Since Bright's description of the uremic syndrome 150 years ago, the kidney has been accorded a key role in the pathogenesis of hypertension, a role reinforced by the finding that in rat models of genetically determined spontaneous hypertension the hypertension often can be transplanted with the kidney1,2,3. The development of hypertension in rats after reduction of renal mass4,5 has led to the hypothesis that reduction of filtration area by glomerular loss6,7 plays a major role in the development of hypertension. This study analyzes the relationship between glomerular numbers and blood pressure (BP) in a new model of genetic hypertension, the Prague hypertensive rat (PHR) and its normotensive counterpart (PNR), rat strains bred from a single parent pair and thus genetically closely related8.
METHODS
Experiments were carried out on PNR and PHR (both sexes, age 7 to 53 weeks). On at least three separate days prior to the experiment, systolic BP was measured by tail-cuff sphygmomanometry. Glomeruli were stained in vivo in anesthetized (40 mg/100 g body weight i.p. sodium pentobarbital) rats as described elsewhere9. In later experiments, the staining procedure was dispensed. After 20 to 30 minutes, the animal was sacrificed by injection of an overdose of anesthetic, and both kidneys were removed rapidly and decapsulated, weighed, cut into small pieces and (when stained) fixed in 1% NH4Cl, macerated (50% HCl for 2 hours at room temperature), rinsed, and transferred to a flask containing 100 ml H2O. The kidney fragments were then dispersed on a magnetic stirrer at low speed for 8 to 10 hours, and the glomeruli in 40 
l aliquots were counted under microscope (at least 20 aliquots from each animal); the mean was used as a single value for statistical analysis. Preliminary analysis showed that the prerequisites for multiple analysis of variance were not met, so partial correlation analysis was performed instead.
RESULTS AND DISCUSSION
Prague hypertensive rats were heavier and had larger kidneys and higher BP than PNR (152 
20 vs. 102 17, P < 0.001). PHR had 19% fewer glomeruli than PNR, which had approximately 31,000 glomeruli per kidney. Males were heavier and had larger kidneys than females. Female PNR had slightly fewer glomeruli than males. There was no significant correlation between BP and total number of glomeruli, except in PHR males Figure 1. Because the lower end of the BP distribution of PHR overlapped with the upper end of that of PNR, it was possible to obtain subsets of PHR and PNR with the same mean BP. These had, however, the same difference in glomerulus count Figure 2. Similarly, subsets of PHR and PNR with the same mean glomerulus count had the same difference in BP as the whole group Figure 2.
Figure 1.
Systolic blood pressure (BP; tail-cuff sphygmomanometry) as a function of the total number of glomeruli (both kidneys) in male (circles) and female (triangles) Prague hypertensive (PHR; solid symbols) and normotensive (PNR; open symbols) rats. The straight lines are the regression lines: (long dashed line) PHR male; (short dashed line) PHR female; (dots spaced apart) PNR male; (dots spaced together) PNR female.
Full figure and legend (15K)Figure 2.
Glomerulus count and systolic blood pressure (BP; mean SEM) in subsets of Prague hypertensive (PHR; solid symbols) and normotensive (open symbols) chosen either for same mean BP (squares) or same glomerulus count (triangles).
Prague normotensive rats had the same number of glomeruli as normotensive rats of other strains10,11,12. PHR had significantly fewer, consistent with one study on spontaneously hypertensive rats13. This is, however, not a universal finding, as several studies have not found any difference10,11,12. If (high) BP and (reduced) glomerular number are causally related, one would expect to find a negative correlation between the two parameters, BP and number. This was the case only for PHR males. Moreover, in the regions of the BP and glomerular number distributions where values for the two strains overlap, subsets of PHR and PNR with the same mean BP had the same difference in glomerulus count. Similarly, subsets of PHR and PNR with the same mean glomerulus count had the same difference in BP as the whole group. Thus, the absolute number of glomeruli does not appear to be a direct determinant of BP in PNR and PHR over seven weeks of age, although neither a possible effect at an earlier age nor a possible sex linkage can be excluded. The lack of consistency in the literature suggests that findings with acute reduction in renal mass (by ablation and/or infarct) are not immediately relevant to genetically determined hypertension. Other possible causal mechanisms, such as genetically determined changes in transporter or receptor proteins14,15,16, humoral mechanisms1, and vascular abnormalities,10 must also be considered.
References
- HELLER, J, SCHUBERT, G, HAVLICKOVA, J, THURAU, K: The role of the kidney in the development of hypertension: A transplantation study in the Prague hypertensive rat. Pflügers Arch 1993 425: 208–212, | Article | ChemPort |
- FERRARI, P, BARBER, BR, TORIELLI, L, FERRANDI, M, SALARDI, S, BIANCHI, G: The Milan hypertensive rat as a model for studying cation transport abnormality in genetic hypertension. Hypertension 1987 10: I32–I36, | PubMed |
- RETTIG, R: Does the kidney play a role in the aetiology of primary hypertension? Evidence from renal transplantation studies in rats and humans. J Hum Hypertens 1993 7: 177–180, | PubMed |
- ANDERSON, S, MEYER, TW, RENNKE, HG, BRENNER, BM: Control of glomerular hypertension limits glomerular injury in rats with reduced renal mass. J Clin Invest 1985 76: 612–619, | PubMed | ISI | ChemPort |
- BRENNER, BM: Nephron adaptation to renal injury or ablation. Am J Physiol 1985 249: F324–F337, | PubMed | ISI | ChemPort |
- BRENNER, BM, ANDERSON, S: The interrelationships among filtration surface area, blood pressure, and chronic renal disease. J Cardiovasc Pharmacol 1992 19 (Suppl 6): S1–S7,
- BRENNER, BM, GARCIA, DL, ANDERSON, S: Glomeruli and blood pressure: Less of one, more the other? Am J Hypertens 1988 1: 335–347, | PubMed | ISI | ChemPort |
- HELLER, J, HELLEROVA, S, DOBESOVA, Z, KUNES, J, ZICHA, J: The Prague hypertensive rat: A new model of genetic hypertension. Clin Exp Hypertens 1993 15: 807–818, | PubMed |
- BANKIR, L, HOLLENBERG, NK: In vivo staining of the kidney with Alcian blue: An adjunct to morphological and physiological studies. Renal Physiol 1983 6: 151–155, | PubMed | ISI | ChemPort |
- KETT, MM, ANDERSON, WP, BERTRAM, JF, ALCORN, D: Structural changes in the renal vasculature in the spontaneously hypertensive rat: No effect of angiotensin II blockade. Clin Exp Pharmacol Physiol Suppl 1996 3: S132–S135, | PubMed |
- DE KEIJZER, MH, PROVOOST, AP, MOLENAAR, JC: Glomerular hyperfiltration in hypertensive fawn-hooded rats. Renal Physiol Biochem 1988 11: 103–108, | PubMed |
- STERZEL, RB, LUFT, FC, GAO, Y, SCHNERMANN, J, BRIGGS, JP, GANTEN, D, WALDHERR, R, SCHNABEL, E, KRIZ, W: Renal disease and the development of hypertension in salt-sensitive Dahl rats. Kidney Int 1988 33: 1119–1129, | PubMed | ISI | ChemPort |
- SKOV, K, NYENGAARD, JR, KORSGAARD, N, MULVANY, MJ: Number and size of renal glomeruli in spontaneously hypertensive rats. J Hypertens 1994 12: 1373–1376, | PubMed | ISI | ChemPort |
- KELLY, MP, QUINN, PA, DAVIES, JE, NG, LL: Activity and expression of Na(+)-H+ exchanger isoforms 1 and 3 in kidney proximal tubules of hypertensive rats. Circ Res 1997 80: 853–860, | PubMed | ISI | ChemPort |
- FUNDER, JW, KROZOWSKI, Z, MYLES, K, SATO, A, SHEPPARD, KE, YOUNG, M: Mineralocorticoid receptors, salt, and hypertension. Recent Prog Horm Res 1997 52: 247–260, | PubMed | ISI | ChemPort |
- CORVOL, P: Liddle's syndrome: Heritable human hypertension caused by mutations in the beta subunit of the epithelial sodium channel. J Endocrinol Invest 1995 18: 592–594, | PubMed |
