Cardiopulmonary Baroreflex in NaCl-Induced Hypertension in Borderline Hypertensive Rats

Borderline hypertensive rats fed an 8% NaCl diet develop increased arterial pressure in association with increased cardiopulmonary baroreflex sensitivity compared with rats fed a 1% NaCl diet. We performed experiments to localize the site of sensitization within the cardiopulmonary baroreflex. To determine whether decreased cardiopulmonary baroreflex sensitivity, as seen in other models of NaCl-induced hypertension, develops later in the course of the disease, we studied an older backcross population derived from borderline hypertensive rats and Wistar-Kyoto rats. Anesthetized borderline hypertensive rats fed 1% and 8% NaCl diets were volume-loaded while right atrial pressure, afferent vagal nerve activity, and renal sympathetic nerve activity were recorded. In 28- to 30-week-old backcross rats fed an 8% NaCl diet, renal sympathetic nerve activity, natriuresis, and diuresis were measured before and during volume loading. Renal sympathetic nerve activity was analyzed with the sympathetic peak detection algorithm. Increases in afferent vagal nerve activity and renal sympathoinhibition were similar in borderline hypertensive rats on either diet during a right atrial pressure rise of 3 mm Hg. In backcross rats, correlations between arterial pressure and renal sympathoinhibition, natriuresis, or diuresis were not found. During volume loading, the peak height of synchronized renal sympathetic nerve discharges decreased while their frequency increased. Attenuated renal sympathoinhibition during acute increases in intravascular volume is not involved in the development or maintenance of NaCl-induced hypertension in borderline hypertensive rats. Renal sympathetic nerve activity decreases because of a reduction in the number of active renal sympathetic nerve fibers.