Beta Cell - Function

Function

The primary function of a beta cell is to store and release insulin. Insulin is a hormone that brings about effects which reduce blood glucose concentration. Beta cells can respond quickly to spikes in blood glucose concentrations by secreting some of its stored insulin while simultaneously producing more.

Control of Insulin Secretion

• Voltage gated calcium ion channels and ATP sensitive potassium ion channels are embedded in the cell surface membrane of beta cells.
• The ATP sensitive potassium ion channels are normally open and the calcium ion channels are normally closed.
• Potassium ions diffuse out of the cell, down its concentration gradient, making the inside of the cell more negative with respect to the outside (as potassium ions carry a positive charge). At rest, this creates a potential difference across the cell surface membrane of -70mV.
• When the glucose concentration outside the cell is high, glucose molecules move into the cell by facilitated diffusion, down its concentration gradient.
• These glucose molecules are quickly used in metabolism to produce ATP which increases the ATP to ADP ratio.
• The ATP sensitive potassium ion channels close when this ratio rises. This means that potassium ions can no longer diffuse out of the cell.
• As a result, the potential difference across the membrane becomes more positive (as potassium ions are accumulating inside the cell).
• This change in potential difference opens the voltage gated calcium ion channels which allows calcium ions from outside the cell to diffuse in, down its concentration gradient.
• When the calcium ions enter the cell, they cause vesicles containing insulin to move to and fuse with the cell surface membrane, releasing insulin by exocytosis.

Other Hormones Secreted by Beta Cells

• C-peptide which is secreted into the bloodstream in equimolar quantities to insulin. C-peptide helps to prevent neuropathy and other vascular deterioration related symptoms of diabetes mellitus. A practitioner would measure the levels of C-peptide to obtain an estimate for the viable beta cell mass.

• Amylin, also known as islet amyloid polypeptide (IAPP). The function of amylin is to slow the rate of glucose entering the bloodstream. Amylin can be described as a synergistic partner to insulin, where insulin regulates long term food intake and amylin regulates short term food intake.