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Journal of Experimental Biology 158,477-493 (1991)
Published by Company of Biologists 1991

L-Proline Transport Systems of Starfish Pyloric Caeca

GREGORY A. AHEARN ¹ and RACHEL D. BEHNKE ¹

¹ Department of Zoology, University of Hawaii at Manoa, Honolulu, HI 96822, USA

Purified brush-border membrane vesicles (BBMV) of starfish [Pycnopodia helianthoides (Brandt)] pyloric caecal epithelium were prepared by magnesium precipitation in order to characterize the possible role of this organ in amino acid transport. L-[³H]proline uptake by these vesicles was Na⁺-dependent and greater at pH7.5 than at pH5.5. L-Pipecolate was a competitive inhibitor of L-proline influx into these BBMV, exhibiting a K_i value of 0.02 mmol l^-1. The amino acid inhibitors, L-pipecolate, L-alanine and L-leucine were used as test substrates to block L-prohne influx by the IMINO, NBB and L transport systems, respectively, in order to estimate the contribution of each process to total L-prohne entry into pyloric caecal cells. The carrier-mediated transport constants for L-prohne transfer by these three systems were: K_t=0.18mmoll^-1 (IMINO), 0.13mmol l^-1 (NBB) and (0.21mmol l^-1 (L); J_max= 1310 pmol mg^-1 protein 30 s^-1 (IMINO), 360 pmol mg^-1 protein 30 s^-1 (NBB) and 470pmol mg^-1 protein 30s^-1 (L). L-Proline influxes through both the IMINO and NBB systems were sigmoidal functions of the external [Na⁺], while transfer by the L system was Na⁺-independent. Multiple sodium ions (e.g. 2 or 3 Na⁺/L-proline) appear to be associated with L-proline transport by both Na⁺-dependent transport systems, but the nature of this association (i.e. activation or energization) is unclear. Results suggest that starfish pyloric caecal epithelium possesses a similar array of L-proline transport proteins to those found in similar cell types of mammalian intestine or kidney, providing tentative support for an absorptive function for this organ.

Key words: pyloric caeca, starfish, Echinodermata, brush-border membrane vesicles, epithelium, Na⁺-dependent, cotransport, L-proline, gastrointestinal physiology, Pycnopodia helianthoides

Accepted on March 8, 1991

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