Contamination of a cardiac sarcolemmal preparation with endothelial plasma membrane.

Preparation of sarcolemma from whole rabbit heart using the method of Jones et al. (Jones,L.R., Besch, H.R., Fleming, J.W., McConnaughey, M.M. and Watanabe, A.M. (1979) J. Biol. Chem. 254, 530-539) results in a 46-fold purification of the endothelial plasmalemma-specific marker angiotensin converting enzyme. This implies contamination of the sarcolemma with vascular endothelial plasmalemma. During preparation of sarcolemma from sheep heart, using the same method, angiotensin converting enzyme copurified with the general plasma membrane marker (Na+ + K+)-ATPase. The ratio of myocyte to endothelial plasma membrane in the final preparation is therefore similar to that in the whole heart homogenate. Ultrastructural analysis has shown that the myocyte/endothelial surface area is 70:30 in whole cardiac muscle. Comparison of angiotensin converting enzyme activity of an endothelial plasma membrane fraction with that of whole heart sarcolemma suggests an upper limit of 42% for endothelial contamination. Contamination by endothelial plasmalemma was dramatically reduced by preparing sarcolemma from myocytes produced by proteolytic disruption of whole hearts. Following disruption, myocytes were separated from non-muscle cells by sedimentation through 0.5 M sucrose. Sarcolemma prepared from sheep cardiac myocytes had approximately 15-fold less angiotensin converting enzyme activity than whole sheep heart sarcolemma but comparable ouabain-inhibitable (Na+ + K+)-ATPase activity.

Solubilisation and reconstitution of the rabbit skeletal muscle sarcoplasmic reticulum K+ channel into liposomes suitable for patch clamp studies.

Sarcoplasmic reticulum (SR) membrane vesicles have been prepared from rabbit skeletal muscle and solubilised using K+ cholate. Solubilised membrane proteins were reconstituted into small asolectin liposomes by dialysis against cholate-free solution. Large liposomes were produced by freezing and thawing at -80 degrees C and room temperature, respectively. The liposomes were assayed for the SR K+ channel using the patch clamp technique. Channel density was modulated by varying protein:lipid ratios during reconstitution. Channels inserted into the membrane with a preferred orientation. The solubilised and reconstituted channel behaves ohmically over the holding potential range +/- 70 mV and has a conductance of 178.4 +/- 4.4 pS (mean +/- SE, n = 37) in 200 mM KCl. The channel has a selectivity sequence of K+ greater than NH4+ greater than Rb+ greater than Na+ and K+ conductance is blocked by hexamethonium and decamethonium. The opening probability of the reconstituted channel is voltage dependent. The conductance and gating characteristics displayed by the solubilised and reconstituted channel correlate well with those previously observed following the fusion of native SR membrane vesicles with planar phospholipid bilayers.

The characterization of a monovalent cation-selective channel of mammalian cardiac muscle sarcoplasmic reticulum.

Rabbit cardiac muscle sarcoplasmic reticulum (SR) was isolated and separated into ryanodine-sensitive and -insensitive fractions (L.R. Jones and S.E. Cala, J. Biol. Chem. 256:11809-11818, 1981). Vesicles of cardiac SR were incorporated into planar phospholipid bilayers by fusion and the channel activity of the membrane studied under voltage-clamp conditions (C. Miller, J. Membrane Biol. 40: 1-23, 1978). Both fractions contain a monovalent cation-selective three-state channel. In the presence of 75 mM K2SO4, the fully open state (beta) conductance of this channel is 157.2 +/- 30 pS and the sub-state (alpha) conductance is 100.7 +/- 21 pS. Both open states display the same selectivity sequence for monovalent cations, i.e. K+ greater than NH+4 greater than Rb+ greater than Na+ greater than Li+ and may be blocked by the skeletal muscle relaxants decamethonium and hexamethonium. Block occurs when the compounds are added to either side of the membrane. The properties of the cardiac SR cation channel are compared with those of the previously reported monovalent cation-selective channels of mammalian and amphibian skeletal muscle SR.

Structural aspects of the sarcoplasmic reticulum K+ channel revealed by gallamine block.

We have studied single-channel conductance fluctuations of K+ channels present in the sarcoplasmic reticulum (SR) membrane systems of rabbit cardiac and skeletal muscle. K+ conductance through the channels is reversibly blocked by gallamine. Conductance block occurs only from the trans side of the channel and is resolved as a smooth reduction in the open state conductance. At a fixed K+ concentration, conduction decreases with increasing gallamine concentration and the data can be fitted to a single-site inhibition scheme. The degree of block seen at a constant gallamine concentration decreases as K+ concentration is increased, indicating competition between gallamine and K+. Gallamine block is voltage dependent, the degree of block increasing with increasing negative holding potential. Quantitative analysis of block yields a zero voltage dissociation constant of 55.3 +/- 16 microM and an effective valence of block of 0.93 +/- 0.12. We conclude that gallamine blocks by interacting with a site or sites located at an electrical distance 30-35% into the voltage drop from the trans side of the channel. This site must have a cross-sectional area of at least 1.2 nm2. The results of this study have been used to modify and extend our view of the structure of the channel's conduction pathway.