Effect of insulin on liver pyruvate kinase in vivo and in vitro.

The relative rate of pyruvate kinase synthesis in alloxan-diabetic rats was Y3 of that observed in controls and increased 3to 4-fold upon treatment with insulin for 3 days. Similar changes in the catalytic activity were observed: 120 + 40, 650 + 100, and 230 ± 79 units of pyruvate kinase/liver in diabetic rats, diabetic rats maintained on insulin for 3 days, and controls, respectively. Thus, changes in the rate of synthesis of liver pyruvate kinase may be a significant mechanism which results in an altered level of liver pyruvate kinase activity in vivo. The effect of insulin on liver pyruvate kinase levels was investigated in vitro by use of primary cultures of rat liver parenchymal cells. Hepatocytes freshly isolated from livers of control rats contained 14.1 _ 2.0 (n = 9) units of pyruvate kinase activity/mg of DNA. When maintained in a medium containing insulin (10-8 M), the pyruvate kinase activity of hepatocytes remained constant throughout the 7to 14-day culture periods. In contrast, in cells maintained in the absence of insulin, the pyruvate kinase activity decreased progressively with time with an apparent first order decay with a half-life of 2 days. Hepatocytes from diabetic rats initially have a low pyruvate kinase activity, 3.2 units/mg of DNA. When these cells were maintained in medium containing 10 6 M insulin, the pyruvate kinase activity remained low for the initial 3 days, then rapidly increased 7-fold between days 3 and 7 and finally decreased back to normal levels by day 11. The relative rate of liver pyruvate kinase synthesis remained constant for 2 days, increased 3-fold on day 3, and remained elevated throughout the remainder of the 7-day culture period. Likewise, administration of insulin in vivo had increased the relative rate of liver pyruvate kinase synthesis; however, effects could be seen within 1 day. The more rapid response of liver pyruvate kinase to insulin in vivo may be the result of an altered systemic endocrine status in addition to a direct effect of insulin on the pyruvate kinase activity in liver cells. The 2or 3-day lag period observed with hepatocytes from diabetic rats or control hepatocytes cultured 24 h in the absence of insulin may be due to the time required for the transcription, translation, and post-translational control of pyruvate kinase synthesis.