J Am Soc Nephrol 13: 27–34, 2002 Preglomerular and Postglomerular Resistance Responses to Different Levels of Sympathetic Activation by Hypoxia KATE M. DENTON, AMANY SHWETA, and WARWICK P. ANDERSON Department of Physiology, Monash University, Clayton, Victoria, Australia. Abstract. This study investigated the effects of graded reflex P Ͻ 0.05) and filtration fraction (FF; P Ͻ 0.01) increased. In increases in renal sympathetic nerve activity (RSNA) on renal response to moderate hypoxia, calculated preglomerular preglomerular and postglomerular vascular resistances. With (ϳ20%) and postglomerular (ϳ70%) resistance both in- the use of hypoxia to reflexly elicit increases in RSNA without creased, but only the increase in postglomerular resistance was affecting mean arterial pressure, renal function and stop-flow significant (P Ͻ 0.05). In contrast, severe hypoxia decreased pressures were measured in three groups of rabbits before and ERBF (56 Ϯ 8%; P Ͻ 0.01), GFR (55 Ϯ 9%; P Ͻ 0.001), and Ϯ Ϯ after exposure to room air and moderate (14% O2) or severe glomerular capillary pressure (32 1 versus 29 1 mmHg; Ͻ (10% O2) hypoxia. Moderate and severe hypoxia increased P 0.001), with no change in FF, reflecting similar preglo- RSNA, primarily by increasing the amplitude of the sympa- merular (ϳ240%; P Ͻ 0.05) and postglomerular (ϳ250%; P Ͻ thetic bursts rather than their frequency. RSNA amplitude 0.05) contributions to the vasoconstriction and a decrease in Ϯ Ͻ Ϯ Ͻ Ͻ increased by 20 6% (P 0.05) and 60 16% (P 0.05), calculated Kf (P 0.05). These results provide evidence that respectively. Moderate hypoxia decreased estimated renal reflexly induced increases in RSNA amplitude may differen- blood flow (ERBF; 26 Ϯ 7%; P ϭ 0.07), whereas estimated tially control preglomerular and postglomerular vascular glomerular capillary pressure (32 Ϯ 1 versus 34 Ϯ 1 mmHg; resistances. Previous renal micropuncture studies have suggested that elec- Physiologically induced graded increases in renal sympathetic trical stimulation of the renal nerves predominantly constrict nerve activity (RSNA) cause progressive renal vasoconstriction the preglomerular vessels (1–4); this is not surprising, because (11,13,23), but the effects of hypoxia on preglomerular and post- the afferent arteriole is much more densely innervated than the glomerular vascular resistances have not been measured. This is efferent arteriole (5). However, electrical stimulation may not of particular interest, given our recent identification of two distinct fully mimic physiologic changes in nerve activity. For exam- nerve types that are differentially distributed to afferent and ef- ple, electrical stimulation at supramaximal voltages activates ferent arterioles (24). Type I axons almost exclusively innervate all nerves simultaneously, and the frequency at which the the afferent arteriole and have larger axon diameters with atypical nerves are fired is altered, whereas it is now clear that the varicosities (5), whereas type II axons are distributed equally to number of nerves activated during a physiologic burst of nerve both afferent and efferent arterioles and resemble those innervat- activity may vary widely (6–18). The bursting pattern of renal ing other blood vessels with typical fusiform varicosities (5). The sympathetic nerve activity can be differentiated into changes in distribution of the two types of nerves to the renal arterioles raises frequency (rate) of nerve firing and changes in amplitude the possibility that preglomerular and postglomerular vascular (number of nerves recruited during the burst) (6,9,19). Physi- resistances may be regulated differentially, depending on the ologic stimuli such as hypoxia, air jet stress, white noise (sound nerve type activated. We hypothesize that increased type I acti- that has a relatively wide continuous range of frequencies of vation would constrict the afferent arteriole, whereas type II nerve uniform intensities), and thermal stimulation have been shown activation would constrict both afferent and efferent arterioles. to reflexly increase the amplitude of the sympathetic bursts to To investigate this hypothesis, we have used two levels of the kidney (i.e., increased recruitment of nerves) without sig- hypoxia (14% and 10% O2) as our stimulus, which is known to nificantly changing the frequency of the discharges produce graded increases in RSNA and renal vascular resistance (11,12,20,21). Speculation has therefore arisen that different (11,23). Importantly, these levels of hypoxia do not alter mean patterns of activation of the renal nerves may produce differ- arterial pressure (MAP), therefore avoiding the potentially con- ential changes in renal function (22). founding influence of renal autoregulation. We have determined the relative changes in preglomerular and postglomerular resis- tances to the hypoxic stimuli, using stop-flow pressure (SFP) Received December 20, 2000. Accepted July 5, 2001. measurements to estimate glomerular capillary pressure. Correspondence to Dr. Kate Denton, Department of Physiology, P.O. Box 13F, VIC, Australia, 3800. Phone: 61-3-9905-2503; Fax: 61-3-9905-2566; E-mail: [email protected] Materials and Methods 1046-6673/1301-0027 Rabbits Journal of the American Society of Nephrology Experiments were performed on male rabbits of a multicolored Copyright © 2001 by the American Society of Nephrology English strain (n ϭ 20; mean, 3.0 Ϯ 0.1 kg; range, 2.5 to 3.5 kg). The 28 Journal of the American Society of Nephrology J Am Soc Nephrol 13: 27–34, 2002 experiments were performed in accordance with the Australian Code gram over 2-s periods was defined as total RSNA. Changes in voltage of Practice for the Care and Use of Animals for Scientific Purposes of the nerve signal above a defined threshold were classified as RSNA and were approved in advance by the Monash University Standing discharges (11). Committee on Ethics in Animal Experimentation. Micropressure measurements were taken during the baseline and gas periods; micropressures were not measured during the recovery Preparation period. The micropressures were pooled for each period, with 3 to 6 On the experimental day, catheters were placed in the ear central SFP measurements and 3 to 5 proximal tubular pressures being taken arteries and marginal veins under local anesthetic (Xylocaine, 0.5% in each period. The micropressures were measured by use of a wt/vol lidocaine; Astra Pharmaceuticals, North Ryde, NSW, Austra- servo-null pressure measuring device (Micropressure System, model lia). Conscious arterial pressure was measured for 20 min. Anesthesia 900A; WPI, Sarasota, Florida). was then induced by intravenous administration of pentobarbitone MAP, heart rate, total RSNA, RSNA frequency, and RSNA am- sodium (90 to 150 mg plus 0.1 to 0.2 mg/kg per min intravenously; plitude were all continuously recorded throughout the experiment and Nembutal, Boehringer Ingelheim, Artarmon, NSW, Australia) and the were sampled by use of an analog-to-digital data acquisition card ϩ rabbits ventilated (small animal ventilator, Model 683; Harvard Ap- (Lab-PC , National Instruments, Melbourne, Australia). Calibrated paratus, Holliston, MA). During surgery and throughout the experi- signals were displayed on screen and saved to disk as 2-s averages of ment, fluid was infused intravenously at 0.17 ml/min per kg (Hart- each variable by use of a program written in the LabVIEW graphical mann’s solution [compound sodium lactate]; Baxter Healthcare, programming language (National Instruments) (11). Toongabbie, NSW, Australia) to replace fluid losses. Surgery was performed on a heated table, and esophageal temperature was main- Measurements and Calculations tained between 36°C and 38°C throughout the experiment via a MAP, heart rate, total RSNA, frequency of RSNA, and RSNA servocontrolled infrared lamp (Digi-Sense Temperature Controller; amplitude were averaged over each clearance period. GFR was esti- Cole Palmer, Vernon Hills, IL). The rabbit was placed in an upright mated via the clearance of [3H]-inulin, and estimated renal blood flow crouching position, and a left flank incision made to expose the left (ERBF) as the clearance of [14C]-para-aminohippurate corrected for kidney. Silastic catheters were inserted into both ureters for the hematocrit. Renal vascular conductance was calculated as ERBF collection of urine, and the left kidney was placed in a micropuncture divided by MAP. Filtration fraction was calculated as GFR divided by cup. Before the kidney was prepared for micropuncture, the renal estimated renal plasma flow. Plasma renin activity (PRA) was deter- nerves were identified and a 3- to 5-mm length freed from the mined by RIA (26). Urinary sodium concentrations were determined surrounding tissue and carefully placed across a pair of hooked silver by flame photometry (Model 943; Instrumentation Laboratory, Milan, recording electrodes. The nerve and electrode were insulated from the ␲ Italy). Plasma oncotic pressure ( a) was calculated from arterial surrounding tissue and effluent from the micropuncture bath by use of plasma protein concentration (Ca), determined by use of the Lowry Wacker Sil-Gel (Wacker-Chemie, Munich, Germany). The kidney assay (25). Estimated glomerular capillary pressure (Pgc) was calcu- was then prepared for micropuncture as described elsewhere (25). ϩ ␲ lated as SFP a. Preglomerular resistance was estimated as (MAP After completion of surgery, [3H] inulin (4-␮Ci bolus plus 300 Ϫ Ϫ Pgc)/ERBF and postglomerular resistance as Pgc/(ERBF GFR).