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, which, unlike traditional tests, permits delivery of portal infusions in 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 onset. Jugular infusions of isotonic saline or glucose and portal infusions of isotonic saline were without effect. For all effective portal glucose infusions, intake was suppressed by approximately 30% of baseline values. Because the duration (and quantity) of effective portal glucose infusions varied by a factor of 10, we conclude that intraoral intake suppression under these conditions is all or none in nature. Whether an intake suppression was obtained depended more on when the infusion 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 effective, whereas 3.0 ml infused for the 30 min before intake was without effect. These results suggest that the liver participates in the control of future intake but not in the termination of an ongoing meal. The temporal requirement for intake suppression should be considered in analyses of the metabolic, hormonal, and/or neural mechanisms that underlie the liver's contribution to intake control.