METHODS

Animals and surgical procedures. Experiments were carried out on 18 overnight-fasted mongrel dogs (18-25 kg) of either sex that had been fed a high protein diet (Wayne dog chow, Wayne Lab-Blox, Allied Mills, Inc., Chicago, IL) for 3-4 wk prior to their use. Sixteen days before each experiment, a laparotomy was performed under general anesthesia (sodium pentobarbital, 25 mg/kg), and a Silastic catheter (Dow Corning Corp., Midland, MI) was inserted into a tributary of the splenic vein. The tip of the catheter was advanced until it was located at the junction of the splenic and portal veins. A similar catheter was inserted into the femoral artery following a cut-down over the left inguinal area, and its tip was positioned in the left iliac artery. After both catheters were inserted, they were filled with saline containing heparin (200 U/ml; Abbott Laboratories, North Chicago, IL), and their free ends were knotted and placed in a subcutaneous pocket so that complete closure of both skin incisions was possible. Two weeks after surgery, blood was withdrawn to determine the leukocyte count and hematocrit of the animal. The only animals used were those that had 1) a leukocyte count below 16,000 mm3, 2) a hematocrit above 38%, 3) a good appetite (consuming> two-thirds of the daily ration), and 4) normal stools. On the day of an experiment the subcutaneous ends of the catheters were freed through a small skin incision made under local anesthesia (2% lidocaine, Astra Pharmaceutical Products, Inc., Worcester, MA). The contents of each catheter were aspirated, and saline was infused through them at a slow rate (0.1 ml/min) until the experiment was begun. The splenic catheter was used for hormone infusion and the arterial catheter for blood sampling. Angiocaths (18 gauge; Abbott) were inserted percutaneously into the two cephalic veins as well as the right saphenous vein. The right and left cephalic veins were used for [14C] alanine and [3H] glucose infusion, respectively, whereas the saphenous vein was used for SRIF infusion. After completion of preexperimental