Developmental Appearance of Sodium Channel Subtypes in Rat Skeletal Muscle Cultures

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Abstract
22Na influx was measured in the established muscle cell line L-6 and in primary rat skeletal muscle cultures following activation of sodium channels by veratridine and sea anemone toxin II. Inhibition of the activated channels by tetrodotoxin (TTX) was analyzed with computer-assisted fits to one- or two-site binding models. In L-6 cultures, two inhibitable sodium channel populations were resolved at all ages in culture: a TTX-sensitive (K = 0.6-5.0 .times. 10-8 M) and an insensitive population (Ki = 3.3-4.9 .times. 10-6 M). In primary rat muscle cultures, the sensitivity of the toxin-stimulated channels to TTX changed with time in culture. In 4-day-old cultures, a single sodium channel population was detected using TTX (Ki = 2.4 .times. 10-7 M). A single population was also found in 6-day-old cultures (Ki = 5.3 .times. 10-7 M). By day 7 in culture, the inhibition of 22Na influx by TTX could be resolved into two components with high- and low-affinity sites for the toxin (Ki = 1.3 .times. 10-9 M and 9.6 .times. 10-7 M). We conclude that a single, toxin-activated sodium channel population with low affinity for TTX exists at early stages, whereas a second, high-affinity population evolves with time in primary rat muscle cultures. The expression of a high-affinity site apparently does not require ongoing neuronal involvement and may reflect an intrinsic property of the muscle cells.