Method for in vivo calibration of renal sympathetic nerve activity in rabbits

A major difficulty of recording from peripheral sympathetic nerves is that microvolt values reflect characteristics of the recording conditions and limit comparisons between different experimental groups. In this study we assessed methods of calibrating renal sympathetic nerve activity (RSNA) in conscious rabbits. Calibration values were obtained from maximum RSNA responses to nasopharyngeal stimulation, airjet stress or unloading baroreceptors. Curves relating RSNA to blood pressure were produced by raising and lowering blood pressure with vasoactive drugs. To assess whether normalization would eliminate differences between RSNA curves which were most likely due to recording conditions, rabbits were first divided into two groups with high or low basal microvolt levels of RSNA, then again into two groups with high or low heart rate. In both cases, curves were similar if values were normalized by nasopharyngeal stimulation or by the upper plateau value. In hypertensive rabbits, where the baroreflex is suppressed, only the nasopharyngeal method showed this attenuated pattern. This method also eliminated the 50% decay in basal RSNA measured over 5 weeks. We conclude that expressing RSNA in terms of the maximum response to nasopharyngeal stimulation provides a calibration method suitable for comparing nerve activity over the long term as well as showing valid differences in baroreflex curves between different experimental groups.