Heart rate-arterial blood pressure relationship in conscious rat before vs. after spinal cord transection

This experiment quantified the initial disruption and subsequent adaptation of the blood pressure (BP)-heart rate (HR) relationship after spinal cord transection (SCT). BP and HR were recorded for 4 h via an implanted catheter in neurally intact, unanesthetized rats. The animals were then anesthetized, and their spinal cords were severed at T1–T2 ( n = 5) or T4–T5 ( n = 6) or sham lesioned ( n = 4). BP was recorded for 4 h daily over the ensuing 6 days. The neurally intact rat showed a positive cross correlation, with HR leading BP at the peak by 1.8 ± 0.8 (SD) s. The cross correlation in unanesthetized rats ( n = 2) under neuromuscular blockade was also positive, with HR leading. After SCT at T1–T2, the cross correlation became negative, with BP leading HR, and did not change during the next 6 days. The cross correlation also became negative 1–3 days after SCT at T4–T5, but in four rats by day 6 and thereafter the cross correlation progressively reverted to a positive value. We propose that the positive cross correlation with HR leading BP in the intact rat results from an open-loop control that depends on intact supraspinal input to sympathetic preganglionic neurons in the spinal cord. After descending sympathetic pathways were severed at T1–T2, the intact vagal pathway to the sinoatrial node dominated BP regulation via the baroreflex. We suggest that reestablishment of the positive correlation after SCT at T4–T5 was attributable to the surviving sympathetic outflow to the heart and upper vasculature reasserting some effective function, perhaps in association with decreased spinal sympathetic hyperreflexia. The HR-BP cross correlation may index progression of sympathetic dysfunction in pathological processes.