A. C. Barger

Redistribution of Renal Blood Flow Produced by Furosemide and Ethacrynic Acid

Outer medullary blood flow (85Kr method) is markedly reduced by furose-mide and ethacrynic acid injected into the renal artery of unanesthetized dogs; juxtamedullary cortical flow is also decreased. Anatomic localization of the redistribution was verified by autoradiography and by silastic injection studies. The injected specimens demonstrated a relative increase in the resistance of the peritubular capillaries of the juxtamedullary cortex and outer medulla, the blood supply of the juxtamedullary tubules, Henles loops and collecting ducts; the vasa recta were dilated. Total renal blood flow (Doppler Flowmeter) decreased 10% 1 min after injection, and then gradually began to increase at 2 to 3 min, reaching levels 25 to 30% above control as cortical blood flow increased. The vasodilatation in the cortex is probably localized in the pars radiata. The onset of the diuresis coincided with the decrease in renal blood flow, and persisted after total renal blood flow returned to control value. Thus, the time course of the dicresis correlated more closely with the sustained decrease in juxtameduiary and outer medullary blood flow.

Interactions between intrarenal epinephrine receptors and the renal baroreceptor in the control of PRA in conscious dogs.

We investigated the relationship between plasma renin activity (PRA) and renal arterial pressure (RAP) during (1) control conditions, (2) intravenous epinephrine infusion, and (3) intrarenal epinephrine infusion. In five uninephrectomized conscious dogs maintained on a low-salt diet, we controlled the RAP with an inflatable constricting cuff around the renal artery. (1) The control stimulus-response curve of the renal baroreceptor consists of a relatively flat section (slope = −0.03 ng Al/ml/hr per mm Hg) at RAPs close to control pressure and a much steeper section (slope = −0.48 ng Al/ml/hr per mm Hg) at RAPs below a threshold pressure of approximately 75 mm Hg. (2) Intravenous infusion of epinephrine at a dose that yields physiological plasma concentrations shifts the stimulus-response curve to the right by 10–15 mm Hg without altering either slope. (3) Intrarenal epinephrine infusion also displaces the stimulus-response curve to the right; intrarenal and intravenous epinephrine infusions that produce similar renal arterial epinephrine concentrations shift the response curve by the same amount. (4) However, as previously reported, intravenous administration of epinephrine decreases mean aortic pressure (MAP) (93 to 88 mm Hg, P < 0.05) and increases PRA (1.1 to 1.6 ng Al/ml per hr, P < 0.05), while intrarenal epinephrine has no effect on either MAP or PRA. Taken together, these results demonstrate that both infusions act intrarenally, increasing the threshold pressure, but that only intravenous epinephrine has an additional systemic effect, lowering MAP and, therefore, RAP. The combination of an increase in threshold pressure and a decrease in RAP causes the RAP to fall below the threshold pressure, raising PRA. These observations explain why PRA increases during intravenous but not intrarenal infusion.

Antibodies as Specific Renin Inhibitors: Studies with Polyclonal and Monoclonal Antibodies and Fab Fragments

The use of antirenin antibody and Fab may provide a more specific physiologic tool and potential therapeutic agent than the existing pharmacologic inhibitors. The antibody combining site, by virtue of its larger size than organic compounds, has the capacity for a larger number of intramolecular contacts with its ligands, thus allowing increased selectivity and affinity. Renin-specific Fab obtained through immunization with purified dog renal renin has been studied. Fab had no effect on blood pressure in the sodium replete conscious dog but induced systemic hypotensive responses in the sodium deplete animal or during acute renovascular hypertension. These responses were accompanied with complete suppression of plasma renin and angiotensin II levels. The vasodepressor responses of Fab were comparable to the converting enzyme inhibitor, teprotide. However, the latter induced a greater renal vasodilator effect. Since antibodies and their fragments derived from immune sera are limited in their application to physiologic study with respect to lack of homogeneity, reproducibility and limitation of quantity, we obtained monoclonal antibodies to purified human renal renin. Fusion HR3 yielded 14 renin-antibody secreting clones. Six clones have been isolated and characterized. Isotypes include IgG1 and IgM. Kds range from 6 X 10(-9) to 7.5 X 10(-10)M renin concentration. Five clones produce antibodies with antienzymatic activity. IC50s of these antibodies range from 1 X 10(-6) to 6 X 10(-8)M. All antibodies are renin-specific and do not bind several nonrenin enzymes and proteins examined. They are also highly species specific, i.e., do not crossreact with mouse, dog, bovine or rat renins but recognize hog renin. These catalytic-site directed antibodies may be highly potent and specific tools for physiologic studies in subhuman primates and man.

Stimulus-response curve of the renal baroreceptor: effect of converting enzyme inhibition and changes in salt intake.

We investigated the effect of converting enzyme inhibition (CEI) on the relationship between renal perfusion pressure (RPP) and steady-state plasma renin activity (PRA) in uninephrectomized conscious dogs on normal-salt (80 meq Na+/day) and low-salt (10 meq Na+/day) diets. Stimulus-response curves for the renal baroreceptor were determined by measuring the steady-state PRA while the RPP was lowered and then held constant by an inflatable cuff placed around the renal artery. On each diet the control stimulus-response curve can be described by two lines intersecting at a threshold pressure; in the higher pressure range PRA is relatively insensitive to changes in RPP, while in the lower pressure range PRA is very responsive to changes in RPP. On the normal-salt diet CEI significantly increases the sensitivity of PRA to RPP in the responsive range without affecting the threshold pressure itself or the values of PRA at pressures greater than the threshold pressure. On the low-salt diet CEI also increases the sensitivity of PRA to RPP significantly in the responsive range; we were unable to determine the effect of CEI on PRA at RPPs greater than the threshold pressure in the low-salt state because CEI causes a significant drop in blood pressure under these circumstances. The effect of CEI was significantly greater in the dogs on the low-salt diet than in the dogs on the normal-salt diet. Thus, CEI and salt depletion interact synergistically to increase the sensitivity of the renal baroreceptor only in the responsive range of the stimulus-response curve, i.e., at renal perfusion pressures below the threshold pressure. (Circulation Research 1987;61:670-677)

Plasma atrial natriuretic factor during chronic thoracic inferior vena caval constriction.

The effects of chronic constriction of the thoracic inferior vena cava (TIVCC) on plasma atrial natriuretic factor (pANF) were studied in conscious dogs (n = 5). TIVCC decreased left and right atria] pressure and led to a decrease in pANF concentration from 199 ± 12 to 104 ± 14 pg/ml while plasma renin and vasopressin concentrations increased. These hormonal changes were associated with a significant fall in sodium excretion to <5 meq/day. pANF remained suppressed during chronic TIVCC as the dogs expanded their extracellular fluid volume and developed ascites. Acute release of TIVCC resulted in abrupt increases in left and right atrial pressure but only a modest rise in pANF from 96 ± 16 to 185 ± 45 pg/ml. The magnitude of the rise in pANF (twofold) contrasted sharply with the eightfold increase in sodium excretion that occurred over the first 24 hours. Our data suggest that decrease in atrial pressure below normal results in a decline in pANF, which, acting in concert with the activated renin-angiotensin system and vasopressin, may contribute to sodium retention. On the other hand, during acute release of TIVCC, which markedly increased atrial pressure and sodium excretion, pANF only returned to control levels. These data suggest that ANF release may be attenuated during chronic reduction in atrial pressure and also raise a question concerning the magnitude of the primary role of ANF in this natriuretic response.