With only 19 amino acids, duramycin is the smallest known polypeptide that has a defined 3-dimensional binding structure. Duramycin binds phosphatidylethanolamine (PtdE) at a 1:1 ratio with high affinity and exclusive specificity. As an abundant binding target, PtdE is a major phospholipid and accounts for about 20% of the phospholipid content in mammalian cellular membranes. PtdE is externalized to the surface of apoptotic cells and also becomes accessible in necrotic cells because of compromised plasma membrane integrity. Given the unique physicochemical properties of duramycin and the availability of PtdE in acute cell death, the goal of this study was to develop and evaluate 99m Tc-duramycin as a novel molecular probe for imaging PtdE. Methods: Duramycin is covalently modified with succinimidyl 6-hydrazinonicotinate acetone hydrazone (HYNIC) and labeled with 99m Tc using a coordination chemistry involving tricine-phosphine coligands. The retention of PtdE-binding activities was confirmed using competition assays with PtdEcontaining liposomes. The blood clearance, pharmacokinetics, and biodistribution of 99m Tc-duramycin were measured in rats. Finally, 99m Tc-duramycin binding to acute cell death in vivo was demonstrated using a rat model of acute myocardial infarction induced by ischemia and reperfusion and confirmed using autoradiography and histology. Results: HYNIC-derivatized duramycin with 1:1 stoicheometry was synthesized and confirmed by mass spectrometry. The radiolabeling efficiency was 80%-85%, radiochemical purity was 78%-89%, and specific activity was 54 GBq. The radiotracer was purified with high-performance liquid chromatography radiodetection before use. The specific uptake of 99m Tc-duramycin in apoptotic cells, compared with that in viable control cells, was enhanced by more than 30-fold. This binding was competitively diminished in the presence of PtdE-containing liposomes but not by liposomes consisting of other phospholipid species. Intravenously injected 99m Tc-duramycin has favorable pharmacokinetic and biodistribution profiles: it quickly clears from the circulation via the renal system, with a blood half-life of less than 4 min in rats. The hepatic and gastrointestinal uptake were very low. 99m Tc-duramycin is completely unmetabolized in vivo, and the intact agent is recovered from the urine. Combined with a fast clearance and low hepatic background, the avid binding of 99m Tc-duramycin to the infarcted myocardium quickly becomes conspicuous shortly after injection. The uptake of radioactivity in infarcted tissues was confirmed by autoradiography and histology. Conclusion: 99m Tc-duramycin is a stable, low-molecular-weight PtdE-binding radiopharmaceutical, with favorable in vivo imaging profiles. It is a strong candidate as a molecular probe for PtdE imaging and warrants further development and characterization.

The asserted dominant role of the kidneys in the chronic regulation of blood pressure and in the etiology of hypertension has been debated since the 1970s. At the center of the theory is the observation that the acute relationships between arterial pressure and urine production—the acute pressure-diuresis and pressure-natriuresis curves—physiologically adapt to perturbations in pressure and/or changes in the rate of salt and volume intake. These adaptations, modulated by various interacting neurohumoral mechanisms, result in chronic relationships between water and salt excretion and pressure that are much steeper than the acute relationships. While the view that renal function is the dominant controller of arterial pressure has been supported by computer models of the cardiovascular system known as the “Guyton-Coleman model”, no unambiguous description of a computer model capturing chronic adaptation of acute renal function in blood pressure control has been presented. Here, such a model is developed with the goals of: 1. representing the relevant mechanisms in an identifiable mathematical model; 2. identifying model parameters using appropriate data; 3. validating model predictions in comparison to data; and 4. probing hypotheses regarding the long-term control of arterial pressure and the etiology of primary hypertension. The developed model reveals: long-term control of arterial blood pressure is primarily through the baroreflex arc and the renin-angiotensin system; and arterial stiffening provides a sufficient explanation for the etiology of primary hypertension associated with ageing. Furthermore, the model provides the first consistent explanation of the physiological response to chronic stimulation of the baroreflex.

Bugenhagen SM, Cowley AW Jr, Beard DA. Identifying physiological origins of baroreflex dysfunction in salt-sensitive hypertension in the Dahl SS rat. Physiol Genomics 42: 23-41, 2010. First published March 30, 2010 doi:10.1152/physiolgenomics.00027.2010.-Salt-sensitive hypertension is known to be associated with dysfunction of the baroreflex control system in the Dahl salt-sensitive (SS) rat. However, neither the physiological mechanisms nor the genomic regions underlying the baroreflex dysfunction seen in this rat model are definitively known. Here, we have adopted a mathematical modeling approach to investigate the physiological and genetic origins of baroreflex dysfunction in the Dahl SS rat. We have developed a computational model of the overall baroreflex heart rate control system based on known physiological mechanisms to analyze telemetry-based blood pressure and heart rate data from two genetic strains of rat, the SS and consomic SS.13 BN , on low-and high-salt diets. With this approach, physiological parameters are estimated, unmeasured physiological variables related to the baroreflex control system are predicted, and differences in these quantities between the two strains of rat on low-and high-salt diets are detected. Specific findings include: a significant selective impairment in sympathetic gain with high-salt diet in SS rats and a protection from this impairment in SS.13 BN rats, elevated sympathetic and parasympathetic offsets with high-salt diet in both strains, and an elevated sympathetic tone with high-salt diet in SS but not SS.13 BN rats. In conclusion, we have associated several important physiological parameters of the baroreflex control system with chromosome 13 and have begun to identify possible physiological mechanisms underlying baroreflex impairment and hypertension in the Dahl SS rat that may be further explored in future experimental and modeling-based investigation. computational model; baroreceptor reflex; autonomic nervous system THE DAHL SALT-SENSITIVE (SS) rat is an extensively studied model of human salt-sensitive hypertension. Consomic substitution studies involving the introgression of whole chromosomes from a salt-resistant rat strain into the isogenic background of the SS strain, congenic substitution studies involving the introgression of smaller regions of chromosomes, and other genetic studies have led to the identification of many quantitative trait loci (QTLs) associated with this disease (12, 36, 44, http://www.pga.mcw.edu). However, phenotypes collected from many of these studies often involve relatively gross measurements such as blood pressure and heart rate, which provide little information on the underlying physiology. Many protective QTLs have been shown to interact epistatically (i.e., in a nonlinear or nonadditive manner) so that these measurements are unable to distinguish various combinations of introgressed QTLs (36). It is also likely that many QTLs have been left unidentified because of these interactions. Thus, these types of measurements become dimi...

Despite extensive study over the past six decades the coupling of chemical reaction and mechanical processes in muscle dynamics is not well understood. We lack a theoretical description of how chemical processes (metabolite binding, ATP hydrolysis) influence and are influenced by mechanical processes (deformation and force generation). To address this need, a mathematical model of the muscle cross-bridge (XB) cycle based on Huxley’s sliding filament theory is developed that explicitly accounts for the chemical transformation events and the influence of strain on state transitions. The model is identified based on elastic and viscous moduli data from mouse and rat myocardial strips over a range of perturbation frequencies, and MgATP and inorganic phosphate (Pi) concentrations. Simulations of the identified model reproduce the observed effects of MgATP and MgADP on the rate of force development. Furthermore, simulations reveal that the rate of force re-development measured in slack-restretch experiments is not directly proportional to the rate of XB cycling. For these experiments, the model predicts that the observed increase in the rate of force generation with increased Pi concentration is due to inhibition of cycle turnover by Pi. Finally, the model captures the observed phenomena of force yielding suggesting that it is a result of rapid detachment of stretched attached myosin heads.

Alterations in energetic state of the myocardium are associated with decompensated heart failure in humans and in animal models. However, the functional consequences of the observed changes in energetic state on mechanical function are not known. The primary aim of the study was to quantify mechanical/energetic coupling in the heart and to determine if energetic dysfunction can contribute to mechanical failure. A secondary aim was to apply a quantitative systems pharmacology analysis to investigate the effects of drugs that target cross-bridge cycling kinetics in heart failure-associated energetic dysfunction. Herein, a model of metabolite- and calcium-dependent myocardial mechanics was developed from calcium concentration and tension time courses in rat cardiac muscle obtained at different lengths and stimulation frequencies. The muscle dynamics model accounting for the effect of metabolites was integrated into a model of the cardiac ventricles to simulate pressure-volume dynamics in the heart. This cardiac model was integrated into a simple model of the circulation to investigate the effects of metabolic state on whole-body function. Simulations predict that reductions in metabolite pools observed in canine models of heart failure can cause systolic dysfunction, blood volume expansion, venous congestion, and ventricular dilation. Simulations also predict that myosin-activating drugs may partially counteract the effects of energetic state on cross-bridge mechanics in heart failure while increasing myocardial oxygen consumption. Our model analysis demonstrates how metabolic changes observed in heart failure are alone sufficient to cause systolic dysfunction and whole-body heart failure symptoms.