Binding and action of insulin and glucagon in monolayer cultures and fresh suspensions of rat hepatocytes

Abstract

Insulin and glucagon binding, and the subsequent stimulation of amino-acid transport, were investigated in adult-rat hepatocytes. Cells were used either in suspension shortly after isolation, or as monolayers after 20 h of culture in a serum-free medium. At 37°C, hepatocytes in monolayer cultures bound 2.5 times as much insulin and glucagon as did freshly isolated cells, owing to an increase in the total number of binding sites per cell. For both hormones, these differences could be accounted for mainly by a greater number of low-affinity binding sites in primary cultured hepatocytes compared with freshly isolated cells. Exposure of hepatocytes to insulin or glucagon for 2–3 h at 37°C in a medium free from amino acids increased the capacity (primary cultures) or induced the emergence (fresh suspensions) of a similar high-affinity component ( K m ∼ 1 mM) of α-aminoisobutyric-acid (AIB) transport. Primary cultured hepatocytes were more sensitive to insulin (half-maximal effect occurred with insulin at ∼0.3 nM) than freshly isolated cells (half-maximal effect ∼0.7 nM) for the stimulation of AIB transport, whereas the dose-response curves were virtually indistinguishable for the glucagon stimulation of AIB transport in both preparations of cells (half-maximal effect occurred with glucagon at ∼1.5 nM). These results indicate that, despite differences in the apparent insulin- and glucagon-binding capacities (which involved mainly a low affinity site), both freshly isolated and primary cultured (20-h monolayers) hepatocytes behave similarly in response to insulin and glucagon with regard to the stimulation of amino acid transport.