1. Glycogen phosphorylase (a form, in rapidly freeze-clamped samples) and glucose release were measured in the perfused liver, in response to a range of concentrations of adrenaline, [8-arginine] vasopressin (anti-diuretic hormone) and angiotensin II. 2. All three hormones increased phosphorylase a activity by about 10 mumol/min per g of fresh liver, which was more than sufficient to explain concomitant glucose release (1-2mumol/min per g). 3. Minimally effective concentrations which activated phosphorylase were: adrenaline, 10nM (2ng/ml); vasopressin, 40pM (40pg/ml, 15 muunits/ml); angiotensin II, 60pM (60pg/ml). 4. Glycogen synthase activity was inhibited by adrenaline and vasopressin but not significantly by angiotensin II. 5. Vasoconstriction observed with adrenaline and angiotensin II (but not vasopressin) might explain part of the activation of phosphorylase, since equivalent vasoconstriction (in separate perfusions) activated phosphorylase, did not stimulate glucose output or inhibit synthase. 6. The potency of these effects suggests that all three hormones can stimulate hepatic glycogen degradation in vivo (by direct hepatic action). It is proposed that hormones, and ischaemia, stimulate glycogen degradation to provide glucose phosphates for disposal within the liver cell, as well as for release as free gluose.

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