APPLICATION OF A MECHANISTIC MODEL TO STUDY COMPETITIVE-INHIBITION OFAMINO-ACID-UPTAKE BY THE LACTATING BOVINE MAMMARY-GLAND

A mathematical model is used to describe uptake by a countertransport system and subsequent flow of three amino acids (AA), Phe, Val, and Me t, fi om arterial blood to milk protein in the mammary gland of a lact ating cow. The model suggests that total uptake of all AA is higher th an net uptake and that a large proportion of the incoming AA is releas ed from the cell directly back to blood. The model is used to predict which of the three AA is Limiting the rate of milk protein synthesis a nd the response to increased arterial concentration of the first-limit ing AA. Simulations are performed to predict possible outcomes of seve ral experimental protocols to AA infusion, which might be used to test in vivo the responsiveness of the bovine mammary gland to an altered arterial concentration of AA. Of the three AA considered, arterial Met concentration appears to be first-limiting. The infusion profile that gives the greatest response in milk protein synthesis rate alters the arterial profile of AA such that it is identical to that of proteins originating in the mammary gland. Model construction can be simplified by acknowledging normal biological constraints.