Angiotensin II and the maintenance of GFR and renal blood flow during renal artery narrowing.

The time course of the renal blood flow and GFR responses to narrowing of the renal artery in conscious dogs is reviewed. The initial response to this threat to renal perfusion is renal vasodilatation, but within minutes a secondary vasoconstriction mediated by angiotensin II begins to develop. Angiotensin II-mediated contraction of mesangial cells is also demonstrable, but this does not apparently reduce the filtration surface area of the glomerular capillaries. The intrarenal effects of angiotensin II restore GFR back to normal within one to two weeks, by which time circulating plasma angiotensin II levels are no longer elevated. In contrast to its effects on GFR, angiotensin II has minimal effects on renal blood flow after stenosis. This may be because, (i) blood flow is mainly determined by the hydraulic resistance of the stenosis; (ii) renal vasoconstriction has relatively little effect on flow due to the particular hemodynamic properties of the stenoses, and (iii) a major site of action of angiotensin II may be within the glomerulus. Thus angiotensin II has a homeostatic role in the maintenance of GFR during renal artery narrowing and one component of this role may involve mesangial contraction.