ACUTE VOLUME EXPANSION AND SALT-LOADING STUDIES IN RATS - THE ROLE OFATRIAL-NATRIURETIC-PEPTIDE AND CATECHOLAMINES

Differences have been postulated for the mechanism of natriuresis due to atrial natriuretic peptide (ANP), salt loading with high salt diet (HS) and acute volume expansion (AcVE), in particular between AcVE and ANP based on the observed synergism between the two. Therefore the ef fects of and the interaction between the three were investigated in ra ts. ANP and AcVE produced the same natriuresis in HS as in normal salt (NS) rats and, in both, the actions of ANP and AcVe were significantl y additive showing similarity in mechanisms. Synergism [(AcVE + ANP) - AcVE] was, however, present only in the NS rats. Proximal tubular sod ium transport was the same with AcVE and ANP + AcVE suggesting that sy nergism is a property of more distal nephron segments. In conscious HS rats, plasma ANP was significantly less but natriuresis was higher th an in NS rats. ANP therefore probably has some causative role in the n atriuresis of AcVE but none in that of IIS loading. Urinary dopamine w as significantly increased by HS and further increased by AcVE in both NS and HS rats, the relationship between dopamine and natriuresis bei ng significantly positive (r(2) = 0.328) reaching equivalent levels in both NS and HS rats. Systemic benserazide prevented the increase in u rinary dopamine but only attenuated the natriuresis of AcVE. We conclu de that I-IS does not potentiate the natriuresis of AcVE or ANP, syner gism between AcVE and ANP is not a proximal tubule event and dopamine accounts for significant natriuresis of VE in addition to other natriu retic factors.