Renin, aldosterone and cortisol in the thoroughbred horse.

To assess the renin-aldosterone system in a large mammal, simultaneous morning activities of plasma renin and concentrations of aldosterone and cortisol were measured in 25 thoroughbred horses. Renin was relatively low in all horses (0.16 +/- 0.02 (S.E.M.) ng angiotensin I/ml per h), levels of aldosterone in plasma were 527 +/- 130 pmol/1 and levels of cortisol in plasma were 141 +/- 11 nmol/l. Levels of aldosterone were significantly correlated with levels of renin in all horses (r = 0.62, P less than 0.001) but not with those of cortisol, and renin was negatively correlated with age in male horses (r = -0.54, P less than 0.05). Horse plasma renin had a pH optimum of 6.0. These data suggest that the thoroughbred horse has a functioning renin-aldosterone system characterized by levels of plasma renin activity that are much lower relative to those of other mammalian species.