Voltage-gated potassium channel in the context of "Ionized calcium"

Graded potentials are changes in membrane potential that vary according to the size of the stimulus, as opposed to being all-or-none. They include diverse potentials such as receptor potentials, electrotonic potentials, subthreshold membrane potential oscillations, slow-wave potential, pacemaker potentials, and synaptic potentials. The magnitude of a graded potential is determined by the strength of the stimulus. They arise from the summation of the individual actions of ligand-gated ion channel proteins, and decrease over time and space. They do not typically involve voltage-gated sodium and potassium channels, but rather can be produced by neurotransmitters that are released at synapses which activate ligand-gated ion channels. They occur at the postsynaptic dendrite in response to presynaptic neuron firing and release of neurotransmitter, or may occur in skeletal, smooth, or cardiac muscle in response to nerve input. These impulses are incremental and may be excitatory or inhibitory.

Calcium ions (Ca) contribute to the physiology and biochemistry of organisms' cells. They play an important role in signal transduction pathways, where they act as a second messenger, in neurotransmitter release from neurons, in contraction of all muscle cell types, and in fertilization. Many enzymes require calcium ions as a cofactor, including several of the coagulation factors. Extracellular calcium is also important for maintaining the potential difference across excitable cell membranes, as well as proper bone formation.

Plasma calcium levels in mammals are tightly regulated, with bone acting as the major mineral storage site. Calcium ions, Ca, are released from bone into the bloodstream under controlled conditions. Calcium is transported through the bloodstream as dissolved ions or bound to proteins such as serum albumin. Parathyroid hormone secreted by the parathyroid gland regulates the resorption of Ca from bone, reabsorption in the kidney back into circulation, and increases in the activation of vitamin D₃ to calcitriol. Calcitriol, the active form of vitamin D₃, promotes absorption of calcium from the intestines and bones. Calcitriol also plays a key role in upregulating levels of intracellular calcium, and high levels of this ion appear to be protective against cancers of the breast and prostate. The suppression of calcitriol by excessive dietary calcium is believed to be the major mechanism for the potential link between dairy and cancer. However, the vitamin D present in many dairy products may help compensate for this deleterious effect of high-calcium diets by increasing serum calcitriol levels. Calcitonin secreted from the parafollicular cells of the thyroid gland also affects calcium levels by opposing parathyroid hormone; however, its physiological significance in humans is in dispute.