EARLY DISTURBANCE OF BAROREFLEX CONTROL OF HEART-RATE AFTER TAIL SUSPENSION IN CONSCIOUS RATS

Experiments were performed on conscious chronically instrumented rats maintained on tail suspension to determine the time course of changes in baroreceptor control of heart rate produced by this procedure. Pres ser responses were elicited by bolus injections of graded doses of phe nylephrine and sodium nitroprusside, permitting evaluation of the tota lity of the sigmoidal curve relating mean arterial pressure to heart r ate. Compared with control rats maintained at 0 degrees, rats maintain ed at 20 degrees using tail suspension for 24 h showed a significant r eduction in reflex gain (-3.9 +/- 0.1 vs. -5.8 +/- 0.3 beats/min; P < 0.001) together with a reduction in the upper (472 +/- 11 vs. 512 +/- 5 beats/min; P < 0.01) and lower (270 +/- 3 vs. 284 +/- 2 beats/min; P < 0.01) plateaus of the sigmoidal curve. In three groups of rats, the sigmoidal curve was studied successively after returning for 1, 24, a nd 48 h at 0 degrees. The observed change in the gain of the reflex re turned toward control values after 48 h, whereas the other parameters characterizing the sigmoidal curve did not change significantly. Trans ient but significant modifications of heart rate (tachycardic response ) after the immediate return to the horizontal position were observed. The study provides evidence that 1) a significant change of the total ity of the sigmoidal curve characterizing baroreceptor control of hear t rate occurs very early after tail suspension in rats and 2) the gain of the reflex is restored during the 48 h after release of tail suspe nsion, whereas the other parameters characterizing the curve, particul arly the plateaus, remain altered.