Sodium depletion induces adrenergic potentiation at prejunctional site

The objective of this study was to examine the effect of sodium depletion on basal sympathetic adrenergic vascular tone and vasoconstrictor responses to adrenergic nerve stimulation and norepinephrine in pentobarbital-anesthetized dogs. Mean arterial pressure, tibial blood flow, and tibial vascular resistance were not significantly different between sodium-replete and sodium-deplete dogs. However, plasma renin activity (PRA) was 11-fold higher in sodium-deplete dogs (P less than 0.01). Sympathetic denervation in sodium-replete and -deplete dogs resulted in a similar decrease in tibial vascular resistance. Nerve stimulation at 0.25 and 0.5 Hz caused 40 +/- 5 and 48 +/- 5% decreases, respectively, in tibial blood flow in sodium-replete dogs and 71 +/- 3 and 77 +/- 3% decreases, respectively, in sodium-deplete dogs (P less than 0.01). Responses to norepinephrine administered intra-arterially were similar in both groups. Neither captopril nor saralasin reduced the potentiated responses to nerve stimulation in sodium-deplete dogs. Bilateral nephrectomy 24 h prior to the experiment reduced PRA to an undetectable level in both sodium-replete and -deplete dogs and equalized the responses to nerve stimulation and norepinephrine in both groups. Because bilateral nephrectomy, but neither captopril nor saralasin, abolished the potentiated responses in sodium depletion, a renal hormone other than renin or vascular angiotensin II may be responsible for this prejunctional adrenergic potentiation.