Splanchnic control of vasopressin secretion in conscious rats

Saline solutions (NaCl, 2 ml, pH 7.4, 10-598 mosmol/kgH2O) were infused over 4 min in conscious rats, via tail artery catheter or intragastric tube. Intragastric infusions of hyper- and hypotonic solutions caused, within 14.4 +/- 2.2 min, a maximal increase and decrease, respectively, of plasma vasopressin (AVP) relative to time controls (r = 0.97; P less than 0.00001) without affecting systemic plasma osmolality (r = -0.09; P less than 0.92). Mean changes of plasma AVP between 11 and 21 min were also correlated with the osmolality of gastric infusion (r = 0.72; P less than 0.000001), whereas systemic osmolality was unchanged (r = 0.14; P less than 0.42). Systemic infusions caused within 9.0 +/- 2.0 min a maximal change in both plasma AVP (r = 0.82; P less than 0.00001) and systemic osmolality (r = 0.97; P less than 0.00001). However, mean changes of plasma AVP between 11 and 21 min weakly correlated with the osmolality of systemic infusions (r = 0.27; P less than 0.20), although correlations between mean changes of systemic osmolality and the osmolality of systemic infusions were significant (r = 0.72; P less than 0.00001). Lack of correlations with mean arterial pressure and heart rate suggest that hemodynamic changes did not mediate the AVP responses. Pretreatment with atropine methyl bromate (2 mg/kg) abolished the AVP response to gastric but not systemic infusions of hypertonic saline. These results indicate that a splanchnic cholinergic receptor mechanism modulates AVP secretion during a moderate gastric intake of salt or water.