Isotonic saline

Renal hemodynamic, urinary excretion and cardiovascular (MAP and heart rate) responses to 180 min of isotonic saline (0.9% sodium chloride, 154 mmol/l, Baxter Healthcare, USA) were examined on a separate day. Similar to the time-control series, following a 1-h equilibration period, cardiovascular and renal function were measured over 60 min to establish baseline before commencement of isotonic saline infusion at 0.13 ml/kg/min equating to 20 µmol Na⁺/kg/min for 180 min (i.v) followed by a 180 min period of recovery, with blood and urine samples collected as described above. A 5 ml blood sample was also collected at end of baseline, saline infusion and recovery period for analysis of plasma renin activity (PRA) via radio-immunoassay (ProSearch International). Urine samples were analyzed for sodium concentration (Beckman Coulter, Monash Medical Centre) and sodium excretion calculated.

On a separate day, the cardiovascular, kidney hemodynamic and excretory responses to isotonic saline infusion (0.9% sodium chloride, 154 mmol/l, Baxter Healthcare, U.S.A.) for 180 min was examined. Following one hour of equilibration, urine samples were collected every 20 min throughout the study with an arterial blood sample (3 ml) taken at mid-point of each urine collection. Baseline data were collected over a period of 60 min. Then isotonic saline infusion was commenced at 0.13 ml/kg/min, which equates to 20 µmol Na⁺/kg/min [27 (link)], for 180 min (i.v) followed by a 180-min period of recovery. During this period both BP (systolic, diastolic and MAP) and HR were measured continuously. In addition, arterial blood samples (5 ml) were collected on EDTA at the end of the baseline, saline load and recovery periods for the assessment of PRA. Within 2–3 days of experimentation, sheep were killed with an overdose of pentobarbitone sodium (100 mg/kg, iv.).

Basal renal function (time control experiment) was determined over a period of 3 hours during vehicle infusion (12 ml/h 0.9% isotonic saline, Baxter Healthcare, USA). A day prior to this, all animals had a catheter inserted into their right and left jugular vein (PE tubing, 1.2 × 1.0, Microtube Extrusions, Australia) for infusion purposes. On the morning of the experiment, a Foley catheter (Size 8 or 10, Uromedical, Australia) was inserted into the bladder for continuous urine collection. ⁵¹Chromium ethylenediaminetetraacetic acid (⁵¹Cr EDTA) and para-aminohippuric acid (PAH), was administered (i.v.) for the determination of glomerular filtration rate (GFR) and renal blood flow (RBF), respectively, via clearance methods as previously described55 (link). An equilibration period of one hour was allowed for the ⁵¹Cr EDTA and PAH to reach steady-state within the plasma. Following the equilibration period, urine samples were collected at 20 minute intervals, with an arterial blood sample (5 ml) collected mid-point, for a period of 3 hours. Blood pressure and heart rate were monitored continuously during this period.