Net metabolism of hormones by portal-drained viscera and liver of lactating holstein cows.

Net flux of insulin, glucagon, somatotropin, somatomedin-C, and somatostatin across the portal-drained viscera and liver of four lactating Holstein cows was measured at 4 and 8 wk postpartum. Cows were fed ad libitum intake a 60:40 corn silage:concentrate diet and milked at 12-h intervals. Milk yield and DM intake were 32.2 and 15.6 kg/d respectively. Twenty-four consecutive measurements of insulin, glucagon, and somatotropin net flux were obtained at 30-min intervals; 12 hourly measurements were obtained for somatomedin-C and somatostatin. Net flux is venous-arterial concentration difference times blood flow. Net flux of somatomedin-C was not detectable across the portal-drained viscera or liver. Somatotropin was removed by total splanchnic tissues. There was net production of somatostatin across portal-drained viscera, but net flux of somatostatin across liver was not significant. On a net basis, removal of insulin and glucagon by liver accounted for 66% of portal-drained visceral production. Within cows and sampling days, increases in net portal-drained visceral production of insulin and glucagon often were mirrored by increases in net removal by liver. As in other species, the bovine liver is an important regulator of circulating concentrations of insulin and glucagon.