Question

Compare and contrast insulin secretion in the beta cells of the pancreas with synaptic transmission. What are similarities and differences with regard to the signal for exocytosis in neurons and pancreatic beta cells, and what causes the signal to occur?

Answer 1

In beta cells, insulin release is stimulated primarily by glucose present in the blood. Insulin secretion involves a sequence of events in β-cells that lead to fusion of secretory granules with the plasma membrane. Insulin is secreted primarily in response to glucose, while other nutrients such as free fatty acids and amino acids can augment glucose-induced insulin secretion. In addition, various hormones, such as melatonin, estrogen, leptin, growth hormone, and glucagon like peptide-1 also regulate insulin secretion. Thus, the β-cell is a metabolic hub in the body, connecting nutrient metabolism and the endocrine system.

Synaptic transmission is the biological process by which a neuron communicates with a target cell across a synapse. Chemical synaptic transmission involves the release of a neurotransmitter from the pre-synaptic neuron, and neurotransmitter binding to specific post-synaptic receptors.

Exocytosis is a process by which a cell transports secretory products through the cytoplasm to the plasma membrane. Secretory products are packaged into transport vesicles (membrane-bound spheres).

Differences in Exocytosos in Neurons and Pancreatic beta cells:

Synaptic vesicle exocytosis occurs in neurons of the nervous system. Nerve cells communicate by electrical or chemical (neurotransmitters) signals that are passed from one neuron to the next. Neurotransmitters are transmitted by exocytosis. Exocytosis in neuronal chemical synapses is Ca2+ triggered and serves interneuronal signalling.

Exocytosis in β-cells does not only result in the release of insulin. The β-cell LDCVs contain a number of substances in addition to insulin. These include Zn2+, adenine nucleotides, Ca2+, glutamate, chromogranin, islet amyloid polypeptide, serotonin and dopamine. Elevated concentrations of glucose within the beta cell ultimately leads to membrane depolarization and an influx of extracellular calcium. The resulting increase in intracellular calcium is thought to be one of the primary triggers for exocytosis of insulin-containing secretory granules