Basal vascular tone in the kidney. Evaluation from the static pressure-flow relationship under normal autoregulation and at maximal dilation in the dog.

In conscious dogs static pressure-flow relationships (I-P curves) were obtained both for the normal autoregulating kidney vasculature and under conditions of maximal vasodilation induced by infusion of papaverine or acetylcholine into the renal artery. Concentration-response curves for the substances infused showed the typical S-shape. Control I-P curves exhibited an autoregulatory plateau; under maximal vasodilation the I-P curves were pressure-passive up to a perfusion pressure of 50 mm Hg, following a power function (1 = a-Pn) with an exponent greater than 1. Under the influence of acetylcholine, renal blood flow was significantly higher than under control conditions even at a perfusion pressure of 20 mm Hg. This indicates that there is an appreciable vascular tone even at a low pressure. The I-P curves under acetylcholine showed a break at about 50 mm Hg, above which the I-P curves were straight lines.