Ethnic differences in the renal dopamine response to an oral salt load

Dopamine is a natriuretic hormone which is synthesised within the kidney. We have previously found that some ethnic groups (e.g. British Caucasians and Thais) show strong positive correlations between 24 h urinary sodium and dopamine outputs. However no such relationship was found in Iranians and black West Africans. We have now studied the effect of oral salt loading (200 mmol) on 24 h urinary sodium and dopamine outputs in individuals from these four ethnic groups. The Caucasians showed a significant increase (35%) in urinary dopamine output after salt loading. However, there was no dopamine response to the salt load in the Ghanaians and the Iranians. The Thais were all vegetarians and fell into two distinct groups with either an increase ('positive responders') or decrease ('negative responders') in dopamine output after salt loading. The negative responders all had a very low mean control sodium output (60 +/- 21 mmol/24 h) compared to the positive responder Thais and the other three ethnic groups which all had mean sodium outputs greater than 115 mmol/24 h. We hypothesize that in some racial groups the uncoupling of the renal sodium-dopamine relationship may be an inherited mechanism to help conserve sodium (as in the Ghanaians and the Iranians) or an adaptive change to being restricted to a low salt diet for a long time (as in the Thai 'negative responders').