Possible contribution of impaired sodium excretion to the development and maintenance of hypertension: a study of the isolated kidneys of the Prague hypertensive rat.
We have shown previously that in the Prague Hypertensive Rat (PHR) "hypertension travels with the kidney" and that the kidney appears to produce an as yet unknown "hypertensogenic" substance. Since enhanced sodium retention could also contribute to this type of hypertension, this possibility was tested in isolated perfused kidneys from PHR and from its normotensive substrain, PNR, bred from the same parent pair as PHR, at two levels of perfusion pressure (PP), i.e 110 and 150 mmHg (where 1 mmHg = 133.3 Pa). Young (6-week-old) and adult (12-week-old) animals of both substrains were used. In young PHR and PNR, there was no significant difference in haemodynamic parameters when the kidneys were perfused at either low or high PP. Surprisingly, water and sodium excretion rates were also the same at both PP values in both substrains, which thus--at this age--do not exhibit the well known "pressure diuresis and natriuresis". In adult PNR, perfusate flow and glomerular filtration rates (GFR) were independent of the level of PP (autoregulation) whereas water and sodium excretion rates were significantly higher at 150 than at 110 mmHg, a finding similar to those found in vivo studies (pressure diuresis and natriuresis). In adult PHR, however, both perfusate flow rate and GFR were pressure dependent: only at the high PP were values of both GFR and perfusate flow obtained which were similar to those in the PNR at low pressure. Sodium excretion was lower, and its tubular reabsorption higher, in PHR than in PNR at both levels of perfusion. Again, in PHR, the higher PP was needed to achieve the same rate of sodium excretion as in PNR at the lower pressure. Thus, the kidneys of PHR retain sodium at a given PP compared with the kidneys from PNR. This may contribute to the development and/or maintenance of hypertension in the PHR.