Inhibition of the carotid baroreflex by urinary bladder distension

The cardiovascular responses to urinary bladder distension include a rise in arterial blood pressure, an increase in vascular resistance and heart rate, which are abolished by sectioning the pelvic and hypogastric nerves. There are indications in the literature that inputs from the arterial baroreceptors and urinary bladder converge at the same autonomic efferent pathways. Recent data provided evidence of a partial inhibitory action of the carotid baroreflex on the reflex cardiovascular responses evoked by bladder distension. In this study, we investigate the influence of urinary bladder distention upon cardiovascular responses elicited by carotid baroreceptor stimulation.

Wistar Rats were anaesthetised with alpha-choloralose (80 mg/kg, i.p.) supplemented as necessary. The carotid sinus was stimulated randomly with different volumes of saline (0.5-2.0 ml). The ureters were ligated and the bladder was cannulated. Arterial blood pressure (BP), ECG, heart rate (HR), and bladder pressure (BLP) were monitored. Baroreceptor stimulation was undertaken on two levels of BLP (2 and 20 cmH2O). The cardiovascular response to baroreceptor activation was assessed as the maximal variation of BP, HR and R-R interval for different volumes of saline injected.

Our results showed that in all tests baroreceptor stimulation evoked a decrease in BP and HR but the magnitude of these responses was significantly different for each volume of saline and for two levels of BLP. The effectiveness and reproducibility of responses to these stimuli were confirmed by injecting a fixed volume of saline into the carotid sinus of two animals at two levels of BLP. In five animals, the variation in BP increased as the magnitude of the stimulus to the baroreceptor increased. However, the response was reduced with increased BLP. The fitted regression line was calculated for all the tests (n = 82; s = 8.2; Yint = 0.62 for BLP = 2 cmH2O; s = 9.5; Yint = 2.9; r = 0.69 for BLP = 20 cmH2O). Similar effects were observed and similar patterns of regression lines were obtained with regard to HR changes (n = 48; s = 13.3; Yint = 2.7; r = 0.82 for BLP = 2 cmH2O; s = 15.8; Yint = 0.89; r = 0.76 for BLP = 20 cmH2O) and interval changes (n = 48; s = 7.1; Yint = 4.3; r = 0.78 for BLP = 2 cmH2O; s = 8.5; Yint = 0.2; r = 0.76 for BLP = 20 cm H2O).

These data suggest that the cardiovascular responses evoked by carotid baroreceptor stimulation are significantly different when the urinary bladder is empty or full with the response diminishing as the urinary bladder is distended.