Insulin (100 μU/ml) increased the electrical potential difference between the inside and outside of fused myotubes from fetal rat hindlimbs 5–8 days after primary culture. Myotubes were hyperpolarized by 3.5 ± 1.2 mV (mean ± SE). Tetrodotoxin (TTX) eliminated spontaneous twitching. With TTX, membrane potentials of myotubes became more negative both in the absence and presence of insulin, and insulin hyperpolarized myotubes by 6.6 ±1.7 mV, which is at least as great as the hyperpolarization this concentration of insulin produced in muscles excised from young adult rats. However, insulin did not significantly stimulate uptake of the glucose analogue 2-deoxy-D-glucose, although there was basal D-glucose transport. Specific insulin receptors were present, demonstrated by radioinsulin binding. In summary, in primary cultures of rat myotubes, there are insulin receptors, insulin hyperpolarizes at least as much as it hyperpolarizes muscle excised from young adult rats, but insulin does not stimulate D-glucose transport.

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