Frequency and amplitude of sympathetic discharges by baroreflexes during hypoxia in conscious rabbits

Sympathetic nerve activity (SNA) from multifiber preparations exhibits two distinct components: the frequency at which discharges occur and their relative amplitude (reflecting the number of activated nerve fibers within each burst). These two components may respond independently to various afferent inputs, indicating separate central controlling processes. We examined the response in the frequency and amplitude of renal SNA to changes in baroreceptor activity and the effect of two forms of hypoxia on this relationship in nine conscious rabbits. Rabbits breathed either room air or one of two hypoxic gas mixtures (10% O2 or 10% O2 + 3% CO2) for 20 min, during which baroreflexes were stimulated by ramp increases and then decreases in arterial pressure with phenylephrine (0.5 mg/ml iv) and nitroprusside (1 mg/ml) (total arterial pressure range induced was 80 mmHg). Hypoxia with 10% O2 significantly increased the resting frequency of SNA before baroreflex modifications from 2.15 +/- 0.18 to 2.82 +/- 0.25 discharges/s and with 10% O2 + 3% CO2 to 3.20 +/- 21 discharges/s. The amplitude of sympathetic discharges was increased 44 +/- 5% over control levels during 10% O2 but was not further increased by the addition of 3% CO2. The baroreflex curve for total SNA (1-s averages of the integrated neurogram) showed a graded response to the two hypoxic stimuli, with significant increases in the upper plateau, gain, and resting point on the curves. However, the baroreflex curve for the frequency or amplitude of sympathetic discharges did not show graded responses to each hypoxic treatment. The frequency baroreflex curve was sigmoidal and not changed from air during 10% O2. During 10% O2 + 3% CO2, the gain (responsiveness) of the curve was increased although the range of frequencies occurring was unaltered. The baroreflex curve for the amplitude showed similar responses to the two hypoxic stimuli, namely, increases in the upper plateau, gain, and resting point of the curve. We conclude that the frequency and amplitude of sympathetic discharges are able to respond differentially to changes in afferent stimuli. Given that alterations in the frequency and recruitment of sympathetic fibers (amplitude) to the kidney may have differing effects, this phenomenon may provide a previously unknown level of renal hemodynamic control through the interaction of specific afferent inputs to the central nervous system.