Pharmacologic responses and spectral analyses of spontaneous fluctuations in heart rate and blood pressure in SHR rats.

The aim of this experiment was to characterize patterns of autonomic control in spontaneously hypertensive (SHR) and normotensive Wistar-Kyoto (WKY) rats by measuring effects of pharmacologic blockers and using power spectral analysis of heart rate variability (HRV) and blood pressure variability (BPV). Resting HRs and BPs were recorded during a 20-min baseline period and over two 20-min periods of sequential autonomic blockade. On one day, animals received atropine followed by atenolol and on the following day, the drug order was reversed. Power spectrum analyses were performed on 30-s blocks of data throughout each of the three 20-min treatment periods (baseline, single blocker, combined blockade) and HRV and BPV were computed over low and high frequency bands. Increases in HR after atropine were significantly (approx. 40%) smaller in SHR rats than in WKYs. This held true even when animals were pretreated with atenolol. In contrast, decreases in HR following atenolol were similar in the two strains. These results indicate that SHRs have reduced vagal tone, but similar cardiac sympathetic tone compared to WKY rats. Virtually all HRV, over both low and high frequency bands, and in both strains, was inhibited by atropine. SHRs had significantly less high frequency HRV which is consistent with the conclusion that they have reduced cardiac vagal activity when compared with WKYs. BPV was not affected by atropine or atenolol but was inhibited by an alpha-receptor (phentolamine). BPV within both the low and high frequency bands was significantly greater in SHRs, suggesting that sympathetic tone to the vasculature is greater in SHR rats.