Identification of amino acid transport systems stimulated by FSH in rat testes.

Testes of 12-day-old rats preincubated (90 min) and incubated (60 min) with FSH in Krebs-Ringer bicarbonate buffer (KRB) increased their uptake of [14C]methylaminoisobutyric acid and [14C]aminoisobutyric acid. The hormone was ineffective in sodium-free KRB. Transport of [14C]aminoisobutyric acid in the presence of methylaminoisobutyric acid (MeAIB; 30 mmol/l) was not stimulated by FSH, and uptake of [14C]cycloleucine was not increased by FSH in any of the experimental conditions used. It was concluded that in immature rat testes FSH affects only the sodium-dependent system which permits the transport of MeAIB (transport system A) and has no effect on the sodium-independent system which prefers leucine (system L) or the sodium-dependent system which does not transport MeAIB (system ASC).