Lameness examinations and radiography of the distal phalanx and associated soft-tissue structures of the front feet of 103 Thoroughbred racehorses, 4 to 9 years old, were performed to determine normal radiographic appearance and morphometry. Of 103 horses examined, 41 were used in the study that were without clinical signs of foot problems or lameness, had raced at least twice prior to radiography, and had raced at least twice more in the 6 months after radiography. Lateromedial and dorsoproximal-palmarodistal radiographic views of each front distal phalanx were used to measure 28 bone and soft-tissue structures, and to evaluate 14 radiographic findings. Significant differences were not observed between left and right digits for any radiographic determination. Mean thickness of the soft tissues dorsal to the distal phalanx, which provides an evaluation of the epidermal laminae, was 14.6 +/- 1.0 mm when measured adjacent to the distal aspect of the distal phalanx. Most horses had straight, smooth hoof walls that were parallel to the dorsal cortex of the distal phalanx. The mean degree of palmar rotation of the distal phalanx was -0.5 +/- 1.3, and none was rotated more than 4 degrees. The dorsal cortex was smooth and straight, without bone deposition or reaction in either digit for only 5 of the 41 horses. Active bone formation was seen unilaterally along the middle portion of the dorsal cortex in 7 horses, and along the distal portion of the dorsal cortex in 4 of the phalanges from 3 horses. New bone formation along the distal dorsal cortex was often accompanied by resorption of the palmar cortex. For 26 of the 31 horses without active bone deposition, smooth inactive bone formation along the midportion of the dorsal cortex was identified in 1 or both distal phalanges. Bone at the solar margin of the distal phalanx was uniformly dense and finely trabeculated, without evidence of resorption or fractures. Severe irregularity of the solar margin was not found in any digit, and the margin of both phalanges was smooth in 8 horses. Various degrees of solar margin irregularity were observed in the other 33 horses. The mean number of vascular canals within the distal phalanx was 8.4 +/- 1.7, and the diameter of the largest canal was 3.4 +/- 0.6 mm. A mean number of 2.0 +/- 1.2 vascular canals was oriented parallel to the radiographic beam on the dorsoproximal-palmarodistal view, and these were termed end-on vessels, because they were visualized as radiolucent dots greater than or equal to 1 mm in diameter in the central portion of the distal phalanx. Racing performance of horses with subtle radiographic signs of laminitis (palmar rotation, hoof wall curvature or undulations, palmar cortical resorption, distal dorsal cortical bone deposition) was poorer than that of horses without these signs. These findings are suggestive of a subclinical laminitis condition, which may influence performance without causing overt clinical signs.

To investigate the determinants of glomerular ultrafiltration, renal micropuncture studies were performed in 9 cats. Mean single nephron glomerular filtration rate (SNGFR), directly measured in outer cortical nephrons, was 29.4 +/- 3.0 nl/min. This was similar to the estimated value for SNGFR (31.3 +/- 4.6 nl/min) obtained by dividing left kidney total glomerular filtration rate (1.41 +/- 0.12 ml/min/kg of body weight) by left glomerular count (175,200 +/- 13,600 glomeruli/kidney). In micropuncture studies performed at mean renal perfusion pressure of 101.3 +/-1.0 mm of Hg, the glomerular capillary hydrostatic pressure was 58.0 +/- 1.4 mm of Hg. The glomerular transcapillary hydrostatic pressure gradient (40.0 +/- 1.8 mm of Hg) exceeded colloid osmotic pressure at the efferent end of the glomerular capillaries (28.4 +/- 2.1 mm of Hg) in all cats studied, indicating existence of positive effective filtration pressure throughout the glomerular capillary bed. These results indicate that glomerular capillary pressure is sufficiently high to prevent forces from reaching filtration pressure equilibrium in feline outer cortical nephrons. Thus, the value of SNGFR in feline nephrons depends on the glomerular transcapillary hydrostatic pressure gradient and the glomerular ultrafiltration coefficient.