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

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Abstract
Pharmacological blockade of the spontaneous electrical activity present in primary cultures of rat myotubes by growth in bupivacaine, tetrodotoxin or KCl increased 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 .mu.M. Growth of myotubes in the presence of 1 .mu.M A23187 [calcimycin] 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. Apparently, there is a Ca2+-mediated negative feedback system in which electrical excitability may be regulated by altering the number of tetrodotoxin-sensitive Na+ channels.