Previous work has shown that chicken erythrocyte transport systems can change with the development of the chicken. Taurine transport via two pathways (Na⁺-dependent and volume-activated) increases in erythrocytes from hatchling to 4-week-old chickens (Porter & Martin, 1992). In contrast, the influx of leucine, lysine and glycine via saturable transport systems is highest in erythrocytes from prehatch chickens and decreases with chicken age (Lerner et al. 1983). Here we report data for changes in the transport of choline and K⁺ (⁸⁶Rb⁺) in erythrocytes from chickens at different developmental ages.

Erythrocytes from 1-day-old and mature chickens were obtained commercially. Erythrocytes from 2-week-old chickens were collected via cardiac puncture, in accordance with UK legislation, into sodium heparin following anaesthesia by intramuscular injection (0.1 to 0.3 ml depending on size) of Rompan, Ketaset and phosphate-buffered saline at a ratio of 1:2:3. Chickens were then killed humanely by cervical dislocation. Erythrocyte influx measurements for choline and K⁺ were made by estimating the uptake of [¹⁴C]choline and ⁸⁶Rb⁺ at 41°C, for 15 and 20 min, respectively.

Choline transport in chicken erythrocytes was predominantly via saturable transport systems, which could be competitively inhibited by dodecyltrimethyl-ammonium. Transport was highest in erythrocytes from 1-day-old chickens and declined with chicken age. The K_m and V_max values for choline transport were 306 ± 24 µM and 38 ± 4 µmol (10¹² cells h)^-1 in erythrocytes from 1-day-old chickens, 224 ± 7 µM and 21 ± 4 µmol (10¹² cells h)^-1 in erythrocytes from 2-week-old chickens and 49 ± 11 µM and 0.44 ± 0.08 µmol (10¹² cells h)^-1 in erythrocytes from mature chickens, respectively (mean ± S.E.M.; n = 3).

Under isotonic conditions and in the presence of a 5 mM external K⁺ concentration, the total unidirectional influx of K⁺ (⁸⁶Rb⁺) was highest in erythrocytes from 1-day-old chickens and declined with chicken age. Ouabain-sensitive influx values, which can be attributed to the Na⁺-K⁺ pump, were 1.78 ± 0.29, 0.96 ± 0.15 and 0.33 ± 0.02 mmol (10¹² cells h)^-1 for erythrocytes from 1-day-old, 2-week-old and mature chickens, respectively (mean ± S.E.M.; n ▓ge│ 3). Under isotonic conditions, bumetanide-sensitive influxes, which can be attributed to Na⁺-K⁺-2Cl^– cotransport, were only present in erythrocytes from 1-day-old chickens. However, stimulated by hypertonic conditions, bumetanide-sensitive components were essentially identical in erythrocytes from 1-day- and 2-week-old chickens (1.00 ± 0.16 and 0.98 ± 0.16 mmol (10¹² cells h)^-1) and decreased in erythrocytes from mature chickens (0.27 ± 0.07 mmol (10¹² cells h)^-1; mean ± S.E.M.; n ▓ge│ 3). Hypotonic conditions stimulated an ouabain and bumetanide-insensitive K⁺ (⁸⁶Rb⁺) influx, which again was essentially identical in erythrocytes from 1-day- and 2-week-old chickens (1.32 ± 0.16 and 1.23 ± 0.19 mmol (10¹² cells h)^-1) and decreased in erythrocytes from mature chickens (0.35 ± 0.07 mmol (10¹² cells h)^-1; mean ± S.E.M.; n ▓ge│ 3).

We conclude that both choline and K⁺ (⁸⁶Rb⁺) fluxes decrease in erythrocytes from chickens with increasing age. However, the patterns of decrease are complex in nature, probably involving differential expression of kinetically distinct transporters.

We thank The Wellcome Trust (grant no. 058230) for financial support.