Enhanced sympathetic pressor responses to intracerebrovascularly infused saline in awake salt-loaded rats.

Osmotic minipumps were used to infuse hypertonic saline intracerebroventricularly into conscious rats fed high-salt diets to determine if combining high-salt intake with ICV saline infusion would elevate blood pressure more than either procedure alone. Mean aortic pressures became progressively elevated in all rats infused with saline and the magnitude of the elevation was significantly larger in those fed high-salt diets instead of the regular chow. Added pressor effects of high-salt feeding were unaffected by pharmacologic vasopressin or angiotensin blockade thereby indicating that humoral mediation by either peptide was not involved. A general increase in cardiovascular reactivity also seemed unlikely since pressor responsiveness to injected norepinephrine did not differ between rat groups. Sympathetic hyperactivity was considered to be a logical explanation because enhancement by high-salt feeding was abolished following either ganglion blockade with pentolinium or chemical sympathectomy with 6-hydroxydopamine. These findings are compatible with the interpretation that high-salt feeding enhanced pressor responses to centrally-infused saline by increasing sympathetic vasomotor tone.