Chronic Dog Model Research Articles

An N-/L-Type calcium channel blocker, cilnidipine, reduces electrical and structural atrial remodeling associated with atrial fibrillation in a chronic dog model

Drugs that suppress AF-promoting remodeling are currently lacking. Cilnidipine is a clinically available Ca 2+ channel blocker that inhibits N-type channels which regulate autonomic function. To assess the effect of cilnidipine on AF-related atrial remodeling. 3 groups of 8 dogs were followed for 3 weeks: – AF maintained by 600 bpm atrial tachypacing treated with placebo (AF + P) ; – cilnidipine 10 mg twice a day (AF + C) ; – instrumented control dogs (Sham). At terminal open-chest study, typical electrophysiological remodeling was observed ( Fig. 1 A) with AF + P: reduced left atrial (LA) effective refractory period (ERP: 69 ± 14 ms vs. 126 ± 15 Sham); enhanced AF vulnerability (inducibility by single extrastimuli: (77 ± 19 vs. 2 ± 6%); and increased AF duration after burst pacing (12.7 ± 10.1 min vs. 0.1 ± 0 min). All these changes were suppressed in AF + C: (ERP 90 ± 15; vulnerability 52 ± 27%; AF duration 5.3 ± 9.1 min). Ex vivo optical mapping ( Fig. 1 B) showed that conduction velocity was decreased in AF + P (68 ± 6 cm/s) vs. Sham (119 ± 9 cm/s, P 0.01); AF + C was protective (97 ± 9 cm/s). SCN5A was decreased in AF + P ( P < 0.01) but not in AF + C vs. Sham ( Fig. 1 C). GJA1 and Connexin43 were down regulated in AF + P ( P < 0.05), but not in AF + C, vs. Sham. Fibrous tissue content (Masson Trichrome) was increased in AF + P (17 ± 4%) vs. Sham (3 ± 1%, P < 0.001) ( Fig. 1 D); AF + C reduced fibrosis (10 ± 4%, P < 0.01). This study shows for the first that cilnidipine, a commercially available N-/L-type Ca 2+ channel blocker, attenuates AF-induced electrical and structural remodeling, introducing a novel potential therapeutic target for this unmet need.

Inositol-1,4,5-Trisphosphate and Ryanodine-Dependent Ca2+ Signaling in a Chronic Dog Model of Atrial Fibrillation

Ca²⁺ signaling regulation plays an important role in triggering and/or maintaining atrial fibrillation (AF). Little is known about the relationship of the inositol-1,4,5-triphosphate receptors (InsP₃Rs) and ryanodine receptors (RyRs) in left atrium to chronic AF. In this study, we investigated the expression and function of InsP₃R₁, InsP₃R₂and RyR₂in a chronic dog model of AF. AF was induced in 6 dogs by rapid right atrial pacing for 24 weeks, and a sham procedure was performed in 5 dogs (control group). The intact left atrial myocytes were used to examine the expression and function of InsP₃Rs, RyRs by BODIPY^O,R TR-X ryanodine, heparin-fluorescein conjugate, and were stimulated by caffeine, ATP to release Ca²⁺ through RyRs, InsP₃Rs separately. We also assessed the molecular components of left atrial tissue underlying the amount of RyR₂, InsP₃R₁and InsP₃R₂ determined by RT-PCR, immunohistochemistry and Western blot analysis. In the chronic AF group, the Ca²⁺ released through RyRs is not altered, but the Ca²⁺ released through InsP₃Rs increased significantly. RyR₂ distributed in cytosol of myocytes, cellular membrane; its expression significantly decreased in AF group compared to controls. InsP₃R₁ distributed in cytosol, InsP₃R₂ distributed not only in cytosol, cellular membrane, but also in nuclear envelope and intercalated discs. The InsP₃R₁ and InsP₃R₂ expression significantly increased in chronic AF group compared to controls. These results indicated that in a chronic dog model of AF, the expression and function of RyR₂ down-regulated; on the contrary, the expression and function of InsP₃R₁, InsP₃R₂ up-regulated, and InsP₃R₂ may be the major InsP₃Rs, which regulate intracellular or even intercellular Ca²⁺ signal transmission.

Nongenomic Renal Effects of Aldosterone

There is increasing evidence for a dependency on aldosterone in the development of hypertension.1 How aldosterone influences the physiology that determines blood pressure or for that matter promotes the pathophysiology of high blood pressure turns out to be less than straightforward. There is the genomic pathway that begins with the binding of aldosterone to the classical mineralocorticoid receptor (MR) and ends with sodium reabsorption at the distal nephron. But aldosterone can convey nongenomic influences as well, actions that have received less attention, possibly in part because they seem counterintuitive to our genomic view of biology. Adding to the complexity of aldosterone’s actions is the fact that there are also nonepithelial (nondistal nephron) targets of aldosterone, such as the heart, the vasculature, the kidney, and other sites. In this issue of Hypertension , Schmidt et al2 describe a rapid (thus nongenomic) effect of administered aldosterone to increase resistance in renal vasculature (a nonepithelial target) in healthy human subjects. The findings follow on the heels of a rather large body of work on the genomic actions of aldosterone. Aldosterone’s nongenomic effects were first recognized more than a decade before an aldosterone-inducible gene was identified in the late 1990s.3 A nongenomic action characteristically occurs almost immediately, usually within several minutes,4 before sufficient time has elapsed for gene …

Biologic ex-vivopre differentiation of adult bone marrow-derived mesenchymal stem cells for cardiac repair in a chronic dog model of myocardial infarction

Biologic ex-vivopre differentiation of adult bone marrow-derived mesenchymal stem cells for cardiac repair in a chronic dog model of myocardial infarction

Functional mechanisms underlying tachycardia-induced sustained atrial fibrillation in a chronic dog model.

Rapid atrial activation causes electrical remodeling that promotes atrial fibrillation (AF), but underlying mechanisms are incompletely understood. We applied epicardial mapping to evaluate atrial electrophysiology and AF duration in dogs subjected to rapid atrial pacing (400/min). Dogs paced for 1 (P1, n=7), 7 (P7, n=13), or 42 (P42, n=7) days were compared with sham dogs (P0, n=13). Atrial pacing progressively increased AF duration. Atrial effective refractory period (ERP) and ERP accommodation to rate were significantly decreased by pacing, with near-maximal changes within 7 days. Atrial conduction velocity decreased more slowly, with maximum changes at 42 days, contributing to increases in AF duration after ERP stabilized. Stepwise multilinear regression indicated that both wavelength (P=.02) and duration of pacing (P=.0001) were independent determinants of changes in AF duration. Mean atrial fibrillation cycle length (AFCL) at 112 recording sites decreased with increased duration of rapid pacing (P<.001), and the SD of AFCL increased progressively (P<.0001), together accounting for 72% of the variance in AF duration. Increases in AFCL variability were due to regionally determined differences in AFCL changes caused by rapid pacing. The number of zones of reactivation per cycle of AF increased as AF became more sustained, consistent with multiple-wavelet reentry. Rapid atrial activation causes time-dependent decreases in ERP, conduction velocity, and wavelength, which, along with increased regional heterogeneity, provide a substrate for AF. The conduction abnormalities and increased regional heterogeneity previously noted in patients with AF may be a consequence, as well as a cause, of the tachyarrhythmia.

Tachycardia-induced changes in Na+ current in a chronic dog model of atrial fibrillation.

We have previously shown that chronic rapid atrial activation (400 bpm) reduces atrial conduction velocity in dogs, contributing to the development of a substrate supporting sustained atrial fibrillation (AF). However, the cellular and ionic mechanisms underlying these functional changes have not been defined. We applied whole-cell patch-clamp techniques to atrial myocytes from dogs subjected to atrial pacing at 400 bpm for 7 days (P7, n = 6) and 42 days (P42, n = 5) and compared the results with those from sham-operated dogs similarly instrumented but without pacemaker activation (P0, n = 6). Rapid atrial pacing allowed for the induction of sustained AF in 67% and 100% of dogs paced for 7 and 42 days, respectively, and significantly decreased conduction velocity under P7 and P42 conditions. In dogs paced for 7 days, Na+ current (INa) density was reduced by 28% at -40 mV (P < .0001, n = 59 cells). INa changes were even more decreased under P42 conditions, by approximately 52% at -40 mV (P < .0001): from -78.7 +/- 4.6 pA/pF (P0, n = 28 cells) to -37.7 +/- 3.0 pA/pF (P42, n = 43 cells). INa was significantly reduced at all voltages ranging from -65 to -10 mV. Voltage-dependent activation and inactivation properties, activation kinetics, and recovery from inactivation were not altered by rapid atrial pacing; however, inactivation kinetics were slowed. AF duration was related to mean INa in each dog (r2 = .573, P < .001). We conclude that rapid atrial activation significantly reduces both conduction velocity and INa density. Since INa is a major determinant of conduction velocity, our data point to INa reduction as a potentially important mechanism contributing to the substrate for AF in this model.

Comparison of Cyclosporin A Absorption from LCT and MCT Solutions following Intrajejunal Administration in Conscious Dogs

Absorption from the intestine of cyclosporin A (CsA), dissolved in either a medium-chain (MCT) or a long-chain triglyceride (LCT) solution, was investigated in a chronic dog model. Following intrajejunal administration of 20mg of CsA/kg of body weight, absorption, judged by the portalvenous appearance of CsA, was determined by measuring whole blood CsA concentrations in the portalvenous and arterial blood and the portalvenous flow. Appearance of CsA from LCT commenced earlier and attained significantly higher mean peak values (±SEM) in the portalvenous blood (2557 ± 436ng/mL) than from MCT (274 ± 80ng/ mL). Portalvenous concentrations of CsA were always higher than arterial concentrations for both LCT and MCT, suggesting that CsA is transported by portalvenous blood following uptake from the gut. Absorption of CsA, measured over 300 min, was 10 times higher with LCT (9.96 ± 2.00%) than with MCT (0.95 ± 0.21%). This significant difference is believed to result from the formation of mixed micelles which occurs during digestion of LCT but not MCT.

The effect of cyclosporine A on bile secretion in dogs

Cyclosporine A is reported to cause cholestasis, but the evidence is confounded by anesthesia and surgery used in acute experiments. To better investigate the effect of cyclosporine on the liver, bile output was directly measured in three cholecystectomized dogs by cannulating the common duct through a chronic duodenal fistula. Control studies were done 1 month after surgery. Cyclosporine in oral doses of 5, 15, and 50 mg.kg-1.d-1 was then given for consecutive 1-week periods. Twice during each study period, bile output was measured for 5 h in fasted, awake animals: 3 h to establish basal conditions, followed by 2 h of taurocholate infusions at 1 and then 2 mumols.kg-1.min-1. Under basal conditions, bile flow rose with each dose of cyclosporine, increasing 63, 127, and 179% above control with cyclosporine 5, 15, and 50 mg.kg-1,d-1, respectively. Bile flow increased similarly during taurocholic acid stimulation. Cyclosporine had no effect on bile salt or bilirubin secretion. In this chronic dog model isolated from other causes of cholestasis, cyclosporine did not induce cholestasis but rather caused a dose-related choleresis without any change in bile salt secretion.

Effect of large-bowel obstruction on colonic blood flow. An experimental study.

The effect of subacute large-bowel obstruction on the mesenteric circulation was studied in a chronic dog model. Colonic obstruction was produced 40 cm distal to the ileocolic sphincter. Five days later, gut blood flow was measured with 15 microns microspheres, together with hemodynamic and metabolic values. Two other groups provided comparative data: unoperated animals to measure baseline values and sham-operative controls. With adequate hydration, hemodynamic and metabolic values remained stable in the experimental group. There was a two-fold increase (P less than 0.05) in blood flow in the dilated colon proximal to the obstruction site, whereas blood flow to the other organs remained unchanged. These results have relevance for the hemodynamic management and use of primary anastomosis in patients with large-bowel obstruction. Furthermore, these data might implicate increased local bowel blood flow as a contributory factor to the poorer long-term prognosis found in patients with large-bowel cancer presenting with intestinal obstruction.

Deliberate epidural air injection in dogs: a radiographic study.

To investigate whether and to what extent air bubbles are present in the epidural space after injections of air, the authors designed an experiment using a chronic dog model. The epidural space was entered at the sixth or seventh lumbar interspace in six mongrel dogs (weight, 20 to 27 kg). Loss-of-resistance (LOR) to saline was tested to identify the epidural space. Three milliliters of air were injected via a Tuohy needle, followed by 5 ml of contrast material (Isovue 300, Squibb Diagnostics). Anterior-posterior (AP) and lateral radiographs were taken on day 1. On days 2, 4, and 8, the procedure was repeated, omitting the air injection. Radiographs were read by a neuroradiologist. Air bubbles were radiographically demonstrated in all six dogs on day 1, and in three of six dogs on day 2. No air was seen on later films. The air bubbles seen were small and few in number but often located near intervertebral spaces. The authors' findings suggest that injection of large volumes of epidural air should be avoided, particularly when N2O is used as part of the anesthetic or when performing diagnostic peridurography since N2O may expand the volume of existing bubbles and interfere with filling of the epidural space by local anesthetics or contrast material.

Gastric emptying, glucose tolerance, and insulin response after duodenojejunostomy

We investigated the impact of direct jejunal delivery of various meals on gastric empyting, glucose tolerance, and insulin response in a chronic dog model following duodenojejunostomy. Ten beagle dogs underwent duodenal transection 2 cm distal to the pylorus and end-to-side duodenojejunostomy 20 cm distal to the ligament of Treitz. Three months after operation each dog underwent gastric emptying studies using radiolabeled normal saline, 20% glucose solution, and standardized mixed solid meal. Glucose tolerance tests with plasma insulin determinations were obtained using the glucose meal. After duodenojejunostomy both the rapid exponential pattern of emptying of normal saline and the slower linear pattern of glucose emptying seen in intact dogs were preserved. The linear gastric emptying of the solid meal which was slower than gastric emptying of either of the liquid meals was also preserved. Although integrated plasma glucose levels over 2 hr were 484.8 ± 40.4 and 456.6 ± 30.4 mg · hr/dl in intact and duodenojejunostomy dogs, respectively ( P > 0.05), the initial rate of rise of plasma glucose was significantly delayed in the duodenojejunostomy dogs. But integrated plasma insulin levels over 2 hr differed significantly ( P < 0.05) between the intact (71.6 ± 9.2 μU · hr/nl) and duodenojejunostomy (48.3 ± 6.2 μU · hr/nl) dogs. We conclude that duodenojejunostomy (jejunal delivery) preserved the patterns of gastric emptying of saline, glucose, and mixed solid meals; retarded initial plasma glucose response to the glucose meal; and blunted plasma insulin response to the glucose load.

Regional blood flow and water content of the obstructed small intestine.

The effects of small-bowel obstruction on the regional distribution of blood flow and water content were studied in a new chronic dog model. Nonstrangulative obstruction was induced 40 cm proximal to the canine ileocolic valve. Blood flow was measured with 15-micron microspheres and hemodynamic and respiratory values were recorded before (experimental phase I) and five days after (experimental phase II) the induction of the obstruction. Two separate control groups of dogs were studied: baseline controls and sham-operative groups (phases I and II). Hemodynamic and respiratory values were stable in both phases in all groups. A comparison of sham phase-II data with the experimental phase-II data in the 120-cm segment of bowel proximal to the site of obstruction showed an 85% increase in blood flow (range, 49% to 106%); for the 280 cm of bowel proximal to the site of obstruction, there was a 6% increase in water content (range, 5% to 9%), and a 39% decrease in dry bowel weight (range, 34% to 46%). These findings help illustrate the pathophysiologic characteristics of microvascular changes in bowel obstruction, which are likely to have particular clinical significance for patients with cardiopulmonary diseases.

Sustained regional abnormalities in cardiac metabolism after transient ischemia in the chronic dog model

Positron emission tomography allows noninvasive assessment of myocardial blood flow and metabolism, and may aid in defining the extent and severity of an ischemic injury. This hypothesis was tested by studying, in chronically instrumented dogs, regional blood flow and metabolism during and after a 3 hour balloon occlusion of the left anterior descending coronary artery. The metabolic findings after ischemia were compared with the recovery of regional function over a 4 week period. N-13 ammonia was used as a blood flow tracer, and C-11 palmitic acid and F-18 deoxyglucose as tracers of fatty acid and glucose metabolism, respectively. Regional myocardial function was monitored with ultrasonic crystals implanted subendocardially. Regional function improved most between 24 hours and 1 week after reperfusion, but was still attenuated at 4 weeks. The slow functional recovery was paralleled by sustained metabolic abnormalities, reflected by segmentally delayed clearance of C-11 activity from myocardium and increased uptake of F-18 deoxyglucose. Absence of blood flow and C-11 palmitic acid uptake at 24 hours of reperfusion correlated with extensive necrosis as evidenced by histologic examination. Conversely, uptake of C-11 palmitic acid with delayed C-11 clearance and increased F-18 deoxyglucose accumulation identified reversibly injured tissue that subsequently recovered functionally and revealed little necrosis. Thus, recovery of metabolism after 3 hours of ischemia is slow in canine myocardium and paralleled by slow recovery of function. Metabolic indexes by positron tomography early after reperfusion can identify necrotic and reversibly injured tissue. Positron tomography may therefore aid in defining the extent and prognosis of an ischemic injury in patients undergoing reperfusion during evolving myocardial infarction.