THERMAL MODULATION OF CHANNEL CATFISH INTESTINAL DIMENSIONS, BBM FLUIDITY, AND GLUCOSE-TRANSPORT

In light of the direct influence of temperature on metabolic rates and dietary loads of ectotherms, intestinal responses were evaluated by m easuring 1) dimensions, 2) transapical initial rates of transport usin g intact tissues and brush-border membrane vesicles (BBMV), and 3) BBM V fluidity using two size groups of channel catfish (Ictalurus punctat us) acclimated to different water temperatures. Intestines of larger f ish at 15 degrees C were 23% longer and 61% heavier than those at 30 d egrees C. Regardless of assay temperature, rates of glucose uptake by fish held at 30 degrees C relative to those at 15 degrees C were twofo ld higher for intact tissues and over fivefold higher for BBMV. Althou gh rates of BBMV transport were higher for smaller fish, adaptive resp onses were greater for larger fish. Temperature coefficients (Q(10)s) for BBMV transport were higher between 5 and 15 degrees C (3.5-4.5) re lative to 15 to 35 degrees C (1.9-2.0) and maybe partly related to the inability of catfish held at low temperatures to adjust apical membra ne fluidity. Our findings indicate that 1) cold-acclimated catfish mai ntain transport capaci ties by increasing intestinal dimensions, 2) hi gh acclimation temperatures increase rates of uptake by as yet unknown mechanisms, 3) thermal modulation of transport varies among species a nd nutrients, and 4) adaptive responses of ectotherms are different fr om those of homeotherms.