Role of rumen fluid hypertonicity in the dehydration-induced hypophagia ofcows

Three experiments were performed in non-lactating, rumen-fistulated cows to assess the role of rumen fluid hypertonicity in dehydration-induced hypoph agia. First, the course of rumen fluid and plasma osmolality before and aft er an individual test meal was recorded when water was offered ad libitum a nd on the fifth day of a 65% water restriction period. Then, the effects of intraruminal water infusions on food intake were examined in dehydrated co ws. Finally, two doses of the local anesthetic mepivacaine HCl were given i nto the rumen in an attempt to inactivate the osmosensors potentially invol ved in dehydration-induced hypophagia. Water restriction reduced test meal size and increased rumen fluid and plasma osmolality. Despite the smaller m eal, the prandial increase in rumen fluid osmolality was more pronounced du ring water restriction than with water ad libitum. Independent of treatment , the test meal had no effect on plasma osmolality. Intraruminal water infu sions during water deprivation decreased rumen fluid osmolality below the c ontrol level and normalized food intake. Injection of 2 or 4 g mepivacaine/ cow into the rumen did not attenuate dehydration-induced hypophagia. All in all, these results suggest that rumen fluid hypertonicity, perhaps in conc ert with plasma hypertonicity, contributes to the early satiation induced b y dehydration. (C) 2000 Elsevier Science Inc. All rights reserved.