Studies on the inflammatory response during involution of the ovine mammary gland.

Cessation of lactation, initiated by the removal of lambs from their mothers, induced an inflammatory response with elevated numbers of neutrophil leucocytes in mammary secretions being detected within 16 h. Leucocyte numbers were still elevated 7 d later. Infusion of whole milk into non-lactating glands caused no change in leucocyte numbers after 24 h. Involution secretions, however, had the capacity to induce an inflammatory response when infused into non-lactating glands. Daily infusion of endotoxin (100 ng) into non-lactating glands resulted in a marked attenuation of the inflammatory response. Such desensitization did not accompany reinfusion of involution secretion. Responses to involution secretion were enhanced in glands desensitized to endotoxin. The failure of desensitization to develop in glands receiving repeated infusions of involution secretion and the lack of cross-desensitization between involution secretion and endotoxin indicate that a novel mediator of inflammation may be produced during mammary involution in sheep.