Amino acid transport in diaphragms from newborn rats: evidence for insulin resistance.

Diaphragms from rats under 24-h-old did not show the well-known increased transport of alpha-aminoisobutyrate found in older tissues in respone to insulin in vitro. A small effect was apparent by 3 days, and stimulation increased as donor rats aged (up to 4--5 wk). One-day diaphragms also had greater uptake than older tissues, due to both decreased Km and elevated Vmax. The change in insulin sensitivity did not result from alteration in the transport system used by alpha-aminoisobutyrate because uptake showed characteristics of the A system at both 1 day and older. Results suggest instead that the 1-day tissues had been made insulin-resistant by high insulin levels in donor animals. Plasma insulin levels of 1-day-old rats were 5 times those of 5-day animals. Elevating the plasma insulin levels of 5-day or 25- to 35-day rats led to a decreased effectiveness of insulin in vitro in stimulating alpha-aminoisobutyrate transport into their diaphragms. In the older animals, the stimulation was inversely proportional to the plasma insulin level 2 h after insulin injection.