Calcium as a Mediator of Hormone Action

activities and by electron microscopy. The higher-density fractions (1.2-1.5~-sucrose) contained actomyosin fibrils, mitochondria and sacroplasmic reticulum, and were enriched in cytochrome oxidase and Caz+-dependent adenosine triphosphatase. The endoplasmic-reticulum marker, NADPH-cytochtome c reductase, was found throughout the gradient. Lower-density fractions (0.75-1 .OM sucrose) were enriched in plasma-membrane markers, acetylcholinesterase and ouabain-sensitive p-nitrophenyl phosphatase, and in y-aminobutyrate uptake; numerous vesicles were also evident. Muscimol-sensitive y-aminobutyrate binding and a-dihydropicrotoxinin binding were both found in membrane fragments appearing in fractions slightly below (0.9-1 .I M) those showing y-aminobutyrate uptake. Fractions could be obtained which exhibited y-aminobutyrate uptake only, with no muscimol inhibition, or which exhibited binding alone, with 100% inhibition by 10,m-muscimol (Meiners et al., 1977). This supports the suggestion made above that the muscimol-sensitive sites are not related t o the y-aminobutyrate-uptake process. For reasons mentioned above, these sites have properties more consistent with receptor sites. The near coincidence in the gradient of tissue fractions showing both muscimolsensitive y-aminobutyrate binding and picrotoxinin binding [a result which has also been found in mammalian brain (Ticku et al . , 1977)] supports the notion that both binding activities are related to y-aminobutyrate receptor/ionophore macromolecules. Such ligand binding would seem t o provide biochemical assays suitable for purification and further study of the properties of these proteins.