The significance of the action of insulin upon muscle membrane potential.

Two observations placed in juxtaposition may clarify the mechanism of action of insulin. The first is that insulin in contact with intact rat muscle causes net movement of potassium from extracellular fluid into muscle. The second is concerned with the electrical potential difference across the cell membrane of resting muscle which depends in part upon the relative concentrations of potassium on either side of the membrane. Let us consider these observations, beginning with the second. Using an appropriate apparatus, it can be shown that there is a potential difference between electrodes placed inside and outside a muscle fiber. This means that the membrane is polarized. The inside of the fiber is negative. The potential varies in different preparations. In excised frog muscle and in leg muscles in situ in adult rats, the resting membrane potential, ER, is about —90 mV. In excised leg muscles of immature rats EK is generally between —70 and —75 mV. 3