Sodium and potassium permeability of membrane vesicles in a sarcolemma-enriched preparation from canine ventricle

Abstract

Vesicles in a highly enriched sarcolemma preparation from canine ventricle were found to develop membrane potentials in response to outwardly directed potassium and inwardly directed sodium concentration gradients. The magnitude of the potential measured by the fluorescent dye diS-C3-(5) suggested a sodium-to-potassium permeability ratio between 0.2 and 1.0 which is one to two orders of magnitude greater than values obtained for the myocardial cell. Radiotracer techniques were employed to evaluate the permeability coefficients of the isolated cardiac sarcolemma membrane for sodium and potassium under equilibrium conditions (i.e., equal salt concentrations in the intravesicular and extravesicular spaces). The uptake of sodium and potassium was best described by two exponential processes which followed an increment of uptake that occurred prior to the earliest assay time (i.e., 17 sec). The compartment sizes were linear, nonsaturable functions of the cation activity. Evaluation of the rate coefficients of cation uptake by the two exponential processes versus cation activity revealed that sodium influx via the slow process and potassium influx via the fast process varied linearly with cation activity, suggesting that the permeability coefficients were concentration independent for these compartments. Conversely, sodium influx via the fast process exhibited a nonlinear increase with increasing sodium activity, and potassium influx via the slow process appeared to saturate with increasing potassium activity. In general, the permeabilities of the sarcolemma-enriched preparation for sodium and potassium were of equal magnitude. The permeability coefficients were lower than that for the potassium coefficient reported for cardiac cells but are in the range of that reported for sodium.

Ca2+ pumping ATPase of cardiac sarcolemma is insensitive to membrane potential produced by K+ and Cl− gradients but requires a source of counter-transportable H+

“…It begs the question of what ions were moving in the absence of valinomycin. Kuwayama~s (t988) use of the valinomycin carries the implicit assumption that no membrane potential is developed by the KCI gradients, which is in contradiction to the findings of Bartschat et al (1980), Schilling et al (1984) and the present study. If the Goldman-Hodgkin-Katz equation can be used to calculate E,~ in the presence of valinomycin, it should also be used to calculate E~ in the absence of valinomycin; the change in transport rate should be compared to the change in E,~ due to the valinomycin addition.…”

“…This method of Em manipulation has been used in SL vesicles by a number of investigators and has been supported by studies with Em-sensitive fluorescent dyes. For example, Bartschat, Cyr and Lindenmayer (1980) and Schilling et al (1984) used the Era-sensitive fluorescent dye (DiS-C3-(5)) to measure membrane potentials generated by manipulations of the internal and external KC1 concentrations with and without valinomycin. Bartschat et al (1980) recorded a fluorescence decrease reflecting an inside negative membrane potential upon dilution of vesicles loaded with 150 mM KCI into a medium containing 147.5 mM choline CI plus 2.5 mM KC1.…”

Voltage-sensitive calcium flux promoted by vesicles in an isolated cardiac sarcolemma preparation

“…In the presence of the potassium-selective ionophore, valinomycin, membrane potentials that approached the potassium equilibrium potential could be established across the membrane of the isolated cardiac sarcolemma vesicle (Schilling et al, 1984). In the present study, the first experiment was designed to evaluate the capacity of the isolated cardiac sarcolemma vesicles to develop membrane potentials under the conditions which were to 5).…”

“…The mean values of calcium influx in pmol Ca/mg/sec from Fig. 5, panel b were converted to pmol Ca/ cmZ/sec by assuming a vesicular volume of 7.5/xl/mg (Schilling et al, 1984) and an average radius of 50 nm (Van Alstyne et al, 1980). The calcium permeability coefficient (/)ca) at each membrane potential (Era) was calculated by the following equation for unidirectional flux:…”

“…The loaded suspension was placed in solutions of different potassium concentrations to establish potassium diffusion potentials in the presence of valinomycin. Membrane potential was monitored by changes in fluorescence of the dye, diS-Cr(5), as described previously Schilling, Schuil, Bagwell & Lindenmayer, 1984).…”