Abstract
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 [3H]sucrose were largely impermeable to K+, Na+, and Cl-, while sarcoplasmic reticulum vesicles impermeable to [3H]sucrose were readily permeable to K+, Na+, H+, and Cl-. Sarcoplasmic reticulum vesicles were essentially impermeable to Ca2+, 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 Ca2+ fluxes across sarcoplasmic reticulum during cardiac muscle contraction and relaxation.
Highlights
[rnethy63H]choline,~ - [ l - ~ H ] g l u - riched in surface membranes with a buoyant density ocfose, and [fr~ctose-l-~H]sucrosweere purchased from New England
Sarcoplasmic reticulum vesicles were essentially impermeable to Ca2+,M&+,choline+, Isolation ofMembranes-Microsomal membrane fractions derived from canine ventricular tissue were prepared as follows. 50g of muscle were minced and homogenized in 375 ml of 0.3 M sucrose and 20 mM
The pellet was resuspended in 0.3M sucrose diac muscle sarcoplasmic reticulum contains structurceosntaining 25 m~ KC1 and 2.5 m~ Hepes, pH 7.2, and placed on a that facilitate the movement of small univalent iAons.discontinuous sucrose gradient consisting of10 ml of25% (w/w), 10 possiblefunctionof these putativeion-conducting structures may be to allow rapid ionfluxes to counter electrogenic Ca2+fluxes across sarcoplasmic reticulum ml of 30%, 10 ml of 3496, and 2.5 ml of 42% sucrose
Summary
From theDepartments of Biochemistry and Nutrition, and Physiology, School of Medicine, University of North Carolina, Chapel Hill, North Carolina27514. Cardiacmusclesarcoplasmicreticulumappears to evidence that cardiac muscle sarcoplasmic reticulum, like contain channel-like structures that render the mem- skeletal muscle sarcoplasmic reticulum, is permeable to K', brane permeableto small univalent ionsC. anine heart Na', H', and Cl-, suggesting the presence of univalent ionmicrosomes fractionated accordingto buoyant density conducting channels. were examined by Millipore filtration, light scattering, andmembranepotentiaml easurementsE. nzymatic
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