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1 Hypertension Unit, University of Ottawa Heart Institute, Ottawa, ON, Canada
2 Laboratory of Cardiac Growth and Differentiation, Institut de Recherches Cliniques de Montreal, Montreal, QC, Canada
3 Canadian Institute of Health Research, University of Ottawa Heart Institute Foundation, Ottawa, ON, none
* To whom correspondence should be addressed. E-mail: fleenen{at}ottawaheart.ca.
Six weeks old Dahl salt -sensitive (S) and -resistant (R) rats received for 2 weeks an intracerebroventricular (icv) infusion of aldosterone (aldo) (22.5 ng/h) or vehicle containing artificial cerebrospinal fluid (aCSF) with 0.15 M Na+. At 8 weeks, MAP, HR, and renal sympathetic nerve activity (RSNA) were recorded in conscious rats at rest and in response to air-stress, and to icv injection of the 
2 -adrenoceptor agonist guanabenz or ouabain. Baroreflex control of RSNA and HR was estimated using iv phenylephrine and nitroprusside. In Dahl S but not R rats, aldo raised resting MAP by 20-25 mmHg, doubled sympathoexcitatory and pressor responses to air-stress and sympathoinhibitory and depressor responses to guanabenz, and impaired baroreflex function. In Dahl S but not R rats, aldo significantly increased content of ouabain-like compounds (OLC) in the hypothalamus, and attenuated excitatory responses to ouabain. Aldo did not affect water intake, plasma electrolytes, or OLC in plasma and adrenal glands. In another set of 3 groups of Dahl S rats, aldo dissolved in aCSF containing 0.16, 0.15, or 0.14 M Na+ was infused icv for 2 weeks. CSF [Na+] showed only a non-significant increase, but resting MAP increased from 111±3 mmHg in rats with aldo in 0.14 M Na+ to 131±3 and 147±3 mmHg with aldo in 0.15 and 0.16 M Na+, respectively (p<0.05 for both). These findings indicate that in Dahl S rats icv infusion of aldo causes similar central responses as high salt intake i.e. increases in brain OLC content, sympathetic hyper-reactivity and hypertension. The extent of the increase in BP by icv aldo depends on the [Na+] in the vehicle. In Dahl R rats, icv aldo did not increase brain OLC, sympathetic reactivity and BP, suggesting that in this rat strain a decrease in central responsiveness to mineralocorticoids may contribute to its salt-resistant nature.
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