EGF receptor-binding activity in the urine of normal horses and horses affected by chronic laminitis

Abstract

A heterologous radioreceptor binding assay (RRA) has been developed capable of detecting nanogram amounts of epidermal growth factor (EGF) receptor-binding activity in equine urine. The binding parameters of [ 125I]mEGF (murine EGF) to EGF receptors on equine plasma membranes are in good agreement with values from other EGF-RRA systems. The dissociation constant estimated from equilibrium methods (K D = 4 × 10 −10 M) is in reasonable agreement with that determined from the rate constants (K D = 6 × 10 −10 M) and is in good agreement with values determined in other species. The assay is specific for equine EGF (eEGF) receptor-binding activity and capable of detecting less than 0.34 nM eEGF receptor-binding activity in urine. Equine EGF receptor-binding activity in equine urine form adult horses varied widely between samples (8.5 ± 6.5 nM). This variability was somewhat reduced when values were adjusted for dilutional effects using urine creatinine as an indicator (3.6 ± 2.0 nanomoles/g creatinine). No significant differences were demonstrated between the means of EGF binding activity concentrations in clinically normal horses and horses affected by chronic laminitis.