Superior Efficacy of PulsatileVersusContinuous Hormone Exposure on Hepatic Glucose Productionin Vitro*

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Abstract
To elucidate the potency of continuous vs. intermittent exposure to hormonal stimuli, hepatic glucose production of isolated perfused rat livers was monitored in response to glucagon and insulin infusion. Using a nonrecirculating perfusion system, continuous exposure to glucagon (35 pM) induced a rise in hepatic glucose production from basal 0.33 .+-. 0.03 mmol/(96 min .times. 100 g BW) to 0.65 .+-. 0.02 mmol/(96 min .times. 100 g BW), while intermittent exposure (3 min on/off intervals; total dose 50%) to the same glucagon concentration elicited an almost identical rise in hepatic glucose production to 0.59 .+-. 0.12 mmol/l(96 min .times. 100 g BW). Insulin (100 mU/liter) given continuously and intermittently (3 min on/off intervals) inhibited glucagon-stimulated (70 pM) hepatic glucose production to the same extent, i.e. by 37.4% and 41.1%, respectively. Doubling the off period to 6 min and thereby reducing the total hormone dose to 33% did not diminish insulin''s suppressive effect on glucagon-stimulated heptic glucose release (34.6%). When the latter infusion protocol was applied with insulin at 300 mU/liter, hepatic glucose production during the first 40 min of glucagon infusion was more restrained (P < 0.01) than during continuous delivery of 100 mU/liter, although the same amount of insulin was infused per period of time. In parallel, glucagon-stimulated cAMP release was similar suppressed by insulin in all experiments. From this we conclude that the effect on hepatic glucose production of pulsatile administration of glucagon as well as of insulin, depending on the applied time interval of hormone exposure, is equipotent or even superior to the respective hormones'' continuous infusion even if the hormone load is significantly reduced.