Calcium-activated sarcoplasmic factor

Contraction of muscle follows an increase of Ca " in the muscle cell as a result of nerve stimulation. This initiates processes which cause the proteins myosin and actin to be drawn together making the cell shorter and thicker. The return of the Ca " to its storage site, the sarcoplasmic reticulum, by an active pump mechanism allows the contracted muscle to relax (27). Calcium ion, also a factor in the release of acetylcholine on stimulation of nerve cells, influences the permeabiUty of cell membranes activates enzymes, such as adenosine triphosphatase (ATPase), Hpase, and some proteolytic enzymes and facihtates intestinal absorption of vitamin B 2 [68-19-9] (28). 

Relatively large changes in membrane thickness have been demonstrated to alter the function of integral membrane proteins. An example of the magnitude of the change in membrane thickness needed to alter protein function is provided by studies of the sarcoplasmic reticulum calcium ATPase. Activity of this integral membrane protein in bilayers with symmetrically substituted, monounsaturated acyl chains with 16, 18, or 20 carbons is nearly constant. However, when the acyl chains are shortened to 14 carbons or lengthened to 22 carbons, activity is reduced by more than a factor of 3 (Lee, 1998). 

Batrachotoxin is extremely potent in antagonizing axonal transport 198). It would appear that the basis of the effect of batrachotoxin on axonal transport is dependent on interactions with sodium channels and the resultant influx of sodium ions. The effect is blocked by tetrodotoxin. In the mollusc, Aplysia californica, it has been proposed that the inhibition of axonal transport by batrachotoxin is not due to interactions with sodium channels 169), but this interpretation has been questioned 105). Blockade of axonal transport by batrachotoxin reduced uptake and transport of nerve growth factor at distal terminals 41), increased activity of certain muscle lysosomal enzymes 40), and altered uptake of calcium in muscle sarcoplasmic reticulum 264, 265). Batrachotoxin inhibits saltatory movements in neuroblastoma cells 105). This inhibitory effect is blocked by tetrodotoxin. 

---