Insulin appears to act through which kind of receptor?

1. Insulin is a protein hormone, so it is water soluble and must signal from the cell surface. 2. It appears to activate a tyrosine kinase receptor at that surface. 3. The receptor triggers phosphorylation of many substrates inside the cell. 4. Those reactions converge on moving glucose transporter vesicles to the membrane. 5. An intracellular nuclear receptor would require the hormone to cross the lipid bilayer, which a protein hormone cannot do. 6. The G protein route describes the cAMP and calcium systems used by hormones such as glucagon and calcitonin. _Source: OpenStax Anatomy and Physiology (CC BY 4.0), Ch 17 "The Endocrine System", section 17.9 The Endocrine Pancreas_