Spontaneous pressure-flow relationships in renal circulation of conscious dogs

Renal pressure-flow (P-F) relationships are usually evaluated by measuring effects of mechanically induced changes in renal arterial pressure (AP) on renal blood flow (RBF). We devised a method allowing evaluation of renal P-F relationships during normal changes in AP occurring spontaneously in a conscious animal rather than during artificially induced changes in AP. In 18 trials in 6 dogs standing at rest, we measured average AP and RBF for each cardiac cycle over periods of approximately 35 min (approximately 3,100 cardiac cycles/trial). AP and RBF values for each cardiac cycle were expressed as percent change (%delta) from the 35-min average (beat-to-beat changes). Slope and angle of each consecutive beat-to-beat P-F change were calculated and collated into one of eight zones representing the possible physiological mechanisms responsible for concurrent, spontaneous changes in RBF and AP. In a predominance of the cardiac cycles (approximately 43%), the spontaneous AP-RBF relationship was consistent with being mediated by arterial baroreflexes (i.e., increases in AP were accompanied by proportionately greater increases in RBF during 44.4% of cardiac cycles in which AP increased, and decreases in AP were accompanied by proportionately greater decreases in RBF during 41.4% of cardiac cycles in which AP decreased). Blockade of autonomic ganglionic transmission with hexamethonium markedly attenuated this pattern. Our results indicate that renal circulation participates in moment-to-moment control of AP via a predominant baroreflex-like pattern.