Charge movement in overstretched amphibian skeletal muscle cut fibers

Membrane current experiments were done in the absence of any permeable ion (except calcium) and the presence of TTX (tetrodotoxin) and TEA+ (tetraethylammonium) in the external solution and caesium in the internal solution at two different sarcomere lengths (3.8 and 4.56 microns) in skeletal muscle fibers of frog (pipiens). Single cut fibers were voltage-clamped in a double Vaseline gap apparatus, and intramembrane charge movement and slow calcium currents were measured. In order to obtain the charge movement, a conventional control was used (the response to a negative pulse) obtained in normally polarized fiber. A descriptive analysis of the results shows us that at 3.8 microns the fibers respond with slow calcium current and charge movement with the characteristics of charge 1 (Q1), but at a sarcomere length of 4.56 microns, just a slow calcium current is obtained. The absence of intramembrane charge movement in overstretched fibers could suggest a modification in the transverse tubular system geometry obscuring the charge movement phenomenon (the linear membrane capacity in stretched fibers was 13.10 micro F/cm2 as average for 5 fibers and in overstretched ones was 13.61 micro F/cm2 as average for 11 fibers). The presence of slow calcium current in this condition (overstretched fiber) means an electrical connection of the tubule with superficial membrane and suggests that the charge movement does not correspond to gating of the "L" type calcium channel.