Cardiac vagal responsiveness during development in spontaneously hypertensive rats

In the present study we sought to determine the effect of age, hypertension and endogenous angiotensin on the chronotropic responses to vagal stimulation in urethane anesthetized-normotensive Wistar-Kyoto (WKY) and spontaneously hypertensive rats (SHR). After beta-adrenoceptor blockade with atenolol, the right and left vagal nerves were stimulated with graded frequencies between 1 and 32 Hz in 5-, 8- and 22-week-old animals. At all ages and in both strains, there was a strong linear relationship between the degree of bradycardia and the log of the stimulation frequency. At the age of 5 weeks, the bradycardia to stimulation of the right vagus was greater in SHR than that observed in WKY (P<0.05). However, in 8- and 22 week-old animals, no differences were observed between the response to vagal stimulation in WKY and SHR. Thus, there was an age-dependent increase in the response to right vagal stimulation in WKY, but no such trend in SHR. No significant age-dependent changes in left vagal responses were observed in either strain. Left vagal responses were approximately half of the response to right vagal stimulation at all ages in SHR and in 8-22 week WKY, but similar to right vagal responses in 5 week WKY. Administration of the angiotensin converting enzyme inhibitor perindopril, which effectively blocked the formation of endogenous angiotensin, did not affect responses to vagal stimulation at any age, in either strain. These results suggest that the baroreflex vagal deficit observed in adult SHR compared to WKY is not due to a difference in the responsiveness of the cardiac vagal neuroeffector mechanism nor due to an effect of circulating angiotensin II. Furthermore, the enhanced vagal bradycardia observed in very young SHR which was due primarily to the earlier establishment of the adult vagal response pattern may indicate accelerated vagal development in this strain compared to WKY.