Spontaneously hypertensive rats exhibit altered cardiovascular and neuronal responses to muscle contraction

We examined the cardiovascular and ventrolateral medullary neuronal responses to muscle contraction in the spontaneously hypertensive rat (SHR) and normotensive Wistar-Kyoto rat (WKY) control. Cardiovascular, respiratory and ventrolateral medullary neuronal responses to muscle contraction evoked by tibial nerve stimulation were recorded. SHRs exhibited significantly larger drops in arterial pressure compared to WKYs in response to muscle contraction (P < 0.05). Basal ventrolateral medulla neuronal discharge rates were similar between the SHR and the WKY groups. A majority of neurons recorded responded to muscle contraction in both the WKY (77 %; n = 53) and the SHR groups (68 %; n = 62). There was no difference in the percentage of neurons that responded with an increase (approximately 60 %) or decrease (approximately 40 %) in firing rate between hypertensive and normotensive rats. Pulse wave-triggered averaging techniques showed that most neurons that responded to muscle contraction also possessed a basal firing rhythm temporally related to the cardiac cycle (85 % in WKYs, 83 % in SHRs). However, decreases in neuronal firing rates in response to muscle contraction were significantly greater in SHRs than WKYs. Therefore, we conclude that muscle contraction unmasks a hyperexcitability of neurons in the ventrolateral medulla of SHRs that parallels the heightened blood pressure responses.