Three aspects of renal function were measured in the toad, Bufo marinus (N=lO): (1) effect of rate of blood volume expansion on renal functions (UFR; GFR; urine and plasma ion concentrations; and ion excretion rates), (2) effect of hypo- and hyperosmotic blood volume expansions on renal functions, and (3) role of GFR and tubular processes in the differential response of UFR under different osmotic expansion stresses. Renal responses to differential rates of blood volume expansion have not been investigated in amphibians. Rate responses will be analyzed considering effects: ( 1) during infusion (neural, or, short term regulation of extracellular fluid volume) and (2) post infusion (hormonal, or, long term regulation of extracellular fluid volume). Volume expansions were administered with hypoosmotic (0.4%) saline and hyperosmotic (1.4%) saline, and ranged in rate from 4.0 to 20.6 ml/kg/min. This protocol is designed to present volume regulatory mechanisms with increased volume stimuli and different osmotic stimuli. Overall, infusion rate had no significant effects on renal responses measured: urine flow rate (UFR); glomerular filtration rate (GFR); urine and plasma ion concentrations; natriuresis; or kaliuresis. This was true for the infusion period and for the observed post infusion period (90 min). Rate was correlated with GFR in the hypoosmotic group (r=0.30, p=0.04) and natriuresis in the hyperosmotic group (r=0.34, p=0.03). A significant positive correlation was observed between UFR and GFR. Relative to treatment, UFR differed significantly; GFR response was inherently similar despite differences at individual intervals, indicating UFR differences between the treatments is due to tubular processes. Responses to hypoosmotic infusion included a significant diuresis, natriuresis, and a decreased urine sodium concentration, relative to hyperosmotic infusion. At low UFRs the hyperosmotic group produced urine relatively concentrated in sodium. Urine sodium concentration and UFR were positively correlated in the hypoosmotic infusion group -- at high UFRs, kidneys were unable to produce a dilute urine.

Currently published data on the role of the lymphatic system in amphibians are inadequate and contradictory. Estimates of the rate of formation of lymph and the role of the lymph hearts in returning this fluid to the circulation are not based on actual volume determinations but rather estimates derived from changes in hematocrit using published values of plasma and blood volume. The lymph hearts are known to be vital to the maintenance of normal fluid compartment physiology and to increase their rate of activity during episodes of hypovolemic stress. Yet, significant redistribution of body fluids following hemorrage appears to occur in animals without lymph hearts. In this study, plasma and blood volumes were determined by the dye dilution technique using injected Evan's blue dye to label the plasma. Eight intact and 6 animals with their lymph hearts destroyed were hemorrhaged to 78% and 75% of their initial blood volumes, respectively. Changes in blood volume were measured following the hemorrhage by analysis of Evan's blue washout and hemodilution.

The present study documents that baroreceptors located in the pulmocutaneous artery (PCA) are key components in control of lymph heart rate in amphibians. A negative feedback control loop exists between arterial pressure and lymphatic heart rate. The recurrent laryngeal nerve (rLN), which innervates the PCA baroreceptors, transmits information on arterial pressure to integration centers in the central nervous system. Lymphatic heart rate (LHR) is reduced as a result of increases in arterial pressure. This loop was determined using three experimental protocols. First, the correlation between LHR reduction and hormonally induced vasoconstriction was determined. Increases in arterial pressure due to pressor actions of angiotensin II and arginine vasotocin at high concentrations was negatively correlated to LHR. Second, lymphatic heart rate changes due to natural increases in arterial pressure were compared to rate changes due to increase in arterial pressure after bilateral denervation of the rLN. Post-denervation LHR was not affected by natural increase in arterial pressure prior to the establishment of a new resting arterial pressure. Increase in arterial pressure due to administration of vasoconstricting hormones was negatively correlated with LHR following denervation. Third, the effect on LHR due to direct stimulation of the rLN was studied. Stimulation of the rLN caused LHR to stop without increases in arterial pressure. Presumably, this negative feedback loop is present to limit fluid return to the cardiovascular system from the lymphatic system during periods of acute hypertension. Reduction in the return of lymph volume to the cardiovascular system could eliminate potential damage to pulmonary tissues due to high arterial pressures.