Behavior of rats kept under conditions of a shifted light/dark regimen and receiving melatonin

We studied the effect of shift in the natural light/dark regimen (desynchronosis) and treatment with melatonin on behavioral characteristics of rats with different activity in the open-field test. Experiments were performed on 172 Wistar rats kept under conditions of the natural or shifted light/dark regimen. Some animals were intraperitoneally treated with 1 ml physiological saline or melatonin in doses of 1 and 2 mg/kg, while others did not receive the injections. Desynchronosis altered the normal rhythm of locomotor activity and abolished the differences between daytime and nighttime activity rats not receiving the injections. The influence of melatonin on locomotor activity of rats maintained under normal or shifted light/dark conditions depended on its dose, time of treatment, and initial behavioral characteristics of animals. Our results indicate that the use of melatonin for treatment of disturbances produced by a shift in the light/dark conditions should be performed taking into account individual behavioral characteristics of the organism.     

Drift of vortex in the myocardium

The behavior of a vortex in rabbit myocardium with artificial inhomogeneity was studied using mapping technique. The inhomogeneity was created by perfusion of a part of the preparation with quinidine solution. Quinidine increased the refractory period of the myocardium and diminished the conduction velocity. It has been found that the vortex drifts along the border of the inhomogeneity because of the difference of refractory periods. The drift velocity was about 4 cm/s, which was five times less than the wave velocity. The direction of the drift was determined by the vector (----omega X----delta R), where ----omega is the angular velocity of vortex rotation, and ----delta R is a gradient of the refractory period.     

Probing field-induced tissue polarization using transillumination fluorescent imaging

Despite major successes of biophysical theories in predicting the effects of electrical shocks within the heart, recent optical mapping studies have revealed two major discrepancies between theory and experiment: 1), the presence of negative bulk polarization recorded during strong shocks; and 2), the unexpectedly small surface polarization under shock electrodes. There is little consensus as to whether these differences result from deficiencies of experimental techniques, artifacts of tissue damage, or deficiencies of existing theories. Here, we take advantage of recently developed near-infrared voltage-sensitive dyes and transillumination optical imaging to perform, for the first time that we know of, noninvasive probing of field effects deep inside the intact ventricular wall. This technique removes some of the limitations encountered in previous experimental studies. We explicitly demonstrate that deep inside intact myocardial tissue preparations, strong electrical shocks do produce considerable negative bulk polarization previously inferred from surface recordings. We also demonstrate that near-threshold diastolic field stimulation produces activation of deep myocardial layers 2-6 mm away from the cathodal surface, contrary to theory. Using bidomain simulations we explore factors that may improve the agreement between theory and experiment. We show that the inclusion of negative asymmetric current can qualitatively explain negative bulk polarization in a discontinuous bidomain model.     

Extracting surface activation time from the optically recorded action potential in three-dimensional myocardium

Optical mapping has become an indispensible tool for studying cardiac electrical activity. However, due to the three-dimensional nature of the optical signal, the optical upstroke is significantly longer than the electrical upstroke. This raises the issue of how to accurately determine the activation time on the epicardial surface. The purpose of this study was to establish a link between the optical upstroke and exact surface activation time using computer simulations, with subsequent validation by a combination of microelectrode recordings and optical mapping experiments. To simulate wave propagation and associated optical signals, we used a hybrid electro-optical model. We found that the time of the surface electrical activation (t(E)) within the accuracy of our simulations coincided with the maximal slope of the optical upstroke (t(F)*) for a broad range of optical attenuation lengths. This was not the case when the activation time was determined at 50% amplitude (t(F50)) of the optical upstroke. The validation experiments were conducted in isolated Langendorff-perfused rat hearts and coronary-perfused pig left ventricles stained with either di-4-ANEPPS or the near-infrared dye di-4-ANBDQBS. We found that t(F)* was a more accurate measure of t(E) than was t(F50) in all experimental settings tested (P = 0.0002). Using t(F)* instead of t(F50) produced the most significant improvement in measurements of the conduction anisotropy and the transmural conduction time in pig ventricles.