Experiments were conducted on male normotensive Wistar rats and SHRs weighing 280–320 g obtained from Vital River Laboratory Animal Technology Co., Ltd. (Beijing, China). The procedures were approved by the Experimental Animal Care and Use Committee of Nanjing Medical University and complied with the Guide for the Care and Use of Laboratory Animals (16). All efforts were made to minimize the number of animals used and their suffering. The rats were anesthetized using an intraperitoneal injection of sodium pentobarbital (50 mg/kg⁻¹) and ventilated with room air using a Harvard 683 Small Animal Ventilator (Harvard Apparatus, Holliston, MA, USA).

The tail artery SBP was measured in conscious rats using a Non-Invasive Blood Pressure monitor (ADInstruments, Bella Vista, Australia). To minimize stress-induced SBP fluctuations, the rats were trained by measuring SBP daily for a minimum of 10 days prior to surgery. The rats were warmed for 10–20 min at 28°C prior to the measurements to allow for detection of tail arterial pulsations and to achieve a steady pulse. The SBP was obtained by calculating the mean of 10 measurements.

A retroperitoneal incision was made and the left renal sympathetic nerve was isolated. The nerve was cut distally to eliminate its afferent activity and was placed on a pair of silver electrodes that were immersed in warm mineral oil. RSNA was amplified using an AC/DC Differential Amplifier Model 3000 (A-M Systems, Inc., Sequim, WA, USA) with a low frequency cut-off at 60 Hz and a high frequency cut-off at 3,000 Hz and integrated at a time constant of 0.1 sec. The raw RSNA, integrated RSNA, MAP and HR were simultaneously recorded on a PowerLab 8SP Data Acquisition System (ADInstruments). The background noise level was determined following sectioning of the central end of the nerve and was subtracted from the RSNA value, as previously described (5).

Hexamethonium hydrochloride was purchased from Sigma-Aldrich (St. Louis, MO, USA) and dissolved in normal saline (Shanghai Baxter Healthcare Co., Ltd., Shanghai, China). The concentrations selected for the current study were 0.2, 1.0, 5.0 or 25.0 mg/kg body weight.

The femoral artery and vein were cannulated under sodium pentobarbital anesthesia to prepare for MAP recordings and the intravenous injection of hexamethonium. Saline or hexamethonium hydrochloride (0.2, 1.0, 5.0 or 25 mg/kg body weight) was then injected into the vein using a PM2000B Cell Microinjector (MicroData Instrument, Inc., South Plainfield, NJ, USA). The effects of hexamethonium on RSNA, MAP and HR were determined by averaging 1 min of the maximal responses. The time course of the maximal effects on RSNA, MAP and HR were determined from the end of the injections to the maximum effect points. The recovery time from the hexamethonium effects on RSNA, MAP and HR were determined from the maximum effect points to the maximum recovery points. The recovery of hexamethonium effects on RSNA, MAP and HR was determined at the maximum recovery points by averaging the parameters for 1 min.

Data were analyzed using SPSS software, version 18.0 (SPSS, Inc., Chicago, IL, USA). Comparisons between two observations were assessed by Student's paired t-test. One-way analysis of variance was used followed by the Bonferroni post hoc test for multiple comparisons. All data were presented as the mean ± standard error. P<0.05 was considered to indicate a statistically significant difference.

The SBP of the tail artery in the conscious state and the baseline MAP of the femoral artery under anesthesia were significantly greater in SHRs compared with Wistar rats. There were no significant differences between Wistar rats and SHRs in body weight or baseline HR (Table I).

Intravenous injection of four doses (0.2, 1.0, 5.0 or 25.0 mg/kg body weight) of hexamethonium significantly reduced the RSNA, MAP and HR in the Wistar rats and the SHRs. There were no significant differences between Wistar rats and SHRs in the RSNA, MAP or HR at the 0.2 or 1.0 mg/kg doses. However, 5.0 and 25.0 mg/kg hexamethonium resulted in greater reductions in the RSNA and MAP in SHRs compared with Wistar rats (Fig. 1). Representative recordings indicate that the intravenous injection of 5.0 mg/kg of hexamethonium reduced the RSNA, MAP and HR (Fig. 2).

There was a significant positive correlation between the alterations in RSNA and MAP response to intravenous injection of hexamethonium in the Wistar rats and SHRs (Fig. 3).

There was no significant difference between the SHRs and Wistar rats in the time course of the maximal effects of the four doses of hexamethonium on the RSNA, MAP or HR (Fig. 4).

There were no significant differences between the SHRs and Wistar rats in the recovery time of the RSNA, MAP or HR at the four doses of hexamethonium investigated (Fig. 5).

RSNA fully recovered at the hexamethonium doses of 0.2 and 1.0 mg/kg and partially recov-ered at doses of 5.0 and 25.0 mg/kg (Fig. 5). MAP and HR were fully recovered at a dose of 0.2 mg/kg and partially recovered at doses of 1.0, 5.0 and 25.0 mg/kg. There were no significant differences between the Wistar rats and SHRs in the recovery time of RSNA at the four doses of hexame-thonium investigated (0.2 mg/kg, 5.0±1.1 vs. 5.2±1.4 min; 1.0 mg/kg, 17.6±4.5 vs. 13.1±1.8 min; 5.0 mg/kg, 23.8±6.4 vs. 16.6±6.1 min; and 25.0 mg/kg, 12.8±3.4 vs. 21.4±4.5 min), MAP (0.2 mg/kg, 7.6±1.1 vs. 21.6±14.1 min; 1.0 mg/kg, 25.9±10.5 vs. 16.9±3.5 min; 5.0 mg/kg, 70.1±11.5 vs. 54.0±24.5 min; and 25.0 mg/kg, 68.6±16.8 vs. 57.0±12.9 min) and HR (0.2 mg/kg, 5.7±1.4 vs. 11.0±1.1 min; 1.0 mg/kg, 16.9 ± 4.2 vs. 16.1±4.5 min; 5.0 mg/kg, 11.8±3.2 vs. 15.6±4.6 min; and 25.0 mg/kg, 15.5±4.8 vs. 27.7±10.4 min) (Fig. 6).