The Function of the Urinary Bladder in Vivo in the Freshwater Rainbow Trout

Two different approaches were employed to examine the possible urine storage and ionoregulatory functions of the urinary bladder in vivo in the freshwater trout. An indirect approach, using non-catheterized fish, involved 'spot sampling' from the bladder to determine urine composition and measurement of PEG-4000 release into surrounding water to quantify urination events. The direct approach employed a new external catheterization technique to collect naturally discharged urine. Both methods demonstrated that trout urinate in intermittent bursts at 20-30 min intervals, and that natural urine flow rate is at least 20% lower and urinary Na and Cl~ excretion rates at least 40 % lower than those determined by the traditional internal bladder catheter technique. The urine is stored for approximately 25 min prior to discharge, and significant ionic reabsorption via the bladder epithelium occurs during this period; a small residual volume is always likely to be retained. We conclude that previous studies using internal bladder catheterization have underestimated the ionoregulatory effectiveness of the entire renal system by preventing the function of the urinary bladder.