Cross-fostering lowers adult blood pressure in the spontaneously hypertensive rat (SHR) (Gouldsborough & Ashton, 2001) and the Dahl salt-sensitive rat (Murphy & McCarty, 1989). The mechanisms involved are unclear, but involve the kidney and the renin-angiotensin system in the SHR (Gouldsborough & Ashton, 2001). As both of these rat models show salt sensitivity, the aim of this study was to determine whether exposure to a normotensive dam is effective in lowering adult blood pressure in a salt-insensitive model, the New Zealand genetically hypertensive (GH) rat (Ledingham et al. 1990).

GH and normotensive (N) rat pups were cross-fostered to a dam of the opposite strain on the day of birth. Systolic blood pressure was measured by tail cuff plethysmography from the age of 6-17 weeks. At 19-20 weeks rats were anaesthetised (thiopentone sodium 100 mg kg^-1) and infused with 0.9 % saline and clearance markers (inulin and para-aminohippuric acid, 100 ml min^-1 I.V.). Following a 3 h equilibration period, urine samples were collected every 20 min and plasma samples (0.5 ml) were collected every hour for a further 3 h. Animals were humanely killed at the end of the experiment.

Cross-fostering GH rats (GHX) significantly reduced their systolic blood pressure compared with naturally reared GH rats up to the age of 9 weeks (GH 192 ± 5 vs. GHX 174 ± 3 mmHg, P < 0.05, repeated measures ANOVA); thereafter blood pressure did not differ. Cross-fostering N rats (NX) increased their blood pressure compared with naturally reared N rats up to the age of 10 weeks (N 124 ± 3 vs. NX 136 ± 3 mmHg, P < 0.001); thereafter blood pressure did not differ.

Cross-fostering had little effect on renal function in adult rats, although GHX rats had a significantly lower (P < 0.05) glomerular filtration rate (Table 1) and renal blood flow than N rats. Electrolyte handling and fluid output were unaltered in either GHX or NX rats compared with naturally reared rats.

These data show that cross-fostering is only effective in lowering blood pressure in the young GH rat, in contrast to SHR and Dahl rats in which cross-fostering permanently lowers blood pressure. Interestingly, cross-fostering increased blood pressure in N rats, which is not seen in control WKY and Dahl salt-resistant strains. Renal function appeared to be unaffected in adult rats in which blood pressure had returned to levels comparable with naturally reared animals.

All procedures accord with current National and local guidelines.