Does the spinal cord generate functionally significant sympathetic activity in the awake rat?

Previous studies reach conflicting conclusions regarding the presence of physiologically significant sympathetic nerve activity (SNA) in conscious rats with transection of the cervical spinal cord (CST). The objective of the current study was to determine whether either spinally generated SNA or nonspecific effects of antagonists are responsible for the natriuresis and decreased heart rate that accompany adrenergic blockade in CST rats. To test the first possibility, adrenergic antagonists (phentolamine and propranolol) were administered to three groups of CST rats: 1) renal denervated, 2) adrenalectomized or sham-adrenalectomized, and 3) rats who received the ganglionic blocker hexamethonium. Neither renal denervation, adrenalectomy, nor ganglionic blockade prevented either the three- to fivefold increase in sodium excretion or the 25-50 beats/min decrease in heart rate previously reported. We then administered combinations of different adrenergic antagonists to CST rats to test for nonspecific effects of the drugs. Whereas idazoxan+propranolol reproduced the natriuresis seen with phentolamine+propranolol, prazosin+yohimbine+propranolol did not. We conclude that there is no evidence for functionally significant spinally generated SNA in conscious CST rats and that the natriuresis observed with phentolamine administration is due to imidazoline binding.