Glomerular and Tubular Functions in Sheep and Goats

¹ From the Department of Surgical Physiology, Walter Reed Army Institute of Research, Walter Reed Army Medical Center, Washington, D. C.

In many species of experimental animals the true equality of the renal clearance of inulin and creatinine has not been demonstrated, and considerable evidence indicates some tubular participation in creatinine excretion. This invalidates the use of creatinine to estimate glomerular filtration rate. We have measured the simultaneous renal clearance of inulin and exogenous creatinine in 23 experiments on seven goats and in eight experiments on two sheep. In both species creatinine clearance, C_Cr, normally exceeded inulin clearance, C_In, at low plasma concentrations of creatinine, P_Cr, and it was depressed both absolutely and relative to C_In by elevations in P_Cr or P_PAH or by intravenous probenecid. In contrast, C_In appeared to be independent of P_In and P_Cr and was not affected by probenecid. At low clearance levels in goats produced by traumatic shock, partial aortic occlusion or phloridzin, C_Cr fell far below C_In but promptly rose above C_In upon restoration of normal clearance levels. These findings indicate both tubular secretion and reabsorption of creatinine. Neither operation appears to be specifically affected by phloridzin. It is believed that tubular secretion of creatinine is an active metabolic process whereas tubular reabsorption is passive and attendant upon a residual tubular permeability to creatinine relative to inulin.