ROLE OF ANGIOTENSIN-II RECEPTOR SUBTYPES IN MEDIATING THE SYMPATHOEXCITATORY EFFECTS OF EXOGENOUS AND ENDOGENOUS ANGIOTENSIN PEPTIDES IN THE ROSTRAL VENTROLATERAL MEDULLA OF THE RABBIT

The presser region in the rostral part of the ventrolateral medulla (V LM) in the rabbit contains a high density of the AT(1) subtype of angi otensin (Ang) II receptor. In this study in anaesthetized barodenervat ed rabbits, we determined the effect of microinjection into the rostra l VLM of the AT(1) receptor antagonist losartan and the AT(2) receptor antagonist PD123319 or? resting arterial pressure and renal sympathet ic nerve activity, and on the cardiovascular responses normally evoked by exogenous Ang II or Ang III in this region. Losartan (1 nmol) abol ished the presser and sympathoexcitatory responses normally evoked by exogenous Ang II, but PD123319 (1 nmol) had little effect on these res ponses. Both losartan (0.1-10 nmol) and PD123319 (0.1-1 nmol) had litt le effect on the resting arterial pressure and renal sympathetic nerve activity, except for a transient sympathoexcitatory response at the h igher doses. In confirmation of previous findings, however, microinjec tion of the non-selective Ang receptor antagonist [Sar(1),Thr(8)]Ang I I (80 pmol) significantly decreased resting arterial pressure and symp athetic nerve activity. These results suggest that the sympathoexcitat ory effects evoked by exogenous Ang II and IU in the rostral VLM are m ediated by AT(1) receptors, but that the tonic sympathoexcitation prod uced by endogenous Ang peptides in the rostral VLM of the rabbit are m ediated by receptors other than AT(1) or AT(2) receptors. (C) 1997 Els evier Science B.V.