The expression of alpha-lactalbumin and casein genes was examined in guinea-pig mammary tissue taken from animals both pre- and post-partum. 2. Analysis of total RNA by RNA excess hybridization with sequence-specific complementary DNA probes demonstrated that alpha-lactalbumin mRNA was present late in pregnancy, and that maximum concentrations were present at parturition. Casein gene transcripts were absent late in pregnancy (62 days), but by parturition were present at concentrations identical to those found at all time points examined throughout lactation. 3. Studies using mammary explants in organ culture showed that tissue from pregnant animals, or animals at parturition, synthesized and secreted only alpha-lactalbumin. After parturition, at the onset of casein synthesis, differential rates of secretion of alpha-lactalbumin and the caseins were observed. 4. The results are discussed in terms of the multiple intracellular mechanisms involved in the regulation of milk protein gene expression in the guinea-pig mammary gland.
