Receptor tyrosine kinases: What is meant by basal phosphorylation of the receptor?

Question

I am reading a journal paper about the insulin receptor. The insulin receptor is a receptor tyrosine kinase, and upon ligand binding, undergoes autophosphorylation of intracellular tyrosine residues. Regarding this paper, the terms "basal phosphorylation" or "phosphorylation in the basal state" of the insulin receptor is frequently used. However, I am unsure what the exact definition of basal phosphorylation is.

Is it the phosphorylation the receptor undergoes under physiological/normal conditions in vivo without the addition of exogenous insulin?

For example in the paper, they treat brain slices from wild-type mice with exogenous insulin, and they report that the phosphorylation of the insulin receptor exceeds the basal state.

Any insights are appreciated.

Answer 1

I think these excerpts are helpful in understanding the authors' use of basal, both in reference to insulin levels and insulin receptor phosphorylation --

As expected, insulin receptors from untreated WT slices are not detectably phosphorylated in the presence of low basal levels of insulin, but become rapidly phosphorylated upon exposure to exogenous insulin. In contrast, insulin receptors from MHCI-deficient $β2m^{-/-}$ $TAP^{-/-}$ hippocampi show significant basal phosphorylation, which is comparable in magnitude to insulin-stimulated phosphorylation in WT slices (Fig. 1A).

By increasing the threshold for insulin receptor activation, MHCI ensures that dynamic insulin sensitivity is preserved, even on a background of basal circulating insulin.

In this context, I would interpret basal phosphorylation to mean the extent of receptor phosphorylation in tissue in the presence of a stable concentration of circulating insulin concomitant with fasting levels of blood glucose.