OUABAIN IN PLASMA FROM SPONTANEOUSLY HYPERTENSIVE RATS

Ouabain has recently been identified in mammalian plasma with an appar ent adrenocortical origin. The objectives of the present study were to determine whether boiled plasma supernatants (BPS) from spontaneously hypertensive rats (SHR) contained elevated levels of material able to inhibit Rb-86 uptake, an indicator of sodium pump activity, compared with Wistar-Kyoto rats (WKY). Furthermore, the effect of increasing di etary calcium content from 1 to 3% on Rb-86-uptake inhibitory activity in plasma was examined. BPS from SHR and WKY consuming 1% calcium con tained sodium pump inhibitory activity equivalent to 16.43 +/- 0.23 an d 5.08 +/- 0.10 ng ouabain/ml, respectively (P < 0.0001). Increasing d ietary calcium intake to 3% reduced plasma ouabainlike activity (OLA) to 9.97 +/- 0.20 ng/ml (P < 0.0001) in SHR but was without effect in W KY (5.39 +/- 0.05; not significant). It was then determined whether th e plasma Rb-86-uptake inhibition could be attributed to authentic ouab ain. In WKY plasma pools the percentage of OLA attributable to authent ic ouabain was 38.0% by radioimmunoassay and 56.7% by Rb-86-uptake ass ay. In SHR these values were 3.8% and <7.1%, respectively. Whereas the data in the present study provide confirmation of the presence of oua bain in rat plasma, ouabain does not account for the elevated OLA in S HR plasma reported here and elsewhere. This hypertensinogenic cardioto nic steroid appears to be appropriately downregulated in SHR rats.