Renal size and composition in hypertensive, obese rabbits

OBJECTIVE: To determine whether the renal growth associated with obesity is due to hypertrophy or hyperplasia. DESIGN: New Zealand white female rabbits were fed either standard rabbit chow (n = 17) or chow foritified with 10% corn oil plus 5% lard (n = 18) for 12-16 weeks. MEASUREMENTS: All rabbits were weighed, and intra-arterial blood pressures were successfully measured at the end of the study in 16 lean and 18 obese rabbits; percent water of entire kidneys (8 lean, 8 obese rabbits) and of defined regions of kidneys (8 lean, 10 obese rabbits) were obtained gravimetrically. Renal hemoglobin, protein and DNA was measured chemically (8 lean, 8 obese rabbits). RESULTS: Kidneys grew in size as the rabbits gained fat. In a series of 8 lean and 8 age-matched obese rabbits, weighing 3.7 +/- 0.1 kg and 5.4 +/- 0.4 kg (P < 0.05), the kidneys were 20% larger in the obese rabbits: 15.0 +/- 0.99 vs 18.0 +/- 2.59 (P < 0.05). Kidney protein was also 20% greater in the obese rabbit: 1.38 +/- 0.06 g/kidney vs 1.66 +/- 0.06 g/kidney (P < 0.05). While total renal DNA was 16% greater in the obese: 18.2 +/- 0.5 mu g/kidney vs 21.1 +/- 0.6 mu g/kidney (P < 0.05), no significant difference existed when the DNA was expressed as mu g/mg protein. Fractional water content of the intact kidney declined with obesity: 78.7 +/- 1.1% vs 76.0 +/- 1.2% (P < 0.05). Conversely, the hemoglobin content of the kidney at autopsy, an estimate of the unstressed vascular volume, increased with obesity: 55 +/- 19 mg/kidney vs 82 +/- 25 mg/kidney (P < 0.05). By contrast, water content of renal parenchyma was constant: 80.8 +/- 1.0% vs 80.9 +/- 1.2% (cortex); 84.0 +/- 0.8% vs 83.6% +/- 2.0% (outer medulla); and 85.7 +/- 0.8% vs 86.0 +/- 2.1% (inner medulla). CONCLUSION: The renal growth associated with obesity was predominantly hyperplastic and was associated with a partial exclusion of fluid from the renal sinus.