Effect of hypocalcic media on chemical sensitivity of toad sartorius muscle.

Membrane potential and spike activity were recorded through intracellular electrodes from toad sartorius muscles superfused in Krebs solution. When this was replaced by a hypocalcic medium (1/20 of normal Krebs Ca2+ concentration), muscles showed extrajunctional sensitivity to acetylcholine, histamine and noradrenaline applied iontophoretically. Normal sensitivity was restored when muscles were bathed in normal calcium media. Changes in chemical excitability of the fibers immersed in hypocalcic solution were related to decreased input membrane resistance. These observations emphasize the role played by Ca2+ on electrical stability and chemical sensitivity of skeletal muscle membrane.