Adenosine increases heart rate and sympathetic nerve activity reflexively in conscious humans through several mechanisms. The purpose of this study was to assess the relative contributions of arterial baroreceptor unloading, carotid chemoreceptor stimulation, and other adenosine-sensitive afferent nerves to these responses. In 12 healthy men, the effect on blood pressure, heart rate (HR), and muscle sympathetic nerve activity (MSNA; peroneal nerve) of lower body negative pressure (LBNP; -15 mmHg (1 mmHg =133.3 Pa)) was compared with the effect of intravenous adenosine (35, 70, and 140 |ag*kg"1*min_1). In eight subjects, the highest dose was reinfused during 100% oxygen to suppress arterial chemoreceptors. Blood pressure reductions during LBNP and adenosine (140 jxg-kg'^min"1) were similar. HR did not change significantly during LBNP (+2 ± 2 beats/min; mean ± SE) but increased at the highest adenosine dose (+25 ± 3 beats/min; p < 0.05). MSNA increased significantly (p < 0.05) during both interventions (+255 ± 82 and +247 ± 5 8 units/100 beats for adenosine and LBNP, respectively), and there was no difference in the MSNA response to these two stimuli (p > 0.1). Oxygen inhibited adenosine-induced increases in HR and MSNA (from +305 ± 99 to +198 ± 75 units/100 beats and from +26 ± 4 to +18 ± 3 beats/min;p < 0.05 for both comparisons). The MSNA response to these combined stimuli was similar to that observed during LBNP. In contrast, the residual HR response (+18 ± 3 beats/min) was significantly greater than the response to LBNP (+2 ± 2 beats/min; p < 0.05). These data indicate that arterial baroreceptor unloading cannot account for the marked adenosine-induced increase in HR, but may be sufficient to explain its effect on MSNA. The effect of 100% oxygen confirms that stimulation of carotid chemoreceptors accounts for approximately one-third of the HR and MSNA response to adenosine. However, other mechanisms, such as stimulation of adenosine-sensitive afferent nerves in other vascular beds, are involved in the HR and possibly the MSNA response.Key words: adenosine, human, heart rate, sympathetic nervous system, baroreflex.Résumé : L 'adénosine augmente la fréquence cardiaque et l'activité nerveuse sympathique de manière réflexe chez les humains conscients et ce, par 1*intermédiaire de divers mécanismes. Le but de la présente étude a été d'évaluer l'influence sur ces réponses d'une décharge des barorécepteurs artériels, d'une stimulation des chémorécepteurs carotidiens, ainsi que d'autres nerfs afférents sensibles à Padénosine. Chez 12 hommes sains, on a comparé l'effet d 'une pression négative des membres inférieurs (PNMI; -15 mmHg (1 mmHg = 133,3 Pa)) avec celui de Padénosine intraveineuse (35, 70 et 140 jig-kg^-m ür1) sur la pression artérielle, la fréquence cardiaque (FC) et l'activité nerveuse sympathique musculaire (ANSM; nerf périonnier). Chez huit sujets, on a réinjecté la plus forte dose d'adénosine en présence de 100% d'oxygène afin de supprimer les chémorécepteurs artériels. Les réductions de press...