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Parabiosis and cross-circulation experiments with spontaneously hypertensive and normotensive rats gave indications for a previously unidentified circulating hypertensive agent. In this study, plasma from normotensive and hypertensive rats was fractionated and the vasopressor action of the corresponding fractions was measured in the isolated perfused rat kidney. One of three vasoactive fractions obtained by gel filtration (Biol-Gel P2) from hypertensive rats showed a significantly higher activity (increase in perfusion pressure by 1502.9 ± 438.9 Pa) than that from normotensive rats (increase in perfusion pressure by 505.4 ± 186.2 Pa, P<0.01). Further chromatographic separations of this fraction revealed that the hypertensive factor is hydrophilic and has no ionic groups or vicinal diol groups. The molecular mass was estimated by dialysis and the matrix-assisted laser desorption/ionization mass spectrometry to be in the range of 1 kDa. The vasopressor is heat resistant and not degradable with trypsin or carboxypeptidase Y. The vasopressor action was not inhibited with the angiotensin-II-receptor antagonist saralasin, the α-receptor antagonist phentolamine, the thromboxane-receptor antagonist carbocyclic thromboxane A2 or the serotonin antagonist ketanserine.

The results confirm the existence of a vasopressor factor in the plasma of hypertensive rats and, in a lower concentration, of normotensive rats, which is possibly related to the pathogenesis of essential hypertension. The chromatographic behaviour suggests that this factor is different from the parathyroid hypertensive factor described recently.