Bovine mammary explant versus primary cell cultures: effect of bovine somatotropin and insulin-like growth factor-I on DNA content and protein synthesis.

Cellular DNA, milk protein content, and protein secretion by bovine mammary explants were compared to cultures of confluent and growing primary bovine mammary secretory cells over 4 d. Explants were obtained at slaughter from eight Holstein cows (120 +/- 35 d lactation). Primary cells were grown to confluence, cryopreserved, thawed, and cultured through five passages. Explants and cells were cocultured with liver and adipose tissue in the presence of somatotropin, insulin-like growth factor-I, and somatotropin + insulin-like growth factor-I. Cellular DNA and milk proteins were assayed using fluorescent probes and flow cytometry. Media proteins were assayed by densitometer scanning of electrophoresis gel bands. DNA content of explant, confluent, and growing primary cells increased similarly through the 96 h incubation. DNA content in G0G1 phase was increased by: (a) insulin-like growth factor-I in explant cells; (b) somatotropin, insulin-like growth factor-I, and their combination in confluent primary cells; and (c) the combination of somatotropin and insulin-like growth factor in growing primary cells. Approximately 65% of explant and confluent primary cells were in the G0G1 or differentiated phase compared to 47% for the growing primary cells. Whey protein content and secretion were similar among cell types. Explant cells contained and secreted more beta-casein than primary cells but secretion trends for beta-casein and k-casein were similar after 48 h for both cell types. Results suggest that primary cell cultures are comparable to explant cultures when used to study mechanisms of DNA and milk protein synthesis and secretion.