Evidence for the involvement of a gastrointestinal peptide in the regulation of glucose uptake in the mammary gland of the lactating rat.

1. A method of obtaining serial arterial and mammary-venous blood samples was used to identify possible factors involved in the regulation of glucose uptake in the gland of the lactating rat. 2. Administration of insulin alone increased the arteriovenous glucose difference across the mammary gland of starved rats, but the time course of the recovery could not account for the restoration of arteriovenous glucose difference observed during refeeding [Page & Kuhn (1986). Biochem. J. 239, 269-274]. 3. A crude extract of the gastrointestinal tract (stomach-ileum) from lactating rats enhanced the change in mammary glucose uptake observed with insulin, but only when large amounts (100 munits/rat) of insulin were used. To achieve a similar recovery of arteriovenous glucose difference using near-physiological amounts (5 munits/rat) of insulin it was necessary to sever the mammary nerves. 4. A peptide fraction (of less than 10 kDa) isolated from the gut extract enhanced the effect of insulin in a similar manner to the crude extract. 5. It is suggested that in addition to insulin at least another component, probably a gut peptide, is required for the restoration of mammary glucose uptake during refeeding. An inhibitory component may also contribute to the regulation of mammary glucose extraction in the lactating rat.