The cumulative fluxes of radioactive sucrose, sodium, and water across a sheet of cat right ventricle were studied simultaneously to obtain the apparent tissue diffusion coefficients for extravascular diffusion at 37°C. The sucrose data fitted the equations for diffusion in tortuous channels in a plane sheet with a tortuosity factor, λ, of 2.11 ± 0.11 (mean ± SE, n = 10). The fit of the earliest data before attainment of steady state was improved by assuming a Gaussian distribution of diffusion path lengths through the extracellular space, but λ was only changed by a few percent. The sucrose diffusion channel contained 0.27 ± 0.03 ml of total tissue water, which is more than measured by others but still less than the expected sucrose space. The steady-state data for sodium agreed with the model for extracellular diffusion using λ and the area available for diffusion for sucrose when sodium equilibration with a dead-end pore volume (presumed to be intracellular) was taken into account. The cumulative flux data for water were monotonic and lacked secondary inflections. Thus the apparent tissue diffusion coefficients for sucrose, sodium, and water were (in 10 −6 cm 2 /s) 1.77 ± 0.23, 5.13 ± 0.68, and 7.39 ± 0.99, respectively, representing a reduction to 23% of the free diffusion coefficient for sucrose and sodium and 22% for water. Keywords tissue diffusion; cat ventricular myocardium; diffusion models; tracer washout; extracellular fluid; dead-end pores; heterogeneous systems; solute transport Fundamental to the analysis of capillary-tissue exchanges is a knowledge of tissue transfer rates by diffusion outside the capillaries (2,30). The purpose of this work was to determine the apparent tissue diffusion coefficients for sucrose, sodium, and water at body temperature in the cat myocardium. The studies were made in a diffusion cell (32) on a sheet of cat myocardium by a method modified from that described by Page and Bernstein (26). These authors found at 23°C that sodium did not enter the cells and that the steady-state diffusion of water, sodium, and sucrose could be accounted for by fluxes occurring only in the extracellular space. Since cell membrane permeability to water is so high that restriction of water diffusion to the extracellular space seemed unlikely, we felt that a study of the three tracers simultaneously at 37°C was needed.
Summary:In a double-blind placebo-controlled study, 130 patients with verified acute myocardial infarction were given magnesium or placebo treatment intravenously immediately upon admission to hospital. The incidence of arrhythmias requiring treatment during the initial week of hospitalization was registered. Serum magnesium concentrations were increased from 0.7 mmol/l to 1.3 mmol/l as a result of the magnesium infusions. This pharmacologically induced hypermagnesemia resulted in a reduction in the incidence of arrhythrmas from 47 % in the placebo group to 21 % in the magnesium group (p=0.003). In the magnesium-treated patients, increments in serum concentrations of magnesium and potassium correlated positively (r=0.47, p
Impulse transmission between closely appositioned cylindrical myocardial preparations , from the ferret, was studied in vitro. One preparation was driven, and when the longitudinal extracellular potential difference between electrodes 80 micrometers apart was made larger than 30-60 mV by increasing the resistance of the extracellular space, ephaptic (i.e. non synaptic) impulse transmission from one preparation to another occurred. In 2 out of 8 pairs of preparations examined the impulse transmission was bidirectional. The latency of the transmission varied from 9 to 369 ms. Various rate dependent blocks (sometimes associated with a Wenckebach phenomenon) were observed at stimulation frequencies above 1 Hz. The experimental situation has pathophysiological properties relevant to the analysis of the genesis of arrhythmias. Ephaptic interaction may be involved in the shaping of the wavefront of excitation under both normal and pathological conditions.
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