Acute blood volume expansion delays the gastrointestinal transit of a charcoal meal in awake rats

Abstract

The present study evaluates the effect of blood volume expansion on the gastrointestinal transit of a charchoal meal (2.5 ml of an aqueous suspension consisting of 5% charcoal and 5% gum arabic) in awake male Wistar rats (200-270 g). On the day before the experiments, the rats were anesthetized with ether, submitted to left jugular vein cannulation and fasted with water ad libitum until 2 h before the gastrointestinal transit measurement. Blood volume expansion by iv infusion of 1 ml/min Ringer bicarbonate in volumes of 3, 4 or 5% body weight delayed gastrointestinal transit at 10 min after test meal administration by 21.3-26.7% (P<0.05), but no effect was observed after 1 or 2% body weight expansion. The effect of blood volume expansion (up to 5% body weight) on gastrointestinal transit lasted for at least 60 min (P<0.05). Mean arterial pressure increased transiently and central venous pressure increased and hematocrit decreased (P<0.05). Subdiaphragmatic vagotomy and yohimbine (3 mg/kg) prevented the delay caused by expansion on gastrointestinal transit, while atropine (0.5 mg/kg), L-NAME (2 mg/kg), hexamethonium (10 mg/kg), prazosin (1 mg/kg) or propranolol (2 mg/kg) were ineffective. These data show that blood volume expansion delays the gastrointestinal transit of a charcoal meal and that vagal and yohimbine-sensitive pathways appear to be involved in this phenomenon. The delay in gastrointestinal transit observed here, taken together with the modifications of gastrointestinal permeability to salt and water reported by others, may be part of the mechanisms involved in liquid excess management.

Neural mechanisms involved in the delay of gastric emptying of liquid elicited by acute blood volume expansion in awake rats

“…We have previously reported that acute BV expansion significantly delays the GE of liquid as well as the gastrointestinal transit in awake rats 5 , 8 –10 – an effect which varied according to the amount of infused volume. Since the greatest GE delay we observed in our previous studies was obtained in rats submitted to a BV expansion equivalent to 5% body weight, 9 we fixed this level of BV expansion in the present study in order to investigate the neural mechanisms involved in the GE delay elicited by acute BV expansion.…”

“…This BV overload protocol has been extensively utilized in our previous studies, both in anaesthetized rats and in awake rats 6 . 8 –10 For investigating neural mechanisms, separate groups of rats were previously submitted to a drug pretreatment or a surgical procedure. In all protocols, the animals were killed at a fixed time interval (10 min) after liquid meal gavage.…”

“…The gut seems to react to acute blood volume (BV) imbalances by adjusting the gastrointestinal (GI) motility 1 –10 as well as its permeability to fluid and electrolytes 11…”

“…Since these variations on the gastroduodenal flow were verified in anaesthetized animals after invasive surgical procedures, we decided to study the effect of acute BV changes on the GI motility of awake rats: BV expansion delays the gastric emptying (GE) and GI transit of liquid meals while BV retraction accelerates GE rates 9 . 10 …”

Variations in gastric compliance induced by acute blood volume changes in anesthetized rats

“…Since the proximal stomach tonus drives gastric fluid emptying, BV changes may modify the flow through the gastroduodenal segment (15). However, the gastric emptying rate and the gastrointestinal transit of a liquid meal in awake rats are decreased by hypervolemia and increased after hypovolemia (16,17). This apparent discrepancy can be understood if one considers the breaking effect of duodenal motility on gastric emptying (15).…”