Acetylcholine receptor in a C2 muscle cell variant is retained in the endoplasmic reticulum

We have examined the properties and intracellular localization of acetylcholine receptors in the C2 muscle cell line and in a variant (T-) that accumulates AChR intracellularly. On immunoblots, the subunit structures of the AChR from wild-type and T- cells were similar except that the gamma and delta subunits of the variant AChR had altered mobilities. Digestion with endoglycosidases H and F demonstrated that this difference results from a failure of high-mannose N-linked oligosaccharides on AChR subunits to be processed to complex forms in the variant. N-linked glycosylation of other proteins in the variant was normal. When examined by immunocytochemistry, the distribution of internal AChR in wild-type cells was consistent with a location both in the endoplasmic reticulum and in the Golgi. Variant cells, however, showed no evidence of Golgi staining. Subcellular fractionation experiments also demonstrated AChR in the Golgi fractions of wild-type cells, but not in those derived from T- cells. We conclude that in T- myotubes most of the AChR fails to be transported out of the endoplasmic reticulum.

A C2 muscle cell variant defective in transport of the acetylcholine receptor to the cell surface

We have previously reported the isolation of variants of the C2 mouse muscle cell line that express reduced amounts of acetylcholine receptors (AChRs) on their surface (Black, R. A., and Hall, Z. W. (1985) Proc. Natl. Acad. Sci. U.S.A. 82, 124-128). One of the variants, T-, makes an approximately normal amount of the AChR but accumulates most of it in an intracellular pool. This pool is stable and does not serve as precursor for surface AChR. Surface levels of insulin receptor and transferrin receptor are normal in T- cells, and a normal proportion of total hemagglutinin is expressed on the surface after infection of the T- variant with influenza virus. Pulse-chase experiments and kinetic analysis show: 1) that T- cells synthesize a normal amount of the alpha subunit but degrade it much more slowly than do wild-type cells; and 2) that newly synthesized alpha subunit is assembled into the AChR at a normal rate. A small fraction of the assembled AChR in T- cells is transported to the surface with normal kinetics, but most of it remains in an internal pool. This variant may provide an important tool for investigation of the factors that regulate AChR assembly and transport to the surface membrane.

Can late abortion be ethically justified?

This paper reviews the practice of late abortion in China and summarizes the arguments for morally justifying the 'one couple, one child' policy.

Developmental changes in lactate dehydrogenase-X activity in young jaundiced male rats

This study was designed to examine the testicular maturation in both jaundiced (jj) and nonjaundiced (Jj) male Gunn rats as indicated by the activity of a testis and sperm-specific enzyme, lactate dehydrogenase-X (LDH-X), and serum testosterone levels. Testicular cytosolic LDH-X activity was determined by oxidation of reduced nicotinamide adenine dinucleotide (NADH). Serum testosterone levels were measured by radioimmunoassay (RIA). Testicular LDH-X activity increased rapidly between 30 and 50 days of age in both Jj and jj Gunn rats. LDH-X activity was significantly lower in the jj Gunn rats at 50 and 60 days of age. Testosterone level was significantly lower in the jj Gunn rats at 50 days of age. LDH-X activity and serum testosterone levels were similar for both genotypes at 180 days of age. The delay of testicular maturation in the jaundiced males seems to be due to lower LDH-X activity and serum testosterone levels.

Characterization of acetylcholine receptor subunits in developing and in denervated mammalian muscle

We have used subunit-specific antibodies to identify and to characterize partially the alpha, beta, gamma, and delta subunits of rat skeletal muscle acetylcholine receptor (AChR) on immunoblots. The alpha subunit of rat muscle is a single band of 42 kDa, whereas the beta subunit has an apparent molecular mass of 48 kDa. Both alpha and beta subunits are glycosylated and contain one or more N-linked oligosaccharide chains that are sensitive to endoglycosidase H digestion. The gamma and delta subunits, on the other hand, each appear as doublets on immunoblots, with apparent molecular masses of 52 kDa (gamma), 48 kDa (gamma') and 58 kDa (delta), 53 kDa (delta'), respectively. In each case, the two bands are structurally related and the lower band is probably the partial degradation product of the corresponding upper band. Each of the four gamma and delta polypeptides is N-glycosylated and contains both endoglycosidase H-sensitive and endoglycosidase H-resistant oligosaccharides. When the AChRs purified from embryonic, neonatal, adult, and denervated adult rat muscles were compared, no differences in the mobilities of alpha, beta, or delta subunits on sodium dodecyl sulfate gels were detected among them, either with or without endoglycosidase treatment. The gamma subunits, which were present in AChRs purified from neonatal, embryonic, or denervated rat muscles, were also identical; no gamma subunit was detected, however, in AChRs of normal adult rat muscle.

Immunological evidence for a change in subunits of the acetylcholine receptor in developing and denervated rat muscle

We used specific antibodies to gamma, delta, and epsilon subunits to characterize acetylcholine receptor (AChR) in extracts and at endplates of developing, adult, and denervated rat muscle. The AChRs in normal adult muscle were immunoprecipitated by anti-epsilon and anti-delta, but not by anti-gamma antibodies, whereas AChRs in denervated and embryonic muscles were precipitated by anti-gamma and anti-delta, but showed little or no reactivity to anti-epsilon antibodies. In immunofluorescence experiments, AChRs at neonatal endplates bound antibodies to gamma or delta, but not epsilon, subunit, whereas those in adult muscles bound antibodies to epsilon or delta, but not gamma, subunit. AChRs at denervated endplates and at developing endplates between postnatal days 9 and 16 bound all three antibodies. We conclude that the distribution of gamma and epsilon subunits of the AChR parallels the distribution of AChRs with embryonic and adult channel properties, respectively.

Gastric processing and emptying of fat

This study was undertaken to determine whether fat leaves the stomach within or bound to the surface of particles of solid food. We studied gastric emptying of fat in 6 human subjects and in 6 dogs with Roux-en-Y duodenojejunostomies so that chyme leaving the stomach could be collected free of bile and pancreatic enzymes. Humans were studied with a duodenal multilumen tube so that phase-specific, nonabsorbed markers in the meal could be tracked with corresponding, phase-specific markers perfused into the duodenum. In this way, we observed the separate time-courses of gastric emptying of the aqueous and solid phases, as well as the extracellular fat (ECF) and intracellular fat (ICF) phases of the meal. In the dogs, all chyme leaving the stomach was collected from a Roux-en-Y fistula and was analyzed directly for aqueous, solid, ECF, and ICF markers in the meal. In both the humans and the dogs, the aqueous phase emptied promptly, whereas the solid, ECF, and ICF phases emptied together, in parallel, after an initial lag. In humans, 22% of the ECF versus 51% of the ICF (p less than 0.02) emptied on or in the solid food particles. In dogs, 81% of the ECF emptied as an oil, 13% emptied on the solid particles, and only 6% emptied as a stable, aqueous emulsion. Sixty-six percent of the ICF emptied in the solid food particles, 20% as a stable, aqueous emulsion, and 14% as an oil. We conclude that most of the ICF empties within the solid food phase, whereas most of the ECF empties as an oil phase.

Effects of viscosity and fluid outflow on postcibal gastric emptying of solids

It is known that the food-filled stomach retains large spheres or pieces of food, allowing the passage of food particles or of plastic spheres with diameters mainly below 2 mm. We have recently shown that spheres having densities greater or less than water emptied from the food-filled canine stomach more slowly than spheres of the same diameter with a density of 1. Thus, hydrodynamics seem to govern gastric emptying of spheres. The present studies were undertaken to determine how altering other hydrodynamic factors, viscosity and velocity of fluid outflow, might affect gastric sieving. Ten mongrel dogs were prepared with chronic duodenal fistulas, which allowed collecting and measuring of emptied spheres and food. The dogs were fed a standard meal of 75 g of steak plus 25 g of 99mTc-labeled chicken liver. Immediately afterward, 50 3.2-mm Teflon spheres were instilled into the stomachs along with 200- or 800-ml volumes of saline or saline plus guar (a viscous polysaccharide). Whether 200- or 800-ml volumes were instilled, the guar significantly sped the emptying of the spheres. Fluid outflow was twice as fast after the 800-ml instillates, but the faster outflow with the 800 ml of saline did not speed emptying of spheres. With the guar instillates, the faster outflow slightly sped the emptying of the spheres and significantly increased the diameter of emptied particles of 99mTc-labeled chicken liver. We conclude that meal viscosity significantly affects gastric sieving.

Effects of gossypol on the estrous cycle and ovarian weight in the rat

The effect of subcutaneously injected gossypol on the estrous cycle and ovarian weights in rats was examined. Daily injections of 5 or 10 mg gossypol/kg/day for 20 days significantly reduced the ovarian weights by 32% and 29%, respectively. Injections of 0.1 and 1.0 mg gossypol/kg/day had no significant effect on ovarian weight. Similarly, ovarian weights were significantly reduced by 37% when hemi-ovariectomized animals were injected with 10 mg gossypol/kg/day for 15 days whereas those of control animals increased in weight by 15%. Animals receiving 5 or 10 mg gossypol/kg/day essentially remained in diestrus by the 15th or the 10th day of injections, respectively. No significant change was found in the liver, adrenal gland or body weight of gossypol-treated rats.

The effects of a myasthenic serum on the acetylcholine receptors of C2 myotubes. I. Immunological distinction between the two toxin-binding sites of the receptor

We have examined the effect of a serum from a patient with myasthenia gravis on the binding of alpha-bungarotoxin (alpha-BuTx) to the acetylcholine receptors (AChRs) of a mouse muscle cell line, C2. After a 2-hr incubation, antibodies in the serum reduced toxin binding to C2 myotubes to a maximal extent of approximately 50%. The degradation of surface AChRs could account for the loss of only 5% of sites during the incubation; the remainder, therefore, must have been lost by blockage of binding. To investigate whether the antibodies blocked specifically one of the two toxin-binding sites that each AChR possesses, we used an analysis based on that of Sine, S. W., and P. Taylor, [1981) J. Biol. Chem. 255: 10144-10156). Although the two sites could not be distinguished by their rates of binding of alpha-BuTx, d-tubocurarine (dTC) inhibition of the initial rate of toxin binding revealed that the sites had affinities for dTC that differed by approximately 30-fold. Incubation with the myasthenic antibodies reduced the number of high affinity dTC sites, without affecting those of low affinity. We conclude that the two toxin-binding sites of the AChR are immunologically distinct.

The effects of a myasthenic serum on the acetylcholine receptors of C2 myotubes. II. Functional inactivation of the receptor

We have investigated the effect of antibodies from a myasthenic serum on the physiological properties of acetylcholine receptors (AChRs) in myotubes of a mouse muscle cell line, C2. The antibodies in this serum blocked the binding of 125I-alpha-bungarotoxin to the myotubes to an extent of about 50%. The antibodies also inhibited the increase in 22Na influx caused by carbamylcholine (CARB). At a concentration of antibody that blocked about 50% of toxin binding, greater than 80% of the AChR-mediated 22Na influx was blocked. The apparent KD for CARB, estimated from the dose-response curve for 22Na influx, was unaffected. The effect of the antibodies was further examined by patch-clamp recording. In greater than 30% of the patches from antibody-treated cells, no channel activity in response to acetylcholine was seen; in contrast, every patch from control cells showed activity. The channels that were seen after antibody treatment were indistinguishable from those seen in normal cells, both in their single-channel conductance and in the kinetic constants used to describe channel opening and closing. We conclude that the antibodies in this serum inhibit the functional response of AChRs in C2 myotubes to acetylcholine and do so by inactivating individual receptors.