythmogenic substrates.

Characterization of pre-existing arrhythmogenic substrate associated with de novo early and late postoperative atrial fibrillation.

BACKGROUND: PoAF is the most common complication after cardiac surgery and may occur in patients with pre-existing arrhythmogenic substrate. Characterization of this substrate could aid in identifying patients at risk for PoAF. We therefore compared intra-atrial conduction parameters and electrogram morphology between patients without and with early- (≤5 days after surgery) and late- (up to 5 years) postoperative atrial fibrillation (PoAF). METHODS AND RESULTS: Epicardial mapping of the right and left atrium and Bachmann's Bundle (BB) was performed during sinus rhythm (SR) in 263 patients (207male, 67 ± 11 years). Unipolar potentials were classified as single, short or long double and fractionated potentials. Unipolar voltage, fractionation delay (time difference between the first and last deflection), conduction velocity (CV) and conduction block (CB) prevalence were measured. Comparing patients without (N = 166) and with PoAF (N = 97), PoAF was associated with lower CV and more CB at BB. Unipolar voltages were lower and more low-voltage areas were found at the left and right atrium and BB in PoAF patients. These differences were more pronounced in patients with late-PoAF (6%), which could even occur up to 5 years after surgery. Although several electrophysiological parameters were related to PoAF, age was the only independent predictor. CONCLUSIONS: Patients with de novo PoAF have more extensive arrhythmogenic substrate prior to cardiac surgery compared to those who remained in SR, which is even more pronounced in late-PoAF patients. Future studies should evaluate whether intra-operative electrophysiological examination enables identification of patients at risk for developing PoAF and hence (preventive) therapy.

Epicardial high-resolution mapping of advanced interatrial block: Relating ECG, conduction abnormalities and excitation patterns.

Background: Impairment of conduction across Bachmann's Bundle (BB) may cause advanced interatrial block (a-IAB), which in turn is associated with development of atrial fibrillation. However, the exact relation between a complete transverse line of conduction block (CB) across BB and the presence of a-IAB has not been studied. Objective: The aims of this study are to determine whether (1) a complete transversal line of CB across BB established by high resolution mapping correlates with a-IAB on the surface ECG, (2) conduction abnormalities at the right and left atria correlate with a-IAB, and (3) excitation patterns are associated with ECG characteristics of a-IAB. Methods: We included 40 patients in whom epicardial mapping revealed a complete transverse line of CB across BB. Pre-operative ECGs and post-operative telemetry were assessed for the presence of (a) typical a-IAB and de novo early post-operative AF (EPOAF), respectively. Total atrial excitation time (TAET) and RA-LA delay were calculated. Entry site and trajectory of the main sinus rhythm wavefront at the pulmonary vein area (PVA) were assessed. Results: Thirteen patients were classified as a-IAB (32.5%). In the entire atria and BB there were no differences in conduction disorders, though, patients with a-IAB had an increased TAET and longer RA-LA delay compared to patients without a-IAB (90.0 ± 21.9 ms vs. 74.9 ± 13.0 ms, p = 0.017; 160.0 ± 27.0 ms vs. 136.0 ± 24.1 ms, p = 0.012, respectively). Patients with typical a-IAB solely had caudocranial activation of the PVA, without additional cranial entry sites. Prevalence of de novo EPOAF was 69.2% and was similar between patients with and without a-IAB. Conclusion: A transverse line of CB across BB partly explains the ECG characteristics of a-IAB. We found atrial excitation patterns underlying the ECG characteristics of both atypical and typical a-IAB. Regardless of the presence of a-IAB, the clinical impact of a complete transverse line of CB across BB was reflected by a high incidence of de novo EPOAF.

Persistence and Distortion of Electrical Activity in the LAA 5 Years After Endovascular Occlusion.

This is the first report demonstrating persistence and distortion of electrical activity in the left atrial appendage 5 years after endovascular occlusion with a Watchman device. Electrical conduction is impaired providing an arrhythmogenic substrate for atrial tachyarrhythmias. Localized inflammation may result in structural and electrical remodeling in these patients. (Level of Difficulty: Intermediate.).

Rhodopsin photoproducts: effects on electroretinogram sensitivity in isolated perfused rat retina.

Isolated perfused retinas of albino rats were exposed to brief saturating flashes of white light which bleached about 50 percent of the rhodopsin present. Transient photoproducts of the reaction could be detected for about 30 minutes. The b-wave threshold increased by some 3 logarithmic units immediately after the flash and remained stable at this level thereafter. This suggests that the longer-lived intermediate products of rhodopsin photolysis do not influence scotopic visual sensitivity.

Retinal capillaries: proliferation of mural cells in vitro.

Capillaries from bovine, monkey, and human retinas maintained in tissue culture produced a monolayer of cells. Autoradiographic and electron microscopic evidence indicated that the mural cells (intramural pericytes) were the cells that proliferated. Since intramural pericytes are damaged selectively in diabetes mellitus, their availability in culture will be useful in seeking means to control diabetic retinopathy.

Retinal dystrophy: development retarded by galactose feeding in spontaneously hypertensive rats.

Retinal photoreceptor cell dystrophies have been widely observed in humans and in animals, but pathogenetic mechanisms are known in only a few such disorders, and successful therapeutic intervention has been reported in fewer still. Spontaneously hypertensive albino rats develop a retinal photoreceptor cell dystrophy with onset late in the first year or early in the second year of life. Between 60 and 70 percent of the animals are affected. A substantial reduction in the prevalence and severity of the dystrophy occurred in such animals whose diet contained 30 percent (by weight) D-galactose. Neither an inhibitor of the enzyme aldose reductase, present in the diet, nor diabetes mellitus, induced by streptozotocin, had any statistically significant influence on the dystrophy. Ambient light and systolic blood pressure levels also did not seem to influence the course of the disorder. The mechanism by which galactose exerts its effect is unknown, but a mutant enzyme with an elevated Michaelis constant (Km) for galactose is plausible.

Ocular complications.

Ocular complications of diabetes in humans are reviewed briefly, and experimental models available for study of the complications are described. Potentially suitable models include not only diabetic animals, but also nondiabetic animals in which analogous lesions have been demonstrated. Many abnormalities of the lens, cornea, iris, and retina comparable to those of diabetes in humans may be observed in diabetic animals, although all abnormalities are not necessarily observed in every species. Retinal changes, in particular, may occur in diabetic animals of several species, but only in large animals (dogs, primates) have saccular capillary aneurysms been reproduced consistently, together with other retinal changes typical of diabetes in humans. A few examples of the uses of animal models are offered, and attention is called to a lack of animal models of proliferative diabetic retinopathy and of rubeosis iridis.

Retinopathy in juvenile-onset type I diabetes of short duration.

We evaluated the prevalence and severity of diabetic retinopathy in 173 juvenile-onset, type I diabetic subjects and 78 nondiabetic controls of similar age, race, and sex distribution by stereoscopic fundus photography and fluorescein angiography, performed by a standardized protocol and evaluated by five expert, masked observers. The overall prevalence of retinopathy was 18% in the diabetic group and 0% in the controls. Retinopathy prevalence increased with duration of diabetes in the diabetic group, with a prevalence of 1% from 0--4 yr after diagnosis, 25% after 5--9 yr, and 67% 10--16 yr after onset of the systemic disease. There was an independent association with age, with little retinopathy before age 15 and a 48% prevalence in older persons. Retinopathy was also found to be independently associated with the following: diabetic "control," evaluated semiquantitatively but on a masked basis; lens opacities; and frequency of daily insulin injections. Among the 166 diabetic subjects who had both angiography and photography, a retinopathy prevalence of 17% was detected by angiography and 11% by photography. This difference was statistically significant (P = 0.01). This study provides baseline data for use in estimating sample size in controlled trials of therapeutic measures to prevent retinopathy in juvenile diabetic populations. The study also supports the hypothesis that long-term hyperglycemia as well as changes (possibly hormonal in nature) associated with puberty are causally related to diabetic retinopathy.

Effects of cardiac glycosides on electrical activity in the isolated retina of the frog.

Ouabain added to physiological salt solutions bathing the isolated frog retina irreversibly abolishes the electrical response to light (the electroretinogram or ERG). The time course of abolition depends on the concentration of ouabain in the medium and the surface of the retina to which it is applied. When the glycoside is placed on the receptor surface, in 7 min the ERG is completely eliminated by 10(-4)M ouabain and more than 90% inhibited by 3 x 10(-5)M ouabain. The effect is slower at lower concentrations and when the solution is applied to the vitreous surface of the retina. The evidence suggests that abolition of the ERG by ouabain is due principally to inhibition of the active transport of sodium: (a) Structurally modified glycosides which are considerably less potent inhibitors of alkali cation-activated ATPase activity in preparations of frog retinal outer segments are also poorer inhibitors of electrical activity in isolated retinas. (b) Replacing much of the sodium in the medium bathing the retina by choline, Tris, or sucrose significantly protects the retina from ouabain. It is suggested that in a standard sodium environment essentially constant activity of the sodium pump is required to prevent rapid and irreversible change. The cellular sites most critically dependent on the sodium pump have not been identified.

Effects of a high-galactose diet on the interphotoreceptor matrix of two strains of rat.

PURPOSE: The authors previously reported that a diet containing 30% galactose retards the development of the late-onset photoreceptor dystrophy in the spontaneously hypertensive (SHR) rat. It was suggested that the dystrophy might result from faulty galactosylation of a critical glycoprotein or glycosaminoglycan (GAG) in the interphotoreceptor matrix (IPM). In the current study, this hypothesis was tested by studying IPM protein and GAG composition in SHR and normotensive Wistar-Kyoto (WKy) control strain rats fed a standard or a high-galactose (30%) diet. METHODS: The authors performed biochemical analyses of the IPM of SHR and of WKy control rats fed either the basal diet or a 30% galactose diet for 14 mo and labeled at the termination of the experiment with 3H-glucosamine and 35S-sulfate. Analyses included high-performance liquid chromatography and two-dimensional sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE) and isoelectric focusing (IEF) of extracted proteoglycans with identification of the GAG by selective enzymatic degradation and immunoblotting of SDS-PAGE gels for interphotoreceptor retinol-binding protein (IRBP), with slot blots for additional quantitation. RESULTS: Although several differences were detected in GAG radiolabeling between the two strains, only one, a decreased overall synthesis of 3H-glucosamine-labeled GAG in the WKy rats, was altered by the high-galactose diet. There was no apparent difference in the protein patterns of the IPM as evaluated by two-dimensional SDS-PAGE and IEF. However, slot blots of that portion of the IPM extracted from the neural retinas showed less reactivity per microgram of protein for IRBP in the galactose-fed animals of both strains than for the rats fed a basal diet. This was statistically significant, however, only in the WKy rats. CONCLUSIONS: The composition of the IPM of these two strains of rat appears similar to the composition of the IPM of other species that have been studied. Whether the quantitative alteration in IRBP that appears to be produced by galactose feeding has functional significance, with particular relevance to retarding the development of the photoreceptor cell dystrophy of the SHR rat, is unclear at this time.

Presumed retinovitreal neovascularization in dystrophic retinas of spontaneously hypertensive rats.

The authors have observed abnormal blood vessels, strongly suggestive of neovascular proliferation, arising from the retinal circulation and extending through the inner limiting membrane of the retina into the vitreous in five spontaneously hypertensive (SHR) rats with severe retinal dystrophy. The animals in whom these presumptive retinovitreal new vessels occurred were all 15 mo of age or older. The new vessels frequently demonstrated thinned and, rarely, fenestrated endothelium, abnormal intracellular junctions, increased numbers of endocytic vesicles, bizarre appearing pericytes, and highly abnormal basement membranes, features that have been observed in retinovitreal new vessels in proliferative retinopathies in humans. Unlike such new vessels arising from the human retinal circulation, however, those that we observed in dystrophic rat retinas were usually surrounded by proliferating retinal pigment epithelial cells within the retinal substance. Unlike the vessels, the pigment epithelial cells did not break through the inner limiting membrane of the retina to enter the vitreous. The pigment epithelial cells that made contact with the internal limiting membrane of the retina demonstrated apical and basal plasma membrane specializations that are typical of these cells in their normal anatomical location, while pigment epithelial cells migrating in cords through the neural retina lacked such specializations. This animal model may be of great value in understanding the mechanisms of retinal neovascularization.

Growth factor localization in choroidal neovascular membranes of age-related macular degeneration.

PURPOSE: Because several polypeptide growth factors are known to influence capillary endothelial cell mitogenesis, the authors investigated the presence of some of these molecules in choroidal neovascular membranes (CNVMs) removed surgically from human subjects with age-related macular degeneration (ARMD). METHODS: The authors performed immunoelectron microscopic studies on surgically removed submacular CNVMs from nine subjects with ARMD and from one subject with ARMD whose eye was studied after death. These were compared with retinal pigment epithelial (RPE) and choroidal tissue from eight normal subjects whose eyes were received after death and one received after massive trauma. RESULTS: RPE cells from the CNVMs were strongly immunoreactive for acidic and basic fibroblast growth factor (aFGF and bFGF) and for transforming growth factor beta (TGF beta). Some of the immunoreactivity was intracytoplasmic, but most was intralysosomal. In addition, some choriocapillary endothelial cells located close to the RPE layer in these CNVMs were immunopositive for bFGF and for FGF receptor. Reaction product for these two substances was located at regular intervals along the endothelial plasma membrane on both the anteluminal and the luminal side of the cells, suggesting a physiological reaction between the growth factor and its receptor. Choriocapillary endothelial cells deeper within the stroma were unreactive to bFGF and FGF receptor antibodies. There was little immunoreactivity for the growth factors in RPE or choriocapillary endothelial cells from normal eyes. The aFGF and bFGF immunoreactivity was highly specific because aFGF positivity was abolished when the antibody was incubated with 10(-6) M aFGF but not a with the same concentration of bFGF, whereas bFGF immunoreactivity was abolished by incubation of the antibody with bFGF but not with aFGF. RPE cells from normal eyes and from eyes affected by ARMD showed strong cytoplasmic immunoreactivity to antibodies for cytoplasmic retinaldehyde-binding protein and superoxide dismutase and weak reactivity to antibodies for vimentin. CONCLUSIONS: These results are consistent with the hypothesis that one or both FGFs are causally related to the development of choroidal neovascularization. The authors have reported similar observations in experimental choroidal neovascularization in pigmented rats after red krypton laser photocoagulation. TGF beta may serve to modulate the effects of these mitogens. The authors suggest that growth factor production is induced in RPE cells after physical or chemical damage. Because of the damage to these cells, FGF molecules can be released from the cells despite the absence of a "signal sequence" in the DNA coding for FGF production.

Effects of endothelin on cultured bovine retinal microvascular pericytes.

PURPOSE: The endothelins, a recently discovered family of peptides, include endothelin-1, the most potent vasoconstrictor known. Retinal microvascular pericytes are thought to be contractile cells analogous to the smooth muscle cells of larger vessels, but the physiologic stimulus for their contraction is unknown. We hypothesized that the endothelins might serve as such stimuli. METHODS: The intracellular free/bound Ca++ ratio increases rapidly immediately before muscle cell contraction. We evaluated changes in this ratio in cultured bovine retinal microvascular pericytes and, for comparison, three other types of ocular cells. We loaded the cells with the calcium-sensitive, fluorescent dye Indo-1. Using a laser cytometer, we monitored the time course of changes in fluorescence of individual cells in response to several putative vasoactive agents, including, in particular, endothelin-1, 2, and 3. RESULTS: Endothelin-1 (ET-1) produced a rapid rise in the free/bound Ca++ ratio, followed by a slow decline. The response occurred at ET-1 concentrations as low as 1 x 10(-12) mol/l, and was graded in amplitude and concentration dependent. After an initial application of ET-1, repeat applications yielded no response. Endothelin-2 was less effective than ET-1, and ET-3 had no effect at all, but both agents blocked the response to ET-1. Several other agents also raised the free/bound Ca++ ratio, but were substantially less effective than ET-1. When any of these agents, except for histamine, was added after even a submaximal concentration of ET-1, no response was observed, but ET-1 applied after these agents produced a large response. Histamine could elicit a rise in the free/bound Ca++ ratio after application of ET-1 to cultured pericytes. The ET-1 response occurred in Ca(++)-free medium and in medium containing 10(-4) mol/l verapamil or nifedipine, indicating that the results we observed are due primarily to the release of free Ca++ from bound intracellular stores. Endothelin-1 produced a similar change in the free/bound Ca++ ratio in cultured bovine RPE cells, but not in retinal microvascular endothelial cells or lens epithelial cells. CONCLUSIONS: ET-1 is at least three orders of magnitude more effective in producing the release of free intracellular Ca++ than other agents tested. It appears to act through specific cell surface receptors, which can be blocked by prior application of other endothelin isopeptides, but not by structurally dissimilar molecules. However, with the exception of histamine, all of these agents appear to act through a common intracellular pathway, because application of ET-1 blocks the subsequent effect of the other agents tested, except histamine. Alternatively, ET-1 may be capable of desensitizing the receptors for these agents without occupying the receptor sites. Because cultured retinal pericytes are extremely sensitive to ET-1 in a response closely linked to muscle cell contraction, ET-1 must be considered a highly plausible agonist for pericyte contraction in vivo.