Effect of exercise, phenylbutazone, and furosemide on the plasma renin activity and angiotensin I in horses.

Circulating concentrations of angiotensin I and plasma renin activity (PRA) were determined in 45 Thoroughbreds before prerace medication (at rest), 1 to 1 1/2 hours before racing but after prerace medication (prerace), within 15 minutes after racing (immediately postrace), and between 1 to 2 hours after racing. Angiotensin I was measured by radioimmunoassay, and PRA was determined by the rate of generation of angiotensin I in plasma incubated at 37 C. Irrespective of prerace medication, there was a marked increase in angiotensin I (0.478 +/- 0.034 to 0.848 +/- 0.051 ng/ml) and PRA (0.191 +/- 0.018 to 0.522 +/- 0.061 ng/hour/ml) from the at-rest samples to the immediate-postrace samples, with the exception of anhidrotic horses in which increases in angiotensin I (0.7131 +/- 0.0794 to 0.8081 +/- 0.0608 ng/ml) and PRA (0.1635 +/- 0.0379 to 0.2176 +/- 0.0437 ng/hour/ml) were not as marked as they were in other horses. Administration of 1 mg of furosemide/kg 2 hours before racing caused an increase in PRA from 0.2438 +/- 0.0324 to 0.3706 +/- 0.1110 ng/hour/ml, whereas horses under the influence of 4 mg of phenylbutazone/kg had a decrease in PRA from 0.2090 +/- 0.0440 to 0.0669 +/- 0.0206 ng/hour/ml. Marked increases in angiotensin I and in PRA did not occur when horses were administered furosemide and phenylbutazone as prerace medication.