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 measuring 1) dimensions, 2) transapical initial rates of transport using intact tissues and brush-border membrane vesicles (BBMV), and 3) BBMV fluidity using two size groups of channel catfish (Ictalurus punctatus) acclimated to different water temperatures. Intestines of larger fish at 15 degrees C were 23% longer and 61% heavier than those at 30 degrees C. Regardless of assay temperature, rates of glucose uptake by fish held at 30 degrees C relative to those at 15 degrees C were twofold higher for intact tissues and over fivefold higher for BBMV. Although rates of PBMV transport were higher for smaller fish, adaptive responses were greater for larger fish. Temperature coefficients (Q10S) for BBMV transport were higher between 5 and 15 degrees C (3.5-4.5) relative to 15 to 35 degrees C (1.9-2.0) and may be partly related to the inability of catfish held at low temperatures to adjust apical membrane fluidity. Our findings indicate that 1) cold-acclimated catfish maintain transport capacities by increasing intestinal dimensions, 2) high acclimation temperatures increase rates of uptake by as yet unknown mechanisms, 3) thermal modulation of transport varies among species and nutrients, and 4) adaptive responses of ectotherms are different from those of homeotherms).