- We recently re-inbred the original colony of SBH-SBN rats, a model of salt-induced hypertension. In the course of phenotyping the new colony, SBH/y were found to excrete a lower urine flow with a higher urine osmolality than SBN/y. Thus disparate water handling between the substrains, a phenotype characteristic of the original colony, was retained throughout the selection procedure and transmitted down the generations to the new colony. As water handling is directly linked to arginine vasopressin (AVP) and in view of potential linkage of this phenotype to salt sensitivity or resistance in terms of the development of hypertension, the AVP axis was further investigated in the new substrains. Basal plasma AVP levels were higher in SBH/y (2.86 +/- 0.22 pg/ml; n = 10) than in SBN/y (1.98 +/- 0.11 pg/ml; n = 10, P < 0.05). Water deprivation for 48 h increased plasma AVP levels severalfold in both substrains to similar levels. Niravoline, a kappa receptor agonist that inhibits central release of AVP, produced at 0.6 and 0.9 mg/kg a more profound diuretic effect in SBN/y than in SBH/y, suggesting greater pituitary release of AVP into the circulation of SBH/y. AVP mRNA contents were compared in SBH/y and SBN/y rats in whole hypothalamic extracts and in the supraoptic (SON) and paraventricular (PVN) nuclei by RNA protection assay. Under basal conditions, AVP mRNA content (in ng) in the hypothalamus of SBH/y was 4.48 +/- 0.52 (n = 29) and of SBN/y was 3.13 +/- 0.35 (n = 30), P < 0.05; in the SON of SBH/y, AVP mRNA content was 3.62 +/- 0.44 (n = 11) and of SBN/y was 2.21 +/- 0.54 (n = 10), P < 0.05; in the PVN of SBH/y, AVP mRNA content was 0.78 +/- 0.16 (n = 9) and of SBN/y was 0.77 +/- 0.13 (n = 11, not significant). Thus the differences in hypothalamic AVP mRNA were primarily in the SON. Water deprivation as well as salt loading (8% NaCl) induced a significant elevation in AVP mRNA content in SBN/y but a blunted response in SBH/y. These data suggest that there is genetically transmitted enhanced hypothalamic expression of the AVP gene in SBH/y compared with SBN/y which results, under basal conditions, in greater pituitary release of AVP, in higher plasma AVP levels, and in increased renal concentrating activity. As AVP has been implicated in various forms of hypertension, these findings render AVP a candidate gene for salt sensitivity or resistance in the Sabra rat model of hypertension.