Electrical activity and cytosolic calcium regulate levels of tetrodotoxin-sensitive sodium channels in cultured rat muscle cells

Pharmacological blockade of the spontaneous electrical activity present in primary cultures of rat myotubes by growth in bupivacaine, tetrodotoxin, or KCl was found to increase the number of voltage-sensitive Na⁺ channels 38-83% as measured by the specific binding of [³H]saxitoxin. The inhibition of spontaneous electrical activity and increase in channel density by bupivacaine displayed an identical dose response, with a half-maximal effect at 3.0 μM. Growth of myotubes in the presence of 1 μM A23187, a Ca²⁺-specific ionophore, resulted in a 30-60% decrease in the number of tetrodotoxin-sensitive channels with no change in affinity for [³H]saxitoxin. A23187 was able to overcome the increase in channel density produced by bupivacaine. These results suggest the presence of a Ca²⁺-mediated negative feedback system in which electrical excitability may be regulated by altering the number of tetrodotoxin-sensitive Na⁺ channels.