Exaggerated response to alerting stimuli in spontaneously hypertensive rats.

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Abstract
The startle response, consisting of behavioral and cardiovascular components, was used to study the reaction of the cardiovascular system to a mild environmental stressor. We used tactile air puff startle to study responses in adult Wistar-Kyoto and spontaneously hypertensive rats. In both strains, air puff elicits a transient motor response with rapid habituation over the test session of 30 trials. Spontaneously hypertensive rats exhibit exaggerated motor responses compared with Wistar-Kyoto rats. Similarly, a 2-3-second duration pressor response was significantly greater in spontaneously hypertensive rats than in Wistar-Kyoto rats (47.7 +/- 2.0 versus 37.1 +/- 1.5 mm Hg, respectively). However, spontaneously hypertensive rats and Wistar-Kyoto rats exhibited strikingly dissimilar heart rate responses. Wistar-Kyoto rats exhibited a transient bradycardia (-42 +/- 7 beats/min) on early trials yielding to tachycardia on later trials (35 +/- 11 beats/min). In contrast, spontaneously hypertensive rats exhibited only tachycardia to all stimuli with an absence of bradycardia. Adrenal medullary secretions chronically modulate cardiac responses in both strains. Sinoaortic denervation did not alter the magnitude or profile of the heart rate responses. Spontaneously hypertensive--Wistar-Kyoto rat differences were not secondary to hypertension because renovascular hypertensive Wistar-Kyoto rats show normal responses to air puff. Four-week-old spontaneously hypertensive rats exhibit enhanced pressor and suppressed bradycardia responses relative to age-matched Wistar-Kyoto rats, indicating chronotropic differences precede development of established hypertension. Our results indicate parasympathetic activation by the mild startle stimuli rather than sympathetic withdrawal allows bradycardia to mask a latent tachycardia in Wistar-Kyoto rats. Spontaneously hypertensive rats exhibit a parasympathetic insufficiency in the startle response to novel alerting stimuli. Thus, mild air puff startle identifies a unique and discriminatory phenotypic difference between inbred normotensive and hypertensive rats.