Sympathetic and parasympathetic influence on blood pressure and heart rate variability in Wistar-Kyoto and spontaneously hypertensive rats.

The relative influence of sympathetic and parasympathetic nervous control of heart rate and heart rate variability, of mean arterial pressure (MAP) and MAP variability was investigated in young spontaneously hypertensive rats (SHR) and Wistar-Kyoto rats (WKY). An on-line computerized system was used for continuous intra-arterial measurements of MAP and heart rate every 2.5 min in freely moving rats with indwelling arterial catheters. Variability was expressed as the standard deviation in each rat. Heart rate and MAP showed clear diurnal rhythms, both in WKY and SHR. Blood pressure variability was clearly higher in SHR than in WKY, both during dark active hours and during light hours of relative sleep, and it did not change in response to either beta-adrenoceptor or vagal blockade. Structural vascular changes with the consequent increase in vascular reactivity may be one explanation for the elevated blood pressure variability in SHR. Heart rate variability was clearly reduced in SHR compared with WKY. This may be due to a reduced tonic vagal discharge in SHR, whereby a vagally mediated tachycardia is eliminated. The vagal withdrawal is part of the defence reaction, which is more easily elicited in SHR than in WKY.