Normal Physiology of Insulin
Release

Normal Physiology of Insulin Release

When blood glucose levels are low, glycolysis in ß-cells is limited by the sparse influx of glucose, and the intracellular ATP/ADP ratio is low. Under these conditions, KATP channels in the membrane of ß-cells remain open, allowing K⁺, but not Na⁺ ions, to freely pass through. The cell membrane of ß-cells also contains energy-dependent ion pumps that pump Na⁺ ions out while pumping K⁺ ions in, thus creating opposing Na⁺ and K⁺ concentration gradients across the cellular membrane. Through the KATP channels, K⁺ ions can flow out of the cell along their concentration gradient, leaving behind an excess of negative charges carried by intracellular proteins. This excess of negative charges creates a negative membrane potential similar to the resting potential seen in nerve cells. The negative membrane potential causes voltage-dependent Ca²⁺ channels in the ß-cell membrane to remain shut and, consequently, the intracellular Ca²⁺ concentration to remain low. Under these conditions, no insulin is released from ß-cells.

When blood glucose levels are high, more glucose is taken up by the ß-cells, glycolysis is increased, and the intracellular ATP/ADP ratio rises. The increased intracellular ATP/ADP ratio triggers closing of the KATP channels in the cell membrane of the ß-cells. Since K⁺ ions can no longer flow out of the cells, the membrane potential becomes more positive, causing the opening of voltage-gated Ca²⁺ channels in the membrane of the ß-cells. Ca²⁺ ions flow in and stimulate the release of insulin from secretory vesicles stored in the cells.