Ion pathways in transverse tubules. Quantification of receptors in membranes isolated from frog and rabbit skeletal muscle

Abstract

The presence of four cation pathways in membrane vesicles isolated from transverse tubules of frog and rabbit skeletal muscle was studied by measuring binding of specific blockers. Transverse tubules purified from frog muscle have a maximal binding capacity for [3H]nitrendipine (a marker for voltage-dependent calcium channels) of 130 pmol/mg of protein; this binding is strongly dependent on temperature and, at 37°C, on the presence of diltiazem. Receptors for [3Hlethylenediamine tetrodotoxin (a marker for voltagedependent sodium channels) and for 12SI-labeled a-bungarotoxin (a marker for acetylcholine-mediated channels) showed maximal binding values of about 5 pmol/mg. The number of sodium-pumping sites in the isolated tubule vesicles, inferred from [3H]ouabain binding, was 215 pmol/mg.'The high purity of this preparation makes feasible the use of these values as a criterion to judge the degree of purity of isolated preparations, and it allows investigation of transverse tubule contamination in other muscle membrane fractions.