Inhibition of hepatic proteolysis by insulin

Abstract

1 Proteolysis was measured as [³H]leucine release from isolated perfused livers from rats, which had been labeled in vivo by an intraperitoneal injection of [³H]leucine about 16 h prior to the perfusion experiment. In livers from fed rats, insulin (35 nM) inhibited [³H]leucine release by 24.5 ± 1.3% (n= 15) and led to an amiloride‐sensitive, bumetanide‐sensitive and furosemide‐sensitive net K⁺ uptake of 5.53 ± 0.31 μmol · g⁻¹ (n= 15). Both the insulin effects on net K⁺ uptake and on [³H]leucine release were diminished by about 65% or 55% in presence of furosemide (0.1 mM) or bumetanide (5 μM), respectively. The insulin‐induced net K⁺ uptake was virtually abolished in the presence of amiloride (1 mM) plus furosemide (0.1 mM). 2 In perfused livers from 24‐h‐starved rats, both the insulin‐stimulated net K⁺ uptake and the insulin‐induced inhibition of [³H]leucine release were about 80% lower than observed in experiments with livers from fed rats. The insulin effects on K⁺ balance and [³H]leucine release were not significantly influenced in the presence of glycine (2 mM), although glycine itself inhibited [³H]leucine release by 30.3 ± 0.3% (n= 4) and 13.8 ± 1.2% (n= 5) in livers from starved and fed rats, respectively. When livers from fed rats were preswollen by hypoosmotic perfusion (225 mOsmol · l⁻¹), both the insulin‐induced net K⁺ uptake and the inhibition of [³H]leucine release were diminished by 50–60%. 3 During inhibition of [³H]leucine release by insulin, further addition of glucagon (100 nM) led to a marked net K⁺ release from the liver (3.82 ± 0.24 μmol · g⁻¹), which was accompanied by stimulation of [³H]leucine release by 16.4 ± 4.6% (n= 4). 4 Ba²⁺ (1 mM) infusion led to a net K⁺ uptake by the liver of 3.2 ± 0.2 μmol · g⁻¹ (n= 4) and simultaneously inhibited [³H]leucine release by 12.4 ± 1.7% (n= 4). 5 There was a close relationship between the Ba²⁺ or insulin‐induced net K⁺ uptake and the degree of inhibition of [³H]leucine release, even when the K⁺ response to insulin was modulated by bumetanide, furosemide, glucagon, hypotonic or glycine‐induced cell swelling or the nutritional state. 6 The data suggest that the insulin‐induced net K⁺ uptake involves activation of both NaCl/KCl cotransport and Na⁺/H⁺ exchange. The findings further suggest that modification of insulin‐induced intracellular net K+ accumulation parallels the antiproteolytic potency of the hormone, consistent with a role of hormone‐induced ion movements across the plasma membrane in mediating the antiproteolytic effect of insulin in liver.