INHIBITION OF HEPATIC PROTEOLYSIS BY INSULIN - ROLE OF HORMONE-INDUCED ALTERATIONS OF THE CELLULAR K+ BALANCE

1. Proteolysis was measured as [H-3]leucine release from isolated perfused livers from rats, which had been labeled in vivo by an intraperitoneal injection of [H-3]leucine about 16 h prior to the perfusion experiment. In livers from fed rats, insulin (35 nM) inhibited [H-3]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-mu-mol . g-1 (n = 15). Both the insulin effects on net K + uptake and on [H-3]leucine release were diminished by about 65% or 55% in presence of furosemide (0.1 mM) or bumetanide (5-mu-M), 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 [H-3]leucine release were about 80% lower than observed in experiments with livers from fed rats. The insulin effects on K + balance and [H-3]leucine release were not significantly influenced in the presence of glycine (2 mM), although glycine itself inhibited [H-3]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 [H-3]leucine release were diminished by 50 - 60%. 3. During inhibition of [H-3]leucine release by insulin, further addition of glucagon (100 nM) led to a marked net K+ release from the liver (3.82 +/- 0.24-mu-mol . g-1), which was accompanied by stimulation of [H-3]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-mu-mol . g (n = 4) and simultaneously inhibited [H-3]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 [H-3]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.