Dipping cone to correct optical distortions at liquid surfaces.

A dipping cone attachment for use with a long-working-distance objective is described. The device eliminates degradation of the microscope image which occurs when microelectrodes are applied to single muscle fibers in vitro.     

The putative agrin receptor binds ligand in a calcium-dependent manner and aggregates during agrin-induced acetylcholine receptor clustering.

Agrin derived from Torpedo electric organ induces the clustering of acetylcholine receptors (AChRs) on cultured myotubes. As a first step toward characterizing the plasma membrane receptor for agrin, we have examined agrin binding to cultured myotubes. Agrin binding is saturable as measured by radioimmunoassay and, like agrin-induced AChR clustering, requires extracellular calcium. Immunofluorescence shows that on myotubes incubated with agrin at 4 degrees C, agrin binds in a uniform, finely punctate pattern that correlates poorly with the distribution of AChRs. Myotubes stimulated with agrin at 37 degrees C for greater than or equal to 2 hr show a coclustering of agrin binding sites and AChRs. By contrast, if anti-AChR antibodies are used either to cluster or to internalize AChRs, the distribution and number of agrin binding sites remain unchanged. The aggregation and calcium dependence of the putative agrin receptor may represent important control points in postsynaptic differentiation.     

The actions of verapamil at the neuromuscular junction

1. The actions of the calcium channel blocker verapamil were studied at the neuromuscular junction of the frog Rana pipiens. 2. In the presence of 50 microM verapamil, subthreshold endplate potentials were produced, and the quantal content was reduced by a factor of 3. 3. Verapamil (10-50 microM) also reduced the postjunctional membrane sensitivity as measured by (a) carbachol iontophoresis and (b) miniature endplate potential amplitude. In addition, verapamil had a strong inhibitory effect on the postjunctional membrane response to repetitive iontophoretic application of carbachol. 4. Thus, verapamil has both pre- and postsynaptic actions at the neuromuscular junction.     

Factors in the inactivation of postjunctional membrane receptors of frog skeletal muscle

Several factors which influence the rate of inactivation of muscle postjunctional membrane (PJM) receptors during the sustained application of carbamylcholine (CARB) have been studied by two methods. The rate of inactivation was increased by elevating the tonicity of the bathing medium, by increasing the CARB concentration, by raising the calcium ion concentration, and by substituting SO₄ ⁼ for Cl^- ions in the extracellular fluid. The relative effectiveness of calcium and other divalent cations in receptor inactivation was compared. In the absence of calcium, other divalent cations such as magnesium, strontium, or manganese were not efficient substitutes for calcium. In the presence of calcium, the addition of strontium or manganese ions accelerated the rate of receptor inactivation, but the addition of magnesium (up to 12 mM) inhibited this process. The inactivation of the membrane receptors in denervated muscle fibers was found to be similar to that in innervated muscle fibers. Various factors in PJM receptor inactivation are discussed. It is suggested that PJM receptor inactivation is influenced by the binding of calcium ions to sites on the internal surface of the PJM.