The placental transport of amino acids by Na⁺-dependent systems A and β is important for growth and development of the fetus (Sibley et al. 1999). There are no studies of amino acid transport by the cat placenta or other endotheliochorial placentas. We have recently developed a model to study taurine uptake by cat placental fragments and demonstrated characteristics of system β (Champion et al. 2001). Here we use the fragment model to measure methylaminoisobutyric acid (MeAIB) uptake to examine the functional activity of system A in the cat placenta.

Cat placentas were collected following natural litter production (one placenta per litter), with full cat welfare considerations in place. Dissection of the placenta was begun within 45 min of delivery. Fragments of the lamellar region (~10 mg wet weight), were isolated and tied to hooks in triplicate. MeAIB was used as a non-metabolisable specific substrate for system A. Uptake of [¹⁴C]MeAIB was initiated by immersing the fragments in control Tyrode buffer (containing Na⁺) or Na⁺-free Tyrode buffer (choline replaced Na⁺) containing 0.1 mCi ml^-1 [¹⁴C]MeAIB at 37 °C. Uptake was terminated by washing the fragments in an excess of control or Na⁺-free ice-cold Tyrode buffer, after 5, 10, 15, 20 or 30 min to give a time course of uptake. Following lysis of the fragments in distilled water for 16-20 h, the lysate was counted for radioactivity as a measure of the labelled MeAIB transported into the placental fragments. The tissue was then transferred to 0.3 M NaOH to dissolve. A sample of each NaOH lysate was assayed to establish the protein content of each fragment. Na⁺-dependent MeAIB uptake, attributable to system A, was calculated as the difference between uptake in the presence and absence of Na⁺.

These results show that cat placental fragments exhibit system A-like transport characteristics. Thus system A as well as system β are likely to be important in provision of amino acids to the fetus by the cat placenta.

All procedures accord with current UK legislation.