INTAKE SUPPRESSION AFTER HEPATIC PORTAL GLUCOSE-INFUSION - ALL-OR-NONE EFFECT AND ITS TEMPORAL THRESHOLD

The effects of hepatic portal infusions of isotonic glucose on glucose intake (3.2%) were evaluated with use of the intraoral intake test, w hich, unlike traditional tests, permits delivery of portal infusions i n explicit temporal relationship to intake onset in nondeprived rats. Continuous or discontinuous portal infusions (0.1 ml/min) of isotonic glucose or saline were initiated 0, 30, 60, or 120 min before meal ons et. Jugular infusions of isotonic saline or glucose and portal infusio ns of isotonic saline were without effect. For all effective portal gl ucose infusions, intake was suppressed by approximately 30% of baselin e values. Because the duration (and quantity) of effective portal gluc ose infusions varied by a factor of 10, we conclude that intraoral int ake suppression under these conditions is all or none in nature. Wheth er an intake suppression was obtained depended more on when the infusi on was delivered than on how much was infused. Thus 1.5 ml of isotonic glucose infused between 60 and 45 min before the intake test was effe ctive, whereas 3.0 ml infused for the 30 min before intake was without effect. These results suggest that the liver participates in the cont rol of future intake but not in the termination of an ongoing meal. Th e temporal requirement for intake suppression should be considered in analyses of the metabolic, hormonal, and/or neural mechanisms that und erlie the liver's contribution to intake control.