RESUMEN
We calculated the electrostatic force between a planar interface, such as a planar-supported lipid bilayer membrane, and the tip of a stylus on which another lipid bilayer or some other biomacromolecular system might be deposited. We considered styli with rounded tips as well as conical tips. To take into account the effect of dynamical hydrogen-bonded structures in the aqueous phase, we used a theory of nonlocal electrostatics. We used the Derjaguin approximation and identified the systems for which its use is valid. We pointed out where our approach differs from previous calculations and to what extent the latter are inadequate. We found that 1) the nonlocal interactions have significant effects over distances of 10-15 A from the polar zone and that, at the surface of this zone, the effect on the calculated force can be some orders of magnitude; 2) the lipid dipoles and charges are located a distance L from the hydrophobic layer in the aqueous medium and this can have consequences that may not be appreciated if it is ignored; 3) dipoles, located in the aqueous region, can give rise to forces even though the polar layer is unchanged, and if this is ignored the interpretation of force data can be erroneous if an attempt is made to rationalize an observed force with a knowledge of an uncharged surface; 4) the shape of the stylus tip can be very important, and a failure to take this into account can result in incorrect conclusions, a point made by other workers; and 5) when L is nonzero, the presence of charges and dipoles can yield a force that can be nonmonotonic as a function of ionic concentration.
Asunto(s)
Microscopía de Fuerza Atómica , Modelos Químicos , Fenómenos Biofísicos , Biofisica , Electroquímica , Enlace de Hidrógeno , Membrana Dobles de Lípidos/química , Sustancias Macromoleculares , Soluciones , Termodinámica , Agua/químicaRESUMEN
Static and dynamic chrono-inotropic responses were recorded from both normal and hypertrophic rat auricular myocardium. The slope of the static force-frequency relation for hypertrophic hearts was steeper than that for control hearts. Computer experiments were designed to study the cellular mechanisms underlying the changes in the force-frequency response associated with heart hypertrophy, with the aid of a mathematical model for excitation-contraction coupling in rat heart. A set of equations was derived which permitted to study the effects on the chronoinotropic relations of both the geometrical dimensions of cardiomyocytes and the sarcoplasmic reticulum, and of the variation in activity of mechanisms for Ca movements through the sarcolemma and the sarcoreticular membrane. A comparison of data obtained from simulated and real experiments suggested that the features characteristic of force-frequency relations for hypertrophic heart are a result of an enhanced volume of intracellular Ca-stores rather than of the total volume of the cardiomyocyte.