Renal sympathetic activity in spontaneously hypertensive rats and normotensive controls, as studied by three different methods

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Abstract
Recordings of sympathetic activity from multifiber preparations of renal nerves have produced conflicting results concerning the presence or absence of an increased sympathetic discharge in spontaneously hypertensive rats (SHR) compared to normotensive Wistar-Kyoto rats (WKY). Recordings of single fiber activity to that kidney were performed in anesthetized SHR and WKY in comparison with multifiber recordings in conscious, undisturbed rats. A new method of estimating sympathetic discharge by analyzing the variability of cycle activity in multifiber nerve recordings was also used. The average nerve activity in a great number of cardiac cycles was then expressed in relation (in percent) to the nerve activity in a small number of cardiac cycles with the highest and lowest nerve activity in each rat. Single fiber recordings showed a significantly higher sympathetic activity to the kidneys in SHR (3.8 .+-. 0.3 Hz) than in WKY (1.7 .+-. 0.2 Hz; P < 0.001). Also average cycle activity was significantly higher in conscious SHR (34 .+-. 1%) than in WKY (26 .+-. 2%, P < 0.01). This was due to the larger number of cardiac cycles in SHR with high sympathetic activity while WKY showed more of silent cardiac cycles which lacked nerve impulses. The recordings of rectified multifiber renal nerve activity also showed an elevated sympathetic activity in conscious SHR rats. The increased renal sympathetic activity appears to reflect the primary central nevous hyperreactivity characterizing SHR hypertension. The increased renal sympathetic activity may be of particular importance for the development of primary hypertension in SHR and perhaps also in man.