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Augmented response of renal sympathetic nerve single units to chemoreceptor stimuli in rabbits with angiotensin induced hypertension

We examined the effect of angiotensin (AngII) hypertension on the activity of post‐ganglionic renal sympathetic units to determine whether elevated whole renal nerve activity is due to recruitment or increased firing frequency. Rabbits were treated with AngII for 4 weeks (50ng/kg/min, high dose) or for 8 weeks (20ng/kg/min, low dose) and then chloralose‐urethane anesthetized to test unitary responses to altered chemoreceptor activity. Units were detected from multi‐unit recordings using a novel algorithm that separated units by action potential shape characteristics using templates that matched spikes within a prescribed SD. Resting firing frequency was similar in sham (1.0±0.04 spikes/s, n=144) and high AngII rabbits (1.1±0.04 spikes/s, n=112) but was lower with low AngII (0.65±0.04 spikes/s, n=149). Firing frequency increased markedly to hypoxia (8% O2) in the high AngII group (+103%) but only by +39% in low AngII and +82% in sham group. The increase in firing rate with hypercapnia (8% CO2) was also greatest in the high AngII group whilst 100% O2 suppressed unitary firing in high AngII and sham groups. This study suggests that increases in sympathetic outflow in low dose AngII hypertension are most likely due to recruitment of individual neurons but high dose AngII markedly increases firing frequency to chemoreceptor stimuli. Supported by grants from National Health and Medical Research Council of Australia.