Comparison of renal sympathetic baroreflex effects of rilmenidine and alpha-methylnoradrenaline in the ventrolateral medulla of the rabbit

Objective To determine the influence of imidazoline receptors and alpha(2)- adrenoceptors in the rostral ventrolateral medulla (RVLM) on the renal symp athetic baroreflex. Methods The effects of rilmenidine (4 nmol) and alpha-methylnoradrenaline ( alpha-MNA, 80 nmol) micro-injected into the RVLM of urethane-anaesthetized rabbits previously implanted with renal nerve recording electrodes were exa mined before and after micro-injection of the imidazoline receptor/alpha(2) -adrenoceptor antagonist idazoxan and the alpha(2)-adrenoceptor antagonist 2-methoxyidazoxan (2-MI), Results Rilmenidine and alpha-MNA both lowered mean arterial pressure (MAP) by 28% and renal sympathetic nerve activity (RSNA) by 35%, and reduced RSN A upper plateaus and ranges by 30-70%. Rilmenidine decreased both sympathet ic burst frequency and amplitude while alpha-MNA reduced amplitude only, Ri lmenidine shifted the RSNA baroreflex curve to the left while alpha-MNA shi fted the curve to the right Idazoxan (13 nmol) reversed the hypotension and all RSNA effects of rilmenidine, while 2-MI (4 nmol) increased MAP 18% abo ve the control and also reversed all RSNA parameters. By contrast, 2-MI rev ersed the alpha-MNA-induced hypotension and partially restored RSNA and the upper plateau of the RSNA baroreflex curve. Idazoxan treatment only partia lly reversed the hypotension after alpha-MNA and had no effect on any of th e baroreflex curves, Conclusion Both alpha-MNA and rilmenidine injected into the RVLM of rabbits produce renal sympathetic inhibition, but differences in the location of t he baroreflex curve and the pattern of effects on burst amplitude and frequ ency suggest different mechanisms of action, The effects of idazoxan sugges t that rilmenidine acts via imidazoline receptors. Since 2-MI reversed the actions of alpha-MNA and also rilmenidine, this suggests that alpha(2)-adre noceptor hypotension can be produced in the rabbit RVLM and that rilmenidin e may activate alpha(2)-adrenoceptors, possibly as a result of activating i midazoline receptors, J Hypertens 2000, 18:1263-1276 (C) Lippincott William s & Wilkins.