Permeability of canine cardiac sarcoplasmic reticulum vesicles to K$^+$, Na$^+$, H$^+$, and Cl$^-$.
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J. Biol. Chem. 257 (13): 7704--7711 (July 1982)

Cardiac muscle sarcoplasmic reticulum appears to contain channel-like structures that render the membrane permeable to small univalent ions. Canine heart microsomes fractionated according to buoyant density were examined by Millipore filtration, light scattering, and membrane potential m easurements. Enzymatic analysis and measurement of D-glucose permeation and Na/Ca exchange systems indicated two membrane fractions suitable for the permeability studies, one enriched in surface membranes with a buoyant density of 1.04-1.11 (10-25\% sucrose) and one enriched in sarcoplasmic reticulum with a buoyant density of 1.13-1.15 (30-34\% sucrose). Surface membrane vesicles impermeable to 3Hsucrose were largely impermeable to K$^+$, Na$^+$, and Cl$^-$, while sarcoplasmic reticulum vesicles impermeable to 3Hsucrose were readily permeable to K$^+$, Na$^+$, H$^+$, and Cl$^-$. Sarcoplasmic reticulum vesicles were essentially impermeable to Ca$^2+$, Mg2+, choline+, gluconate-, 1,4-piperazinediethanesulfonic acid (Pipes-), and D-glucose. These results suggest that cardiac muscle sarcoplasmic reticulum contains structures that facilitate the movement of small univalent ions. A possible function of these putative ion-conducting structures may be to allow rapid ion fluxes to counter electrogenic Ca$^2+$ fluxes across sarcoplasmic reticulum during cardiac muscle contraction and relaxation.
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