The following are the main end effects of insulin stimulation:
1. Within seconds after insulin binds with its membrane receptors, the membranes of about 80% of
the body’s cells markedly increase their uptake of
glucose. This action is especially true of muscle cells
and adipose cells, but it is not true of most neurons in the brain. The increased glucose transported into the cells is immediately phosphorylated and
becomes a substrate for all the usual carbohydrate
metabolic functions. The increased glucose transport is believed to result from translocation of multiple intracellular vesicles to the cell membranes;
these vesicles carry multiple molecules of glucose transport proteins, which bind with the cell membrane and facilitate glucose uptake into the cells.
When insulin is no longer available, these vesicles separate from the cell membrane within about 3 to 5 minutes and move back to the cell interior to be used again and again, as needed.
2. The cell membrane becomes more permeable to many of the amino acids, potassium ions, and phosphate ions, causing increased transport of these
substances into the cell.
3. Slower effects occur during the next 10 to 15 minutes to change the activity levels of many more intracellular metabolic enzymes. These effects result mainly from the changed states of phosphorylation of the enzymes.
4. Much slower effects continue to occur for hours and even several days. These result from changed
rates of translation of messenger RNAs at the ribosomes to form new proteins and still slower effects from changed rates of transcription of DNA in the
cell nucleus. In this way, insulin remolds much of the cellular enzymatic machinery to achieve some of its metabolic effects.
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