Insulin Receptor Sites as Membrane Markers during Embryonic Development

Binding of insulin to sea urchin egg plasma membrane has been studied by biochemical and immunocytochemical methods. Unfertilized and fertilized eggs as well as embryos during the first cell division have been used.

• 1. 

  Competition experiments between ¹²⁵I-insulin (1 nM) and an excess of native insulin (30 μM) indicate a specific hormone fixation to membrane crude extracts from unfertilized and fertilized eggs. The magnitude of "specific binding'is comparable to values recorded for mammalian cells.
• 2. 

  Inhibition of insulin fixation by concanavalin A (100 μg/ml) suggests the glycoprotein composition of plasma membrane receptors.
• 3. 

  A 30-min incubation of unfertilized and fertilized eggs in the presence of insulin leads to a significant increase in cyclic AMP content.
• 4. 

  An immunocytochemical method demonstrates that insulin is selectively and specifically bound to the plasma membrane of eggs incubated in the presence of insulin before fixation.

It can be concluded that insulin receptor sites are components of sea urchin eggs plasma membrane. Insulin binding which leads to cyclic AMP accumulation is not deeply modified by fertilization and does not induce visible morphological changes in the eggs.