Reflex cardiovascular responses to somatic stimulation in spontaneously hypertensive rats

This study aimed to test whether the cardiovascular responses to somatic stimulation in spontaneously hypertensive rats (SHR) were enhanced compared with those in normotensive Wistar-Kyoto (WKY) rats, and to examine any role of the impaired baroreflex function in the hypertensive rats.

Experiments were done in anaesthetized SHR (n = 34) and WKY (n = 31). Baroreceptor reflexes were assessed by continuous infusion of incremental doses (5-30 microg/kg per min) of phenylephrine over a 3 min infusion period. Cardiovascular responses to sciatic nerve stimulation (5 s trains, 1 ms pulse duration, 400 microA intensity) were studied before and after baroreceptor deactivation. The latter was achieved either by carotid occlusion and cutting the vagi and aortic nerves (SHR, n = 28 and WKY rats, n = 27), or by complete baroreceptor denervation (SHR, n = 6 and WKY rats, n = 4).

We confirmed that baroreceptor sensitivity was significantly lower in SHR (0.40 +/- 0.05 ms/mmHg) than in WKY rats (0.90 +/- 0.04 ms/mmHg). Sciatic nerve stimulation elicited significantly greater increases in mean arterial pressure (MAP) and in heart rate in SHR than in WKY rats (+32.5 +/- 1.9 mmHg versus +20.2 +/- 1.1 mmHg and +13.5 +/- 1.5 bpm versus +8.0 +/- 1.1 bpm, respectively). Following baroreceptor deactivation, the responses to the same sciatic nerve stimulation of MAP and heart rate in SHR (+38.5 +/- 2.4 mmHg and +15.5 +/- 1.5 bpm) were still significantly greater than those in WKY rats (+29.5 +/- 1.3 mmHg and +11.6 +/- 1.2 bpm).

These results show that cardiovascular responses to sciatic nerve stimulation are increased in SHR compared to WKY rats, and that this increased reactivity to somatic stimuli in hypertensive rats does not depend upon the impairment in baroreflex function demonstrated in this strain.