Functional evidence for sympathetic nerve activity in conscious cervical spinal rats

Experiments in anesthetized animals have demonstrated sympathetic nerve activity after cervical spinal cord transection (CST), yet little evidence exists for sympathetic nerve activity in conscious spinal animals. We determined the effect of CST on mean arterial pressure (MAP), heart rate (HR), and plasma renin activity (PRA) in conscious rats. Next, we tested for the presence of functionally significant sympathetic nerve activity 24 h after CST by measuring MAP, HR, urine flow, urinary sodium excretion (U(Na)V), urinary potassium excretion (U(K)V), and PRA before and for 2 h during administration of adrenergic antagonists (propranolol + phentolamine) or vehicle. CST resulted in significant decreases in MAP (100.0 ± 1.6 to 75.0 ± 1.4 mmHg), HR (353 ± 7.5 to 315 ± 6.6 beats/min), and PRA (2.2 ± 0.3 to 1.2 ± 0.2 ng angiotensin I · ml^-1 · h^-1). Subsequent adrenergic blockade resulted in a further drop in HR (-52 ± 21 beats/min), as well as two- to threefold increases in U(Na)V and U(K)V. We conclude that CST causes an overall decrease in sympathetic drive. However, sympathetic nerve activity to the heart and kidney remains after CST and is responsible for support of HR and retention of sodium and potassium.