GUT BRAIN SIGNALING OF WATER-ABSORPTION INHIBITS VASOPRESSIN IN RATS

The mechanism for inhibition of vasopressin (AVP) by gastric water inf usion was examined in 24- or 48-h dehydrated conscious rats (n = 136 r ats, 255 experiments; mean AVP baseline = 6.3 pg/ml). Gastric water in fusions of 1 (n = 8), 2.5 (n = 19), and 4 ml (n = 10) caused a volume- dependent inhibition of plasma AVP by -0.31, -1.77, and -3.02* pg/ml, with decreases of systemic osmolality of -1.06, -1.52, and -4.26 mos mol/kgH(2)O ( = significant vs. isotonic, Duncan's test). Gastric iso tonic infusions (1-4 ml) had no effect or slightly increased AVP. Syst emic infusions of 1.25 (n = 6), 2.1 (n = 10), and 6.3 mi (n = 8) inhib ited AVP by -0.48, -1.07, and -2.51 pg/ml, with decreases in systemic osmolality of -1.61, -2.77, and -7.21* mosmol/kgH(2)O. Systemic isoto nic infusions (2.1 and 6.3 ml) slightly inhibited AVP by -0.71 and -0. 85 pg/ml. Individual changes in AVP by gastric infusion of 2.5 ml of w ater did not correlate with changes in systemic osmolality, mean arter ial pressure, or heart rate but highly correlated with preinfusion AVP (r = 0.74, P < 0.0001, n = 28). Pretreatment with systemic atropine m ethyl bromate (0.7 mg/rat), which abolishes the AVP secretion to gastr ic hypertonic saline, did not affect the AVP response to gastric water infusion (n = 9). Combination of 2.5 ml of gastric water and systemic hypertonic saline prevented the decrease in systemic osmolality and s till significantly inhibited plasma AW. Maximal inhibition of AVP by g astric water was reduced by 62.6% after lesion of the common hepatic v agal branch (n = 5) relative to shams with identical abdominal surgery (n = 6) and by 62.7 and 72.5% after right (n = 11) and left (n = 8) c ervical vagotomy relative to 12 shams (P < 0.05). The results show tha t 1) gastric water absorption is signaled mainly by splanchnic osmosen sors, 2) water signaling is atropine insensitive, and 3) the major wat er-signaling pathway projects through the common hepatic vagal branch and cervical vagal nerves.