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Exposing one end of a frog's toe muscle (ex. dig. long. IV) in a sucrose gap apparatus to a Ca++ -free Ringer solution resulted in a depolarization of about 15 mV in about 20-25 min (or 40-45 min in a choline Ringer solution) with very little further depolarization with continued exposure (1 hr or more). As little as 0.05mm Ca++ was sufficient to prevent or greatly reduce the depolarization. Means and frequency histograms of membrane potentials recorded with intracellular microelectrodes were plotted for fixed time periods of exposure to 0-Ca++ The mean depolarizations were between 12 and 35mV when tested 15 min after exposure to 0-Ca++ but were only about 10-15mV when first tested after a 1-hr exposure; the maximum fall observed was about 55 mV after 90 min and 4 periods of sampling. The most consistent change produced by 0-Ca++ was a flattening and broadening of the frequency histogram. These results suggest that the large falls (>20 mV) in frog' skeletal muscle membrane potentials in 0-Ca++ solutions seen here and often reported by other workers are an artifact produced by the intracellular microelectrodes on the cell membranes made fragile by the reduction in membrane Ca++.

EFFECTS OF REDUCING THE EXTRACELLULAR CALCIUM CONCENTRATION ON THE RESTING POTENTIAL OF FROG'S SKELETAL MUSCLE FIBERS