Reflex responses on blood pressure and renal nerve activity to local intra-arterial injection of capsaicin in anesthetized dogs.
The present study was performed to examine effects of intra-arterial and intravenous injection of capsaicin on efferent renal sympathetic nerve activity (RNA) in pentobarbital anesthetized dogs. In animals with intact baroreceptors, intra-arterial injection of capsaicin (4 +/- 1 microgram) produced significant increases in mean blood pressure (MBP) and heart rate (HR) by +43 +/- 8 mmHg and +33 +/- 15 beats/min at 20 s after the injection, respectively. There followed a recovery toward the control so that 60 s after the intra-arterial injection changes in MBP and HR were +8 +/- 5 mmHg and +18 +/- 8 beats/min, respectively. RNA showed a biphasic response, comprising of an initial increase (+52 +/- 28% of the control at 20 s) followed by a decrease by -30 +/- 8% of the control 60 s after the injection. Following bilateral cervical vagotomy the initial increase in RNA did not differ significantly from the value of RNA in animals with intact baroreceptors. However, the later decrease in RNA reversed above the control so that 60 s after the injection change in RNA was +5 +/- 10% of the control. Complete baroreceptor denervation showed a similar response on RNA in the vagotomized animals (+49 +/- 9 and +14 +/- 10% of the control at 20 and 60 s after the injection, respectively). In contrast, intravenous injection of capsaicin (6 +/- 1 microgram/kg) resulted in significant decreases in MBP (-19 +/- 11 mmHg) and HR (-9 +/- 4 beats/min). RNA at 20 s after the injection showed a unidirectional decrease by -16 +/- 7% of the control in animals with intact baroreceptors. These responses reversed above the control after cervical vagotomy. Thus, these data indicate that activation of C-fiber afferents in skeletal muscles by intra-arterial injection of capsaicin results in renal sympathoexcitation, and that the later sympathoinhibition is mediated by combined activation of systemic baroreceptors.